U.S. patent application number 12/041857 was filed with the patent office on 2008-11-06 for method for making nanostructured soldered or brazed joints with reactive multilayer foils.
Invention is credited to Etienne Besnoin, Omar Knio, Jiaping Wang, Timothy Weihs.
Application Number | 20080272181 12/041857 |
Document ID | / |
Family ID | 46302061 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080272181 |
Kind Code |
A1 |
Wang; Jiaping ; et
al. |
November 6, 2008 |
METHOD FOR MAKING NANOSTRUCTURED SOLDERED OR BRAZED JOINTS WITH
REACTIVE MULTILAYER FOILS
Abstract
Self-propagating formation reactions in nanostructured
multilayer foils provide rapid bursts of heat at room temperature
and therefore can act as local heat sources to melt solder or braze
layers and join materials. This reactive joining method provides
very localized heating to the components and rapid cooling across
the joint. The rapid cooling results in a very fine microstructure
of the solder or braze material. The scale of the fine
microstructure of the solder or braze material is dependant on
cooling rate of the reactive joints which varies with geometries
and properties of the foils and components. The microstructure of
the solder or braze layer of the joints formed by melting solder in
a furnace is much coarser due to the slow cooling rate. Reactive
joints with finer solder or braze microstructure show higher shear
strength compared with those made by conventional furnace joining
with much coarser solder or braze microstructure. It is expected
that the reactive joints may also have better fatigue properties
compared with conventional furnace joints.
Inventors: |
Wang; Jiaping; (Vienna,
VA) ; Besnoin; Etienne; (Baltimore, MD) ;
Knio; Omar; (Timonium, MD) ; Weihs; Timothy;
(Baltimore, MD) |
Correspondence
Address: |
PATENT DOCKET ADMINISTRATOR;LOWENSTEIN SANDLER PC
65 LIVINGSTON AVENUE
ROSELAND
NJ
07068
US
|
Family ID: |
46302061 |
Appl. No.: |
12/041857 |
Filed: |
March 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10844816 |
May 13, 2004 |
7361412 |
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12041857 |
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10761688 |
Jan 21, 2004 |
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10844816 |
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09846486 |
May 1, 2001 |
6736942 |
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10761688 |
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60469841 |
May 13, 2003 |
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60475830 |
Jun 4, 2003 |
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60201292 |
May 2, 2000 |
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Current U.S.
Class: |
228/234.3 |
Current CPC
Class: |
B23K 28/00 20130101;
B32B 15/017 20130101; B23K 35/0238 20130101; G05B 17/02 20130101;
F24V 30/00 20180501; H05K 3/3494 20130101; B23K 20/00 20130101;
H05K 2203/1163 20130101; B23K 20/08 20130101; B23K 31/02 20130101;
H05K 3/3463 20130101; B23K 35/0233 20130101; B23K 2103/10 20180801;
H05K 2203/0405 20130101; Y10T 428/12493 20150115; B23K 1/0016
20130101; Y10T 428/12986 20150115; B23K 2101/40 20180801; B23K
31/12 20130101; C06B 45/14 20130101; B23K 2103/05 20180801; C23C
14/14 20130101; B23K 1/0006 20130101; C06B 21/0083 20130101; B23K
20/165 20130101; B23K 35/34 20130101; B23K 20/06 20130101; B32B
15/01 20130101 |
Class at
Publication: |
228/234.3 |
International
Class: |
B23K 1/00 20060101
B23K001/00 |
Goverment Interests
GOVERNMENT INTEREST
[0003] The United States Government has certain rights in this
invention pursuant to Award DMI-0115238 supported by NSF.
Claims
1. A method of joining together a first body and a second body
comprising the steps of: disposing between the first body a second
body, a reactive multilayer foil and at least one layer of braze or
solder material adjacent the foil; pressing the bodies together
against the foil; and igniting the foil to melt the braze or
solder, wherein the melted braze or solder has a cooling rate
sufficient to produce a lamellar spacing when solidified of less
than 100 nanometers.
2. The method of claim 1 wherein the reactive multilayer foil is a
freestanding reactive multilayer foil.
3. A method of joining together a first body and a second body
comprising the steps of: disposing between the first body a second
body, a reactive multilayer foil and at least one layer of braze or
solder material adjacent the foil; pressing the bodies together
against the foil; and igniting the foil to melt the braze or
solder, wherein the melted braze or solder has a cooling rate
sufficient to produce a lamellar spacing when solidified of less
than 50 nanometers.
4. A method of joining together a first body and a second body
comprising the steps of: disposing between the first body a second
body, a reactive multilayer foil and at least one layer of braze or
solder material adjacent the foil; pressing the bodies together
against the foil; and igniting the foil to melt the braze or
solder, wherein the melted braze or solder has a cooling rate
sufficient to produce a lamellar spacing when solidified of less
than 10 nanometers.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/844,816, filed on May 13, 2004, which is also a
continuation-in-part of U.S. application Ser. No. 10/761,688 filed
on Jan. 21, 2004, which is a divisional of U.S. application Ser.
No. 09/846,486 filed on May 1, 2001 (issued as U.S. Pat. No.
6,736,942), U.S. application Ser. No. 10/844,816 claims the benefit
of U.S. Provisional Application No. 60/469,841 filed on May 13,
2003 and U.S. Provisional Application No. 60/475,830 filed on Jun.
4, 2003, U.S. application Ser. No. 09/846,486 (issued as U.S. Pat.
No. 6,736,942) claims the benefit of U.S. Provisional Application
No. 60/201,292, filed on May 2, 2000.
[0002] U.S. application Ser. No. 10/844,816; U.S. application Ser.
No. 10/761,688; U.S. application Ser. No. 09/846,486; U.S.
Provisional Application No. 60/469,841; U.S. Provisional
Application No. 60/475,830; and U.S. Provisional Application No.
60/201,292 are each hereby incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0004] The joining of components of the same or different materials
is fundamental in the manufacture of a wide variety of products
ranging from ships and airplanes to tiny semiconductor and optical
devices. Joining by brazing or soldering is particularly important
in the assembly of products composed of metal parts and the
fabrication of electronic and optical devices.
[0005] Typically soldered or brazed products are made by
sandwiching a braze or solder between mating surfaces of the
respective components and heating the sandwiched structure in a
furnace or with a torch. Unfortunately these conventional
approaches often expose both the components and the joint area to
deleterious heat. In brazing or soldering, temperature-sensitive
components can be damaged, and thermal damage to the joint may
necessitate costly and time consuming anneals. Alternatively, the
presence of heat-sensitive components, such as semiconductors
devices, may require low temperature joining that produces weaker
joints.
[0006] Accordingly there is a need for improved methods of joining
products by braze or solder and the improved joined products that
they can produce.
SUMMARY OF THE INVENTION
[0007] The present inventors have determined that the conventional
approach of brazing or soldering using furnaces or torches
inherently produces sub-optimal joints. The furnace or torch heats
not only the joint area but also the bodies to be joined. The
heating of these bodies adjacent the joint area, combined with the
insulating effect of the bodies, slows the cooling of the braze or
solder and produces a joint of enlarged microstructure and weakened
mechanical properties.
[0008] In accordance with the invention, bodies of materials are
joined between mating surfaces by disposing reactive nanostructured
foils between the mating surfaces and adjacent one or more layers
of braze or solder. The composition and thickness of the foils are
chosen, as by thermal modeling techniques, to minimize deleterious
heating of the bodies and to provide an optimal heat profile to
produce a nanostructured joint having superior mechanical
properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The nature, advantages and various additional features of
the invention will appear more fully upon consideration of the
illustrative embodiments now to be described in detail in
connection with the accompanying drawings. In the drawings:
[0010] FIG. 1 is a schematic drawing of a self-propagating reaction
in a multilayer reactive foil.
[0011] FIG. 2 is a schematic diagram illustrating the reactive
joining of two components using a reactive foil and a pair of braze
or solder layers;
[0012] FIG. 3 illustrates the reactive joining of Au-coated
stainless steel shear lap specimens;
[0013] FIGS. 4A and 4B are SEM micrographs of stainless steel
components joined using (A) Al/Ni foil and sheets of freestanding
AuSn solder and (B) conventional furnace soldering.
[0014] FIGS. 5A and 5B are micrographs depicting microstructures of
AuSn solder from (A) reactive foil joining and (B) conventional
furnace soldering;
[0015] FIG. 6 is a graphical illustration plotting predicted
temperature versus time at the center of AuSn solder layers made
with reactive foils in (1) stainless steel and (2) aluminum
joints;
[0016] FIGS. 7A and 7B are micrographs depicting fine lamellar
entectic structure of AuSn solder in (A) reactive foil joined
stainless steel and (B) reactive foil joined aluminum;
[0017] FIG. 8(a) depicts a schematic view of a first reactive
multilayer joining configuration;
[0018] FIG. 8(b) depicts a schematic view of a second reactive
multilayer joining configuration;
[0019] FIG. 9(a) depicts a schematic view of a third reactive
multilayer joining configuration;
[0020] FIG. 9(b) depicts a schematic view of a fourth reactive
multilayer joining configuration;
[0021] FIG. 10(a) depicts exemplary measured temperature profiles
of the reactive multilayer joining configuration of FIG. 9a;
[0022] FIG. 10(b) depicts exemplary predicted temperature profiles
of the reactive multilayer joining configuration of FIG. 9a;
[0023] FIG. 11(a) depicts predicted temperature profiles for an
example of the reactive multilayer joining configuration of FIG.
9b;
[0024] FIG. 11(b) depicts measured and predicted temperature
profiles for an example of the reactive multilayer joining
configuration of FIG. 9b;
[0025] FIG. 12 depicts a schematic view of a reactive multilayer
joining configuration;
[0026] FIG. 13(a) depicts an exemplary graphical display of a
relationship between foil thickness and heat of reaction;
[0027] FIG. 13(b) depicts an exemplary graphical display of a
relationship between foil thickness and front velocity;
[0028] FIG. 14 depicts exemplary graphical results for the reactive
multilayer joining configurations of FIG. 9(b) and FIG. 12;
[0029] FIG. 15 depicts exemplary graphical results for the reactive
multilayer joining configurations of FIG. 9(b) and FIG. 12;
[0030] FIG. 16 depicts a schematic view of a reactive multilayer
joining configuration;
[0031] FIG. 17(a) depicts exemplary predicted temperature profiles
of the reactive multilayer joining configuration of FIG. 16;
[0032] FIG. 17(b) depicts an exemplary measured infrared
temperature distribution of the reactive multilayer joining
configuration of FIG. 16;
[0033] FIG. 17(c) depicts an exemplary measured infrared
temperature distribution of the reactive multilayer joining
configuration of FIG. 16;
[0034] FIG. 18 depicts exemplary graphical results for the reactive
multilayer joining configuration of FIG. 16;
[0035] FIG. 19 depicts exemplary graphical results for the reactive
multilayer joining configuration of FIG. 16;
[0036] FIG. 20 depicts exemplary graphical results for the reactive
multilayer joining configuration of FIG. 16;
[0037] FIG. 21 depicts a schematic view of a reactive multilayer
joining configuration;
[0038] FIG. 22 depicts exemplary graphical predictions for the
reactive multilayer joining configuration of FIG. 21;
[0039] FIG. 23 depicts a schematic view of a reactive multilayer
joining configuration;
[0040] FIG. 24 depicts exemplary predicted temperature profiles of
the reactive multilayer joining configuration of FIG. 23;
[0041] FIG. 25(a) depicts exemplary predicted results of the
reactive multilayer joining configuration of FIG. 23;
[0042] FIG. 25(b) depicts exemplary predicted results of the
reactive multilayer joining configuration of FIG. 23; and
[0043] FIG. 26 depicts a schematic view of a reactive multilayer
joining configuration.
[0044] It is to be understood that these drawings are for purposes
of illustrating the concepts of the invention and, except for the
graphs and micrographs, are not to scale.
DETAILED DESCRIPTION
[0045] This description is divided into three parts. Part I
describes and illustrates reactive foil joining and the resulting
joints. Part II describes a thermal modeling technique useful
optimizing reactive foil joining, and Part III exemplifies the
application of the thermal model to produce superior joints.
References indicated by bracketed numbers are fully cited in an
attached List.
[0046] I. The Method and Resulting Joined Products
[0047] A. Multilayer Reactive Foils and Their Use in Forming
Joints
[0048] Self-propagating exothermic formation reactions have been
observed in a variety of nanostructured multilayer foils, such as
Al/Ni, Al/Ti, Ni/Si and Nb/Si foils .sup.[1-4]. These reactions are
driven by a reduction in atomic bond energy. Once the reactions are
initiated by a pulse of energy, such as a small spark or a flame,
atomic diffusion occurs normal to the layering.
[0049] FIG. 1 schematically illustrates a multilayer reactive foil
14 made up of alternating layers 16 and 18 of materials A and B,
respectively. These alternating layers 16 and 18 may be any
materials amenable to mixing of neighboring atoms (or having
changes in chemical bonding) in response to a stimulus. Preferably
the pairs A/B of elements are chosen based on the way they react to
form stable compounds with large negative heats of formation and
high adiabatic reaction temperatures. A wide variety of such
combinations is set forth in the above referenced U.S. patent
application Ser. No. 09/846,486 which is incorporated herein by
reference.
[0050] The bond exchange generates heat very rapidly. Thermal
diffusion occurs parallel to the layering and heat is conducted
down the foil and facilitates more atomic mixing and compound
formation, thereby establishing a self-propagating reaction along
the foil. The speeds of these self-propagating exothermic reactions
are dependent on layer thickness and can rise as high as 30 m/s,
with maximum reaction temperatures above 1200.degree. C.
.sup.[5].
[0051] Reactive multilayer foils provide a unique opportunity to
dramatically improve conventional soldering and brazing
technologies by using the foils as local heat sources to melt
solder or braze layers and thereby join components. Reactive foil
soldering or brazing can be performed at room temperature and in
air, argon or vacuum.
[0052] FIG. 2 schematically shows the use of multilayer reactive
foil 14 to join together two components 20A and 20B. The reactive
foil 14 is sandwiched between the mating surfaces 21A and 21B of
the components and adjacent one or more layers 22A, 22B of braze or
solder. The reactive foil 14 is preferably a freestanding reactive
foil as described in the aforementioned application Ser. No.
09/846,486 (issued as U.S. Pat. No. 6,736,942) but could be a
coating on one or more of the components. The braze or solder
layers can also be freestanding or coatings on the components.
[0053] Once the components, foil and solder or braze are assembled,
an ignition stimulus 23 is applied to foil 14 produces rapid and
intense heat diffusing as a thermal wavefront through the foil.
[0054] This new reactive joining process eliminates the need for
furnaces or other external heat sources, and with very localized
heating, temperature sensitive components or materials can be
joined without thermal damage. The localized heating offered by the
reactive foils is also advantageous for joining materials with very
different coefficients of thermal expansion, e.g. joining metal and
ceramics. Typically when metals are soldered or brazed to ceramics,
significant thermal stresses arise on cooling from the high
soldering or brazing temperatures, because of the thermal expansion
coefficient mismatch between metals and ceramics. These thermal
stresses limit the size of the metal/ceramic joint area. When
joining with reactive multilayers, the metallic and ceramics
components absorb little heat and have a very limited rise in
temperature. Only the solder or braze layers and the surfaces of
the components are heated substantially. Thus the typical mismatch
in thermal contraction between metallic and ceramics components and
the resulting delamination are avoided and strong metal/ceramics
joints with large areas can be formed by this reactive joining
process.
[0055] In addition, the reactive joining process is fast and
cost-effective, and results in strong and thermally-conductive
joints. Substantial commercial advantages can thus be achieved,
particularly for assembly of microelectronic devices.
[0056] B. Factors Affecting the Microstructure of Joints
[0057] There are many different properties to be considered in
solder or braze joints, such as mechanical, thermal and electrical
properties, depending on the different applications of these
joints. Among these, mechanical properties are often the most
important in many joining application since joints without any
mechanical strength cannot be used in practice. The mechanical
properties that are important to the service behavior of solder or
braze joints include their strength and resistance to fatigue.
Considering the potential use of the reactive joints as
load-bearing components, improvement of mechanical properties of
the joints becomes an even more important issue. In order to
optimize the mechanical properties of the reactive joints, it is
essential to study the microstructure of the solder or braze within
the joint and to understand how the microstructure might affect the
mechanical properties of the joint.
[0058] Reactive joining is such a very rapid process that the total
heating and cooling is completed within less than one second. With
a rapid cooling rate greater than 500.degree. C./second, the
microstructures of the solder or braze materials in reactive joints
might be very different from those obtained from conventional
furnace soldering or brazing. Previous research on reactive joining
has not addressed this issue. The present invention describes the
very different microstructures of solder or braze materials in
reactive foil joints and conventional furnace joints due to
different cooling rates in these two processes and relates the
microstructures of the solder or braze materials with mechanical
properties of these joints.
[0059] It has been shown in literature that cooling rate greatly
affect the microstructures of materials and mechanical properties
are dependent on the microstructures of materials. Since eutectic
lamellar microstructures can be observed in conventional solder
joints, current research about effects of cooling rates and
microstructures on mechanical properties in alloys with lamellar
eutectic microstructure will be reviewed in more details.
[0060] Effect of cooling rate on eutectic microstructures has been
studied in several alloy systems. Sn--Ag--Cu alloy is one of the
most commonly used solder materials in electronics industry.
Noguchi et al. .sup.[6] studied the microstructure of Sn--Ag--Cu
solder ball bonding formed at various cooling rates, 200, 100, 60,
50, and 10.degree. C./min. The lamellar spacing becomes smaller at
faster cooling rate. In this study, the lamellar spacing, ranges
between 400 and 2000 nm. The relationship between the cooling rate,
Rc, and lamellar spacing .lamda., was experimentally determined
as
.lamda.=K/Rc.sup.1/2
[0061] where K is a constant. Kim et al. .sup.[7] also studied the
microstructures of Sn--Ag--Cu alloy prepared under different
cooling rates, 0.012.degree. C./s, 0.43.degree./s and 8.degree.
C./s, showing that the eutectic microstructure was coarsened by
decreasing the cooling rate.
[0062] Cooling rate has similar effect on eutectic microstructures
in other alloys. For example, lamellar spacing in two-phase Ti-48Al
alloys was investigated as a function of cooling rate by Tang et
al. .sup.[8]. It was found that the lamellar spacing is inversely
proportional to the cooling rate. As the cooling rate increases
from 0.1.degree. C./s to 1.degree. C./s, the lamellar spacing
decreases from 2000 nm to 250 nm.
[0063] Mei and Morris .sup.[9] studied the microstructures of
Sn60-Pb40 solder joints which were cooled at different conditions:
quenched in ice water, air-cooled to room temperature in about 5
minutes, and furnace-cooled in about 30 minutes. The furnace cooled
solder joint has a typical lamellar and colony appearance: the two
phases are arranged side by side in long range, differently
oriented arrays that form colonies. In the air-cooled solder joint,
the colony size seems smaller, and the lamellae become shorter. The
quenched solder joint has finer features. Increasing the cooling
rate of a 60Sn/40Pb solder joint disturbs the regular formation of
a lamellar/colony microstructure, and results in a more
fine-grained microstructure.
[0064] It has been found that in several solder alloy, such as
Sn--Ag--Cu, Sn--Pb, Sn--Ag, Sn--Zn, Sn--Bi alloys, finer eutectic
microstructures obtained by increasing the cooling rate result in
higher strength and higher hardness .sup.[7][10][6][11]. In other
eutectic alloys, such as Ti--Al alloy, it was also observed that
finer lamellar spacing and smaller colony size result in higher
strength and hardness .sup.[12][13][14][15] The relationships
between lamellar spacing and colony size and yield strength of the
alloy follow the extended Hall-Petch equation for lamellar
microstructure .sup.[16],
.sigma. y = .sigma. 0 + K .lamda. or .sigma. y = .sigma. 0 + K d
##EQU00001##
[0065] where .sigma..sub.y is the yield stress, .lamda. is the
lamellar spacing, d is the colony size, and .sigma..sub.0 and K are
constants.
[0066] It has been reported in literature that fatigue properties
of materials also depend on microstructures. For example, in
60Sn/40Pb solder alloy .sup.[9], increasing the cooling rate of the
solder joint results in a finer-grained microstructure,
facilitating grain boundary deformation mechanisms and leading to a
longer fatigue life. In a TiAl alloy with lamellar microstructures,
it has been found that in the coarser colony microstructure
(approximate to 1400 .mu.m), the fatigue crack growth threshold
(.DELTA.K.sub.th) is markedly decreased compared with the finer
colony microstructure (90 .mu.n), while the crack growth resistance
remains constant. The fine lamellar spacing (0.2-0.7 .mu.m)
microstructures result in higher .DELTA.K.sub.th and fatigue crack
growth resistance compared to the coarse lamellar spacing
(approximate to 5.5 .mu.m) microstructure. It was suggested that
this higher fatigue resistance is mainly attributed to the higher
number of lamellar interfaces resistant to crack advance, as well
as to the higher closure effects. The colony boundaries and the
lamellar interfaces play an important role in retarding the
advancing crack at room temperature, serving as barriers for the
dislocation movement and as sinks for dislocation pile-ups
.sup.[17].
[0067] C. Experiments Relating Microstructure with Joint
Properties
[0068] The reactive multilayers used in the reactive joining
process are nanostructured materials [1-4, 7-11] that are typically
fabricated by vapor depositing hundreds of nanoscale layers that
alternate between elements with large, negative heats of mixing,
such as borides (e.g. Ti/B), carbides (e.g. Ti/C), silicides (e.g.
Ni/Si and Zr/Si, etc), aluminides (e.g. Ni/Al, Ti/Al and Zr/Al,
etc) or others. The solder or braze materials are commercial
solder/braze alloys used in soldering/brazing industry, such as
AuSn, AgSn, PbSn, Cusil, Incusil or others.
[0069] As an example, here reactive Al/Ni multilayer foils and AuSn
solder were used. The foils were obtained by magnetron sputtering
and the final product of the reaction is the AlNi compound. To
enhance wetting of the foils by the AuSn solder during joining, the
foils were coated with a 1 .mu.m thick wetting layer of Incusil ABA
braze. Joining of gold-coated stainless steel specimens and Al
specimens using Al/Ni foils and AuSn solder will be described in
detail below. The heating and cooling rate of the reactive joining
process were evaluated using infrared imaging. The microstructure
and shear strength of the resulting joints were characterized using
scanning electron microscopy (SEM) and tensile shear lap tests.
[0070] Referring to FIG. 3 stainless steel joints 30 were
fabricated by stacking two sheets of AuSn solder 31A, 31B and one
reactive foil 32 between two stainless steel samples 33A, 33B. The
dimensions of the stainless steel specimens were 0.5 mm.times.6
mm.times.25 mm and were electroplated with Ni and then Au to
enhance bonding. The Ni layer serves to promote adhesion to the
stainless steel after removal of the native oxide, and the Au
coating is designed to prevent surface oxidation and thereby
enhance wetting by molten AuSn solder. These stainless steel
samples were joined at room temperature in air by igniting the
reactive foils under pressure of approximately 100 MPa. The joint
area was approximately 5 mm by 6 mm. Au and Ni plated Al (6061)
specimens were joined in the same way, with joint area of 3
mm.times.6 mm. The smaller joint area was used for the Al specimens
to avoid deformation or fracture of the Al specimens themselves
before the failure of the solder joint, due t0 the low tensile
strength of the Al 6061 alloy.
[0071] In order to estimate the cooling rate of reactive joining,
temperatures in the stainless steel components during the reactive
joining were measured using an infrared camera for the case of a 70
.mu.m Al/Ni foil and 25 .mu.m thick AuSn solder sheets. Before the
joining process, the sides of the stainless steel specimens were
carefully polished to a 6 .mu.l finish and painted white, to ensure
a uniform emissivity. Then the temperatures at the side surfaces of
the components were monitored during the reactive joining using the
infrared camera with a spatial resolution of 108 .mu.m and a
temporal resolution of 0.2 seconds. Based on a series of thermal
profiles, it was estimated that the total heating time is less than
0.2 seconds. After the reaction, temperatures in the stainless
steel specimens decreased very quickly. In the stainless steel
components, at 100 microns from the interface between the solder
layer and the stainless steel, the temperature decreased to
60.4.degree. C. and 38.8.degree. C. at 0.2 seconds and 0.8 seconds
after reaction respectively. Here the cooling rate is very rapid
and is estimated to be >1000.degree. C./second.
[0072] For comparison some stainless steel specimens were joined
using a furnace to heat the AuSn solder instead of using a reactive
foil. In this case, two pieces of stainless steel specimens and one
piece of AuSn solder (25 .mu.m thick) were clamped together and
heated above the melting temperature of AuSn solder in air. Here
the cooling rate is much slower which is about 1.degree.
C./second.
[0073] Cross sections of untested stainless steel joints were
polished to a 1 .mu.m finish and then characterized using scanning
electron microscopy (SEM) in a JEOL microscope. FIG. 4(a) shows two
stainless steel specimens that were joined using two pieces of
free-standing AuSn solder (25 .mu.m thick) and one Al/Ni reactive
foil (80 .mu.m thick). Cracking was observed within the reacted
foils and is attributed to the fact that when the foils react they
contract due to densification; they also contract due to cooling
from the high reaction temperatures. Both sources of contraction
can be constrained by the surrounding material, thereby leading to
cracking. Molten AuSn solder typically flowed into these cracks,
creating a particle composite interface with hard pieces of
reactive foil in a solder matrix. Note that the AuSn solder layers
decreased in thickness from 25 .mu.m to several microns, suggesting
that the majority of the solder flows into cracks and out of the
bond area, due to the applied pressure. The microstructure of the
AuSn solder layer is shown in FIG. 5(a) at higher magnification. A
very fine lamellar eutectic structure is observed, including a
light Au rich phase (.xi. phase, Au.sub.5Sn) and a dark Sn rich
phase (.delta. phase, AuSn). These two phases grow simultaneously
and form parallel plates in grain-like colonies. The spacing
between these plates is about 50 nm.
[0074] FIG. 4(b) shows two stainless steel specimens that were
joined using a piece of free-standing AuSn solder (25 .mu.m thick)
heated in furnace. The thickness of the AuSn solder layer remains
at 25 .mu.m after soldering compared with several micron AuSn
solder left within the joint by reactive joining. The
microstructure of the AuSn solder of the joint formed by melting
solder in a furnace is much coarser as shown in FIG. 5(b) at higher
magnification. This is due to the slow cooling rate in conventional
soldering. The lamellar eutectic microstructure has a very high
interfacial area per unit volume, and therefore is
thermodynamically unstable. The Au-rich phases and Sn-rich phases
grow and eventually rearrange themselves into a coarser equiaxed
structure.
[0075] Study of the microstructure of the AuSn solder layer in
reactive joints and conventional furnace solder joint suggests that
solder material with a much finer microstructure can be obtained
during reactive joining process due to its very rapid cooling
rate.
[0076] Stainless steel joints made by both reactive joining and
conventional furnace soldering were tested in tension at room
temperature using an Instron testing machine and a crosshead speed
of 0.1 mm/min. Shear strengths of these joints were obtained by
dividing the maximum failure load by the joint area. The average
shear strength of the stainless steel joints by reactive joining is
approximately 48.+-.3 MPa. In comparison, the average shear
strength of the stainless steel joints made by conventional
soldering was only 38.+-.1 MPa. The lower strengths of these joints
can be attributed to their coarser microstructure (FIG. 5(b)),
compared to the fine eutectic microstructure (FIG. 5(a)) for
reactive multilayer joints that cool very rapidly. It could also be
attributed to their thick 25 .mu.m AuSn solder layer (FIG. 4(b)),
compared to the several microns thick AuSn solder layer (FIG. 4(a))
in reactive joints. In order to demonstrate that the thickness
factor is not important, some stainless steel joints made by
reactive joining were annealed at the AuSn solder's melting
temperature for 5 minutes. It was found that the average shear
strength of these annealed joints decreases to about 39 MPa,
similar to the average shear strength of stainless steel joints by
conventional furnace soldering. For these joints, the AuSn solder
layer is several microns thick with a coarse microstructure. This
demonstrated that the lower strengths of these joints by
conventional furnace soldering are attributed to their coarser
microstructure rather than their thicker solder layer. It is
expected that if other solder or braze materials are used, finely
microstructured solder or braze materials can also be obtained by
reactive joining due to the rapid cooling rate, and therefore, that
the shear strength of the reactive joints will be higher than those
made by conventional soldering or brazing joints.
[0077] Data in literature show that materials with finer
microstructure might have higher hardness, strength and better
fatigue properties compared with those with much coarser
microstructure. It is expected that the reactive joints with much
finer microstructured solder or braze layer may also have better
fatigue properties compared with conventional furnace joints. This
is a very important advantage for the applications of the reactive
joining process.
[0078] This demonstrated that the very localized heating and very
rapid cooling during reactive joining can not only offer the
ability to join temperature sensitive materials or dissimilar
material but also improve the mechanical properties of the joints
by producing very fine microstructure of the solder or braze
materials.
[0079] In addition, the scale of the fine microstructure of the
solder or braze material is dependant on cooling rate of the
reactive joints which varies with geometries and properties of the
foils and components. For example, the differences in thermal
conductivities of stainless steel and Al also lead to differences
in cooling rate following reactive joining. FIG. 6 is a plot of
temperature versus time at the center of the AuSn solder layer in
both stainless steel joint and Al joint made with two 25 .mu.m
thick AuSn solder layers and one 80 .mu.m thick Al/Ni foil. The
numerical predictions show that following reaction of the foil,
temperature at the center of the solder layer in the stainless
steel joint decreases from 700.degree. C. to 400.degree. C. within
1 ms, with a maximum cooling rate of 0.7.times.10.sup.6.degree.
C./s. It takes 3 ms for the center of the solder layer to cool down
to its melting temperature, 280.degree. C. The cooling rate at the
center of the solder layer is 2.times.10.sup.4.degree. C./s at this
moment. The Al joint cools faster than the stainless steel joint
and temperature at the center of the solder layer decreases from
600.degree. C. to 260.degree. C. within 1 ms, with a higher maximum
cooling rate of 1.1.times.10.sup.6.degree. C./s. It takes 0.7 ms
for the center of the solder layer to cool down to its melting
temperature, 280.degree. C. At this moment, the cooling rate at the
center of the solder layer is 1.1.times.10.sup.5.degree. C./s.
[0080] The difference in cooling rates will impact microstructures
of the solder layers as seen in FIG. 7. The microstructures of the
AuSn solder in an Al joint and a stainless steel joint, both made
with 80 .mu.m thick Al/Ni foils and 25 .mu.m thick AuSn solder
layers, are shown in FIGS. 7(a) and 7(b). In both stainless steel
and Al joints, a very fine lamellar eutectic structure is observed,
including a light Au rich phase and a dark Sn rich phase. The
formation of the fine lamellar structure is due to the very rapid
cooling of the reactive joint. The lamellar spacings of the AuSn
solder in the stainless steel joint and the Al joint are
approximately 30 nm and 20 nm, respectively. The microstructure of
the solder layer, in turn, will impact joint strength if failure
occurs in the solder layer.
[0081] II. A Thermal Modeling Technique Useful for Optimizing the
Microstructure of Reactive Foil Joints
[0082] In one embodiment of this invention, a computational model
formulation is used for choosing a reactive foil that will optimize
the microstructure of a joint. The model is applied by discretizing
(i.e., making mathematically discrete; defining for a finite or
countable set of values; not continuous) an unsteady energy
equation in a computational domain (e.g., including computational
inputs and/or boundaries) that includes one or more properties of
the reactive multilayer foil, the surrounding joining layers (e.g.,
solder and/or braze) and the components to be joined. In one
example, this discretization is implemented by integrating the
model formula set forth herein using as inputs various dimensions
and physical properties of one or more of the reactive multilayer
foil, the surrounding joining layers, and the components, as well
as boundary conditions of the computational domain. One example
includes a two-dimensional discretization in which the domains
representing the foil, joining layers and the components are
rectangular domains, each specified in terms of its length and
thickness.
[0083] The embodiments below provide specific illustration of such
configurations, where a heat release rate corresponds to an
essentially flat self-propagating energy front traveling along the
length of the reactive multilayer foil (e.g., the energy or heat
wave front produced across one or more of the reactive multilayer
foil, the surrounding joining layers, and the components when the
reactive multilayer foil is ignited). For such implementation,
inputs to the computational model include: (a) the dimensions
(length and thickness) of the components, solder and/or braze
layers, and the reactive foil, (b) the density, heat capacity, and
thermal conductivity of the components, (c) the density, heat
capacity, thermal conductivity, heat of fusion, and melting
temperature of the solder and/or braze layers, (d) the heat of
reaction and the propagation velocity, (e) the ignition location,
(f) the density, heat capacity, thermal conductivity, heat of
fusion, and melting temperature of the product of reaction in the
reactive multilayer, and (g) thermal and mass flux conditions on
domain boundaries. Computational solution of the discretized model
equations then provides the transient evolution of the thermal
waves within the foil, the joining layers, and the components.
[0084] For example, application of the model may include providing
the length, width, and thickness of each of a reactive multilayer
foil, a first component, a second component, a first joining layer,
and a second joining layer. Using these respective lengths, widths,
and thicknesses as inputs, as well as thermal and mass flux
conditions on domain boundaries, the formula set forth below is
integrated for each of the reactive multilayer foil, the first
component, the second component, the first joining layer, and the
second joining layer. When integrated, the output is the prediction
of a how an energy or thermal wave front will propagate in each of
the reactive multilayer foil, the first component, the second
component, the first joining layer, and the second joining layer
when the reactive multilayer nanofoil is ignited.
[0085] Any of the aforementioned predictions of the computational
model formulation (e.g., the prediction of how the energy or heat
wavefront will behave in each of the reactive multilayer foil, the
first component, the second component, the first joining layer, and
the second joining layer) may be used to assess the magnitude and
duration of various joining parameters such as melting of the
solder and/or braze layers, the wetting of critical interfaces, and
the thermal exposure of the components. The model can thus predict
insufficient melting of the solder and/or braze, lack of wetting at
critical interface, excessively short melting duration, or
excessive thermal exposure of the components, in which case the
parameters of the reactive joining configuration can be
systematically altered. The model can be reapplied to the altered
configuration to verify whether the parameters are suitable.
Examples include systematic variation of the thickness of the foil
and the thicknesses of the solder and/or braze layers, the heat of
reaction (for instance by altering the composition or
microstructure), and/or the solder material. Such systematic
variation of parameters can be iteratively applied until a suitable
configuration is determined. It should be evident for one skilled
in the art how to generalize such an iterative approach to include
other configuration parameters and iteration methods. For example,
the inputs to the model may be any combination of any of the
physical properties of any of the materials set forth herein.
[0086] Embodiments of the invention include a multi-dimensional
computational code for simulating the reactive joining process. The
code may be run and/or stored on a computer or any other suitable
computer readable medium. The code may be an implementation of a
multi-dimensional transient formulation of an energy equation that
accounts for the properties of the self-propagating reaction as
well as the physical properties of the reactive nanofoil, the
fusible materials, and/or the components. The computational model
formulation consistent with the present invention will next be
described.
[0087] The multi-dimensional transient formula may be based on a
specially-tailored mathematical formulation that combines the
unsteady energy equation:
.rho. .differential. h .differential. t = .gradient. q + ( 1 )
##EQU00002##
[0088] with a simplified description of the reaction front
represented by . In Eq. (1), h denotes the enthalpy, .rho. is the
density, t is time, q is the heat flux vector, and is the heat
release rate. The enthalpy, h, is related to the temperature, T,
through a detailed relationship that involves the material's heat
capacity, c.sub.p, and the latent heat, h.sub.f. The term
represents the rate of heat released by the self-propagating front
as it traverses the reactive foil. The latter is described in terms
of a thin front that propagates in the direction normal to its
surface. The propagation speed is prescribed using either measured
or computed values. Examples of the measured and computed
propagation speeds are shown in FIG. 13(b). The strength of is thus
obtained by combining the known reaction velocity and heat of
reaction for a given reactive foil. Note that is localized within
the front that traverses the nanofoil, and vanishes in within the
one or more fusible materials and/or components.
[0089] The propagation of the heat or energy wave (e.g., evolution
of the temperature) within the configuration, as well as the
evolution of the melting and/or solidification of the one or more
fusible materials, may be determined by integrating Eq. (1) over
the entire configuration. A transient finite-difference
computational model of the above formulation has been developed for
this purpose. The finite-difference discretization is based on
dividing the domain into computational cells of fixed grid size.
Enthalpy is defined at cell centers, while fluxes are defined at
cell edges. Second-order centered-difference approximations are
used to approximate spatial derivatives. This spatial
discretization scheme results in a finite set of coupled ordinary
differential equations (ODEs) that govern the evolution of the
enthalpy at the cell centers. The set of ODEs is integrated in time
using the explicit, third-order Adams Bashforth scheme. Based on
the resulting solution, one can readily determine various
properties of the reactive joining process, including the amount of
solder that melts at a specific cross-section or spatial location,
the corresponding melting duration, as well as the temperature
evolution within the foil, solder or braze layers, and the
components. It should be evident for someone skilled in the art how
to implement various alternative spatial discretizations of
arbitrary order, including as finite-element, spectral-element, or
collocation approximations, as well as various implicit, explicit,
or semi-implicit time-integration schemes.
[0090] Note that, in the case of a one-dimensional (or flat)
reaction front, an equivalent steady formulation of Eq. (1) may be
derived by recasting the equations of motion in a moving reference
system that travels at the same speed as the reaction front (e.g.,
temperature and other measurements may be taken at various
positions along a line that is substantially perpendicular to the
surfaces one or more of the reactive multilayer nanofoil, the
joining layers, and/or the components). This alternative
formulation, however, may have several drawbacks, including
difficulties in specifying the variation of the thermal interface
resistance with temperature (e.g., pre-reaction and/or
post-reaction), in post-processing and data analysis (e.g.,
duration of melting), and in comparison with experimental
measurements. Also note that when the interfaces between adjacent
layers are not initially bonded, the formulation may accommodate a
thermal interface resistance, and account for the variation of the
thermal interface resistance as melting occurs along these
interfaces.
[0091] The simulation results may be used to determine the degree
of melting of the fusible materials that occurs within the reactive
joining process, as well as the time duration over which wetting
occurs at critical interfaces. As used in this application, a
critical interface is an interface that requires wetting in order
to form a suitable bond at the interface. In most cases, a critical
interface is one that is initially unbonded. The critical
interfaces in arrangements may vary depending on the parts (e.g.,
reactive nanofoils, fusible materials, and/or components) and the
configuration of the parts in the particular arrangement.
[0092] FIGS. 8(a) and 8(b) depict examples of critical interfaces.
As shown in FIG. 8(a), one or more fusible materials 80a, 80b may
be pre-deposited onto one or more components 81a, 81b so that a
suitable bond may be provided, prior to chemical transformation
(e.g., ignition) of the nanofoil 82, between the one or more
fusible material 80a, 80b and the one or more components 81a, 81b.
Thus, the critical interfaces in FIG. 8(a) are at the interfaces
83a, 83b between the nanofoil 82 and the fusible materials 80a,
80b, and not at the interfaces 84a, 84b between the fusible
materials 80a, 80b and the components 81a, 81b. For this
arrangement, suitable parts (e.g., reactive nanofoils, fusible
materials, and/or components) may be selected (e.g., taking into
consideration size, shape, and/or composition) and/or particularly
positioned such that, when the reactive nanofoil 82 is chemically
transformed (e.g., ignited), the heat from the ignited reactive
nanofoil 82 may cause only a portion of the layers of the fusible
material 80a, 80b to melt. In other words, the heat from the
ignited reactive nanofoil 82 may not effect a complete melting of
the fusible material 80a, 80b and/or may not effect a melting the
portion of the fusible material 80a, 80b that is bonded to its
respective component 81a, 81b. In this arrangement, the melting of
all of the fusible material 80a, 80b and/or melting of the fusible
material 80a, 80b that is bonded to the component 81a, 81b may be
undesirable for several reasons. First, to generate enough heat to
completely melt the fusible material 80a, 80b, a thicker and/or
more energetic nanofoil 82 (e.g., having a more powerful chemical
composition) may be necessary, which may unnecessarily increase the
cost of the procedure. Second, melting the fusible material 80a,
80b that may be bonded to the component 81a, 81b may weaken the
pre-existing strong bond at the interfaces 84a, 84b between the
fusible materials 80a, 80b and the components 81a, 81b.
[0093] In FIG. 8(b), free-standing sheets of the fusible material
85a, 85b are disposed between the components 86a, 86b and the
reactive nanofoil 87. In this case, both interfaces of the fusible
material 85a, 85b are initially unbonded and, thus, both interfaces
88a, 88b, 89a, 89b of the fusible material 85a, 85b (e.g., the
interface 88a, 88b adjacent the reactive nanofoil 87 and/or the
interface 89a, 89b adjacent the component 86a, 86b) may be
considered critical interfaces 88a, 88b, 89a, 89b. Accordingly, for
this arrangement, suitable parts (e.g., one or more reactive
nanofoils 87, fusible materials 85a, 85b, and/or components 86a,
86b) may be selected (e.g., taking into consideration size, shape,
and/or composition) and/or particularly positioned such that, when
the reactive nanofoil 87 is ignited, the heat from the ignited
reactive nanofoil 87 may cause a substantially complete melting of
the one or more fusible materials 85a, 85b.
[0094] It is understood that the arrangements set forth in FIGS.
8(a) and 8(b) are not limiting, and that some of the aspects set
forth herein may be combined, removed, altered, and/or used to
implement any number of suitable arrangements and/or manufacture
any number of suitable products. Based on the arrangements, what
constitutes a critical interface that needs to be wetted may also
vary. For example, one or more component surfaces may be untreated,
or they may have a treatment layer (e.g., an adhesion underlayer of
Ni and/or Au plating, a layer of a solder or braze, or both, for
example, such that the layer of solder or braze is deposited onto
the adhesion layer). In another example, a free-standing sheet of a
fusible material may be disposed between the nanofoil and each of
the components, however, the free-standing sheet may or may not be
used. In a further example, the reactive multilayer nanofoil may
have one or more fusible layers on one or more sides of the
reactive multilayer nanofoil. In yet another example, one or more
layers of a fusible material may be provided between one or more
reactive multilayers and one or more components. In a yet further
example, one or more reactive multilayers maybe disposed between
one or more components. In such a configuration, the one or more
reactive multilayers may be in direct contact with the one or more
components (e.g., a particular reactive nanofoil may provide
sufficient energy to effect melting of one or more components).
Such a process may be called reactive welding, as opposed to
reactive soldering or brazing. An example of reactive welding is
disclosed in U.S. patent application Ser. No. 09/846,486 filed May
1, 2001 and entitled "Free Standing Reactive Multilayer Foils," the
entirety of which is incorporated herein by reference.
[0095] In a further example, embodiments of the invention may
include combining simulation results with experimental observations
to determine a suitable range of conditions that can be implemented
in a reactive joining method to yield a reactive joint with
suitable joint properties.
[0096] Embodiments of the invention may include any configuration
and combination of any of the aspects set forth herein with respect
to implementing and/or manufacturing suitable reactive joints using
suitable reactive joining methods. One set of embodiment may
include configurations where parts (e.g., one or more reactive
nanofoils, fusible materials, and/or components) are disposed
substantially symmetrically about a reactive nanofoil centerline.
Another set of embodiments may include configurations where parts
are disposed asymmetrically about a reactive nanofoil centerline.
These and other embodiments are described below.
[0097] For embodiments with symmetric configurations, the
thermo-physical properties of any part at corresponding symmetrical
locations on either side of the nanofoil centerline may be
substantially identical. An example may be reactive joining of
components made of substantially the same material and/or using
substantially identical layers of the fusible material. For
embodiments with asymmetric configurations, material properties may
differ at corresponding symmetric locations on either side of the
nanofoil. An example may include the joining of components made of
dissimilar materials and/or reactive joining configurations that
use different braze or solder layers on each side of the reactive
nanofoil. As reflected in the model results and experimental
observations disclosed herein, one of the distinctive features of
the two setups may be that for symmetric configurations heat may be
transported symmetrically with respect to the nanofoil centerline;
a symmetric temperature distribution may accordingly prevail. In
asymmetric configurations, the heat of reaction may be unequally
transported with respect to nanofoil centerline, and an asymmetric
temperature field may be consequently established. As further
disclosed herein, these features may have an impact on thermal
transport during reactive joining, and suggest new arrangements and
configurations to one of ordinary skill in the art.
[0098] The technique described herein has been applied to analyze a
wide variety of symmetric configurations, in particular for
reactive joining of Cu components, Au-plated stainless steel (SS)
components, Ti components, as well as gold-plated Al. Exemplary
results obtained for Cu--Cu joints and for the joining of Au-plated
stainless steel to itself and for Au-plated Al to itself are
provided herein. The methods and results for the Cu--Cu joints and
SS-SS joints are also applicable to other materials.
[0099] The design model is validated by comparing computed
predictions to temperature measurements performed during the
reaction using infrared (IR) thermometry. Results are provided for
the two configurations shown in FIGS. 9(a) and 9(b), showing
reactive joining of two Cu components 90a, 90b in FIG. 9(a) and two
Au-plated stainless steel components 90c, 90d in FIG. 9(b). As
shown in FIG. 9(a), the surfaces 91a, 91b of the components 90a,
90b may be pre-wet with an Ag--Sn solder layer 92a, 92b having a
thickness of approximately 75 .mu.m. The free-standing Ni--Al
nanofoil 99 may have a thickness of about 55 .mu.m, and each side
of the nanofoil 99 may have about 1 .mu.m of Incusil 94a, 94b
deposited thereon. As shown in FIG. 9(b), free-standing sheets of
Au--Sn solder 92c, 92d may have a thickness of about 25 .mu.m and
may be disposed between the reactive nanofoil 93c and the
respective Au-plated stainless steel components 90c, 90d. The
free-standing Ni--Al nanofoil 93 may have a thickness of about 70
.mu.m, and each side of the nanofoil 93 may have about 1 .mu.m of
Incusil 94c, 94d deposited thereon. The materials and/or values
disclosed herein are exemplary only. The present invention is
application to other materials and/or dimensions.
[0100] FIGS. 10(a) and 10(b) contrasts measured and predicted
temperature profiles for the Cu--Cu joint configuration shown in
FIG. 9(a). FIG. 10(a) illustrates the measured instantaneous
temperature profiles at various times and at substantially constant
positions on the Cu--Cu joint configuration during reactive joining
of the Cu components. FIG. 10(b) discloses the predicted
temperature profile at substantially the same constant positions on
the Cu--Cu joint configuration during reactive joining of the Cu
components, taken here at 0 seconds, 200 milliseconds, 1000
milliseconds, 630 milliseconds, 830 milliseconds, and 1030
milliseconds after ignition of the reactive multilayer nanofoil.
Note the close agreement between the measured and computed peak
temperatures. Also note the short duration of the reactive joining
process. As can be seen in FIGS. 10(a) and 10(b), the reactive
joining process is essentially complete within hundreds of
milliseconds of the passage of the front (e.g., the passage of the
heat or energy, usually at its peak magnitude, through various
positions on one or more of the reactive multilayer nanofoil, the
joining layers, and the components).
[0101] FIG. 11(a) shows instantaneous predicted temperature
profiles across the stainless steel joint configuration shown in
FIG. 9(b). Curves are generated at the selected time instants,
corresponding to the moment of passage of the self-propagating
front, and at 0.1 ms, 0.5 ms, 1 ms, 10 ms, 50 ms and 400 ms
afterwards. The results show that the temperature across the joint
decreases very quickly to 48.degree. C. at 400 ms after the passage
of the front, which is comparable with the experimental temperature
measurement of 47.degree. C. FIG. 11(b) shows the evolution of the
temperature in the stainless steel configuration shown in FIG. 9(b)
at 100 microns from the interface between the solder layer and the
stainless steel. Shown are results obtained from both the numerical
simulations (predictions) and the IR (actual) measurements. Note
the close agreement between model predictions and experimental
measurements, the rapid drop of the temperature, and the limited
thermal exposure of the components.
[0102] The model may be applied to systematically investigate the
effect of the nanofoil thickness on the wetting of critical
interfaces, on the melting of the fusible material, and/or on the
thermal exposure of the components. For example, FIG. 12 depicts an
embodiment for the reactive joining of Al-6061T6 components 120a,
120b that may be first coated with a thin Ni underlayer 121a, 121b,
and then an Au layer 122a, 122b. As shown in FIG. 12, free-standing
sheets of Au--Sn solder 123a, 123b may have a thickness of about 25
.mu.m and may be used as the fusible material 123a, 123b. Each side
of the nanofoil 124 may have about 1 .mu.m of Incusil 125a, 125b
deposited thereon The effect of the thickness of the nanofoil 124
on the wetting of the critical interface 126a, 126b between the
solder 123a, 123b and the component 120a, 120b (may or may not
include one or more of layers 121a, 121b, 122a, 122b) may be
analyzed by quantifying the time duration during which the solder
123a, 123b is locally in a molten state. To this end, the thickness
nanofoil 124 may be systematically varied, while other parameters
(e.g., of the nanofoil 124, layers 121a, 121b, 122a, 122b, 125a,
125b, and/or fusible material 123a, 123b) may be fixed.
[0103] As described herein, the model inputs into the computation
model formulation may include the thermophysical properties of the
nanofoil and of the components. For example, the table below
discloses possible inputs such as the thermal conductivity, heat
capacity, and/or density of Al-6061-T6, Au--Sn, Incusil-ABA,
Al--NiV Foil, and/or stainless steel.
TABLE-US-00001 Thermal Heat Conductivity Capacity Density Material
(W/m/K) (J/kg/K) (kg/m.sup.3) Al-6061-T6 167 896 2700 AuSn 57 170
14510 Incusil-ABA 70 276 9700 Al--NiV Foil 152 830 5665 Stainless
Steel 18 500 7990
[0104] Other possible inputs may include the solidus temperature of
Incusil (T.sub.s=878K), the liquidus temperature of Incusil
(T.sub.1=988K), the heat of fusion Incusil (H.sub.f=10792 J/mol),
the solidus temperature of Au--Sn solder (T.sub.s=553K), the
liquidus temperature of Au--Sn solder (T.sub.1=553K), and/or the
heat of fusion of Au--Sn solder (H.sub.f=6188 J/mol).
[0105] Both predicted and measured values based on foil bilayer
thickness are depicted in FIGS. 13(a) and 13(b). FIG. 13(a) shows
how the heat of reaction may be affected by Al--Ni foil thickness
for "thick" foils (e.g., RF16 having about 2000 bilayers) and
"thin" foils (e.g., RF 18 having about 640 bilayers). The lines
depict the predicted heat of reaction given a particular bilayer
thickness of the Al--Ni foil while the circles depict the measured
heat of reaction of bilayers having a particular thickness. Note
that the predicted heat of reactions substantially correlate with
the measured heat of reactions. In a further example, FIG. 13(b)
depicts how front velocity (speed) is dependent on bilayer
thickness. The line shown in FIG. 13(b) depicts the predicted front
velocity given a particular bilayer thickness of the Al--Ni foil
while the circles depict the measured front velocity of bilayers
having a particular thickness. Note that the predicted front
velocities substantially correlate with the measured front
velocities.
[0106] FIG. 14 depicts computed predictions for the amount of
melting of the solder layer as well as the duration of melting at
the critical solder-component interface as a function of nanofoil
thickness. The dashed line represents results that may be obtained
for reactive joining of Al--Al components, for example, as shown in
the configuration depicted in FIG. 9(b), while the solid line
represents results that may be obtained for reactive joining of
Au-plated stainless steel components, for example, as shown in the
configuration depicted in FIG. 12.
[0107] For Al--Al joints, the model predictions in FIG. 14 indicate
that when the nanofoil thickness is smaller than about 35 .mu.m,
only partial melting of the about 25 .mu.m-thick layers of Au--Sn
solder may occur. Accordingly, the duration of melting at the
critical interface between the solder and the component may be
about 0 ms. On the other hand, when a nanofoil having a thickness
substantially equal to or greater than about 35 .mu.m is used, the
entire solder layer may melt and the duration of wetting of the
critical interface (e.g., duration of melting of at least a portion
of the Au--Sn solder layer) may be positive. In particular, the
duration of melting may increase as the nanofoil thickness
increases. The model prediction also indicates that the minimum
nanofoil thickness needed to melt the about 25 .mu.m-thick layer of
Au--Sn solder may be larger for the Al--Al joints than for the
SS-SS joints. Furthermore, for corresponding nanofoil thicknesses
(e.g., greater than about 20 .mu.m), the model predicts that the
duration of melting of the solder layer may be larger (and as the
nanofoil thickness increases, substantially larger) for the SS-SS
joints than for the Al--Al joints. This may be due to the fact that
the thermal conductivity of stainless steel may be much smaller
than that of Al-6061-T6. Consequently, heat may be conducted at a
much slower rate into the SS than in the Al. These predicted
results underscore the need for a careful optimization of the
design, configuration, and/or dimensions of reactive joining
configurations (e.g., nanofoil thickness), based on the
thermophysical properties of the reactive multilayer, of the
fusible materials, and/or of the components.
[0108] Additional numerical predictions of the model (e.g.,
associated with the melting of the fusible material and/or of
wetting of critical interfaces) may be contrasted with additional
experimental measurements, for example, the shear strength of the
reactive joints.
[0109] For example, FIG. 15 shows that the measured shear strength
of the Al--Al joints and/or SS-SS joints may be associated with
and/or dependent on nanofoil thickness. For the present tests, the
foils that are thicker than about 55 .mu.m correspond to the RF16
family (e.g., have about 2000 bilayers), while the foils that are
thinner than about 55 .mu.m correspond to the RF18 family (e.g.,
having about 640 bilayers). The joint strengths were measured using
tensile shear-lap tests. Consistent with the predictions set forth
in FIG. 14, the measurements of FIG. 15 indicate that successful
joints may be obtained when the thickness of the reactive nanofoil
for an Al--Al joint is about 35 .mu.m, and when the thickness of
the reactive nanofoil for a SS-SS joint is about 20 .mu.m.
[0110] Specifically, FIG. 15 shows that Al--Al joints may fail when
the reactive nanofoil is thinner than about 35 .mu.m and/or that
SS-SS joints may fail when the nanofoil thickness is less than
about 20 .mu.m. The measurements set forth in FIG. 15 also show
that the respective joint strengths may steadily increase with
increases in the thicknesses of the respective nanofoils until a
plateau and/or peak strength is reached. Once that peak and/or
plateau is reached, the joint strength may remain constant and/or
no further strength may be imparted to the joint even with
successive increases in nanofoil thickness. For SS-SS joints, the
plateau may be reached when the nanofoil is thicker than about 42
.mu.m, and for Al--Al joints, the peak strength may be reached when
the nanofoil is about 80 .mu.m thick.
[0111] Accordingly, by using the model predictions of FIG. 14 and
the measured results of FIG. 15, one may be able to correlate the
optimal and/or maximum strength of a particular joint with the time
duration during which the solder remains in a molten state at the
critical interface. For example, for the present configurations,
one may be able to conclude that the Au--Sn solder must wet the
critical interface for about 0.5 ms in order to achieve an optimal
and/or maximum strength bond. The bond strength may also be
affected by other parameters of the present configurations, for
example, the peak temperature at the interface between the fusible
material and the component. The predictions and/or corresponding
measurements set forth herein hold for both the Al--Al and SS-SS
joints. It should be evident for one skilled in the art to
generalize the present embodiment to a variety of other material
systems.
[0112] The design approach set forth herein may be applied to
analyze asymmetric configurations (i.e., configurations where
properties of the materials, such as thermal properties, may differ
on different sides of the nanofoil). An example of such an
asymmetric configuration is shown in FIG. 16, which illustrates the
reactive joining of SiC to Ti-6-4, in which the thicknesses of the
Incusil layers that are pre-deposited onto the SiC and Ti may be
held fixed.
[0113] As SiC may have a much larger thermal conductivity than
Ti-6-4, the thermal profile during the reactive joining may be
asymmetric with respect to the nanofoil centerline. Such asymmetry
in the thermal profile of across the SiC and Ti-6-4 assembly is
shown in FIG. 17(a), which graphically shows that the thermal wave
may diffuse faster on the SiC side than on the Ti. Moreover, the
peak temperatures may be generally higher on the Ti side than on
the SiC side. Similar effects (e.g., faster diffusing on the SiC
side than on the Ti side and/or higher peak temperature on the Ti
side than on the SiC side) may be observed by analysis of IR
thermometry images of the SiC--Ti assembly during reactive joining,
exemplary samples of are shown in FIGS. 17(b) and 17(c). FIG. 17(b)
shows an IR image of the configuration at the ignition of the
reactive multilayer nanofoil, while FIG. 17(c) shows an IR image of
the configuration at about 240 ms after ignition. As further
discussed herein, this understanding of the thermal properties of
an asymmetric joining configuration may be used to design new
reactive joining configurations.
[0114] Returning to FIG. 16, the thickness of an Incusil layer 161
that may be pre-deposited onto the Ti 162 may be about 62 .mu.m
thick, while the Incusil layer 163 that is pre-deposited onto the
SiC 164 may be about 100 .mu.m thick. In this particular design
analysis, as set forth below, a parametric study may first be
conducted of the effect of the thicknesses of the braze layers 165,
166 pre-deposited on both sides of the reactive nanofoil 167. To
this end, the thicknesses of the braze layers 165, 166 facing the
SiC (t.sub.1 in FIG. 16) and Ti (t.sub.2 in FIG. 16) may be varied
independently. Meanwhile, the overall thickness (180 .mu.m),
reaction heat (1189 J/g) and reaction velocity (2.9 m/s) of the
nanofoil 167 and the thicknesses of the adjoining layers 165, 166
may be held fixed. The nanofoils used in the analysis of SiC/Ti-6-4
joints may correspond to the RF16 family, whose properties are
shown in FIG. 13. Other inputs to the design model are provided in
the table below.
TABLE-US-00002 Thermal Heat Conductivity Capacity Density Material
(W/m/K) (J/kg/K) (kg/m.sup.3) SiC 130 750 3200 Ti-6-4 6.7 610 4510
Incusil-ABA 70 276 9700 Ni/Al Foil 152 830 5665
[0115] Other possible inputs may include the solidus temperature in
Incusil (T.sub.s=878K), the liquidus temperature of Incusil
(T.sub.1=988K), and the heat of fusion of Incusil (H.sub.f=10,792
J/mol).
[0116] The model computations for FIG. 16 focused on the wetting of
the critical interfaces, which in the present case correspond to
the interfaces 168, 169 between the Incusil layers 165, 166
pre-deposited onto the nanofoil 167 and the Incusil layers 161, 163
pre-deposited onto their respective components 162, 164.
Specifically, for the arrangement shown in FIG. 16, it may be
necessary for the reaction to produce sufficient heat so as to melt
the braze layers 165, 166 that are pre-deposited onto the nanofoil
167, as well as partially melt the braze layers 161, 163 that are
pre-deposited onto the Ti 162 and the SiC 164. In the computations,
we quantify this phenomenon (e.g., melting of the one or more braze
layers) by monitoring the peak thicknesses of the molten braze
layers 163, 161 on the SiC 164 and Ti 162, respectively t.sub.SiC
and t.sub.Ti. The following table shows the various thicknesses
t.sub.SiC, t.sub.Ti of molten braze layers 163, 161 (i.e., amount
of melting of the braze) for various combinations of the
thicknesses t.sub.1, t.sub.2 of the one or more braze layers 165,
166 pre-deposited on the nanofoil 167.
TABLE-US-00003 t.sub.1 (.mu.m) t.sub.2 (.mu.m) t.sub.SiC (.mu.m)
t.sub.Ti (.mu.m) 1 1 19.32 45.95 1 4 19.36 35.05 1 8 19.40 27.03 1
12 19.44 19.87 1 16 19.48 13.84 4 1 15.49 47.54 4 4 15.54 35.39 4 8
15.57 27.24 4 12 15.62 21.03 4 16 15.66 13.99 8 1 11.50 47.95 8 4
11.55 35.63 8 8 11.58 27.38 8 12 11.62 21.15 8 16 11.67 15.11 12 1
7.74 49.55 12 4 7.79 35.98 12 8 7.82 27.58 12 12 7.87 21.31 12 16
7.92 15.26 16 1 3.75 51.31 16 4 3.79 37.45 16 8 3.82 27.83 16 12
3.87 21.51 16 16 3.92 15.45
[0117] FIG. 18 graphically shows the thickness of the molten braze
layer 161, 163 as a function of the one or more braze layers 165,
166 deposited on either side of the reactive nanofoil 167 for the
combinations where an equal thickness of braze 165, 166 is
deposited on either side of the reactive nanofoil 167 (i.e.,
t.sub.1=t.sub.2). The dashed curve shows the amount of melting on
the Ti component and the solid curve shows the amount of melting on
the SiC component.
[0118] Examination of the results in the table above reveals that
the amount of braze 163 out of t.sub.SiC that melts on the SiC
component 164 may depend on the thickness t.sub.1 of the braze
layer 165 on the SiC-side of the nanofoil 167. Specifically,
t.sub.SiC may decrease as t.sub.1 increases. Similarly, the amount
of braze 161 out of t.sub.Ti that melts on the Ti component 162 may
depend on the thickness t.sub.2 of the braze layer 166 on the
Ti-side of the nanofoil 167, and decrease as the latter
increases.
[0119] This effect is graphically depicted in FIG. 18; where both
curves (t.sub.SiC and t.sub.Ti) decrease as one increases the
thickness of the braze layer 165, 166 (e.g., having thickness of
t.sub.1 and t.sub.2) that may be pre-deposited onto the nanofoil
167. This figure also shows that more braze may melt on the Ti
component than on the SiC component (t.sub.Ti>t.sub.SiC). This
prediction may be attributed to the fact that SiC has a much higher
thermal conductivity than Ti-6-4. Combined, the present results
indicate it may be desirable to keep the thickness of braze 165,
166 pre-deposited onto the nanofoil 167 as small as possible. The
results also indicate that, for a nanofoil 167 having a total
thickness (which may or may not include the layers 165, 166) of
about 180 .mu.m having Incusil layers 165, 166 with a thickness of
about 1 .mu.m pre-deposited on both sides of the nanofoil 167,
substantial melting of the braze layers 161, 163 deposited onto
both components 162, 164 may occur. Thus, this configuration
provides a suitable design for the joining process. Based on these
results, it should be obvious for one skilled in the art to design
the thickness of fusible material pre-deposited on the reactive
nanofoil, both to design the joining process as well as to achieve
other effects such as limiting the thermal exposure of the
components.
[0120] The asymmetric arrangement of FIG. 16 may also be used to
examine the effect of overall nanofoil thickness, t.sub.F, on
t.sub.Ti (the thickness of the molten braze layer 161 on the
titanium 162) and t.sub.SiC (the thickness of the molten braze
layer 163 on the silicone carbide 164). In light of the results
above, the thicknesses t.sub.1 (the thickness of the braze layer
165 on the SiC side of the nanofoil 167) and t.sub.2 (the thickness
of the braze layer 166 on the Ti-side of the nanofoil 167) may be
held fixed, t.sub.1=t.sub.2, where, for example, both t.sub.1 and
t.sub.2 may be equal to about 1 .mu.m. As shown in FIG. 19, the
nanofoil thickness t.sub.F was varied between about 60 .mu.m and
about 270 .mu.m, and the computed values of t.sub.Ti and t.sub.SiC
are plotted against t.sub.F. The results show that each of t.sub.Ti
and t.sub.SiC may increase as the nanofoil thickness t.sub.F
increases. For nanofoil thicknesses t.sub.F smaller than about 160
.mu.m, the amount of melting of the braze layers 161, 163 that are
pre-deposited onto the components 162, 164 may be quite small, as
t.sub.Ti and t.sub.SiC may both fall below about 16 .mu.m. On the
other hand, for a nanofoil thickness t.sub.F larger than about 200
.mu.m, the entire layer of Incusil 161 pre-deposited onto the Ti
163 may melt. The present results thus indicate that, for the
configuration of FIG. 16, a suitable and/or desirable nanofoil
thickness to achieve the suitable and/or desired effects may be in
the range of about 150 .mu.m to about 200 .mu.m. A nanofoil
thickness between about 150 .mu.m and about 200 .mu.m may be
suitable and/or desirable because such a nanofoil thickness may
ensure sufficient wetting of critical interfaces 168, 169 and/or
avoid complete melting of the braze layers 161, 163 that are
pre-deposited onto the components 162, 164. Using this methodology,
it should be obvious for someone skilled in the art how to design
the nanofoil thickness, particularly so as to induce melting at
critical interfaces 168, 169, while avoiding this effect at
initially bonded interfaces.
[0121] The asymmetric arrangement of FIG. 16 may also be used to
examine the effect of heat of reaction on the melting of the
fusible material 161, 163, 165, 166 and on wetting at critical
interfaces 168, 169. As mentioned herein, the heat of reaction of
reactive multilayer nanofoils 167 may be controlled using a variety
of means, for example, by varying one or more of the stoichiometry,
the deposition rate (which affects the premix width), and/or the
bilayer thickness, and/or by annealing the nanofoil at moderate
temperature in an inert environment, as discussed in Gavens and
Glocker.
[0122] To illustrate the impact that varying the heat of reaction
may have on melting fusible materials 161, 163, 165, 166 and/or
wetting critical interfaces 168, 169, computed simulations were
conducted with a nanofoil 167 having a fixed thickness t.sub.F of
about 180 .mu.m, and Incusil layers 165, 166, that were
pre-deposited on the nanofoil 167, each having a fixed thickness
t.sub.1 and t.sub.2 of about 1 .mu.m. The front velocity was held
fixed at about 2.9 m/s. With these fixed values, the heat of
reaction was varied in the range between about 800 J/g and about
1600 J/g. Using these inputs, predicted values for t.sub.Ti and
t.sub.SiC were computed from the simulations and are plotted
against the heat of reaction, as shown in FIG. 20. The results
indicate that t.sub.Ti and/or t.sub.SiC may exhibit a strong
dependence and/or correlation with the heat of reaction. For
example, as shown in FIG. 20, when the heat of reaction drops below
about 900 J/g, the results predict that insignificant melting of
the braze layers 161, 163 may occur. As the heat of reaction is
increased beyond about 900 J/g, the results predict that the curves
for t.sub.Ti and/or t.sub.SiC may rise rapidly. In particular, when
the heat of reaction exceeds about 1300 J/g, the results predict
that substantially the entire layer of Incusil 161 pre-deposited
onto the Ti 162 may melt during the reactive joining process. These
results underscore the need and/or benefits of carefully
controlling or characterizing the heat of reaction. For example, in
the present asymmetric configuration set forth in FIG. 16, the heat
of reaction used may preferably fall in the range of about 1160 J/g
to about 1300 J/g. The heat of reaction can be controlled in a
known manner so as to control the amount of melting of the braze
material, to thereby limit the thermal exposure of the components,
and/or to control other related results and/or effects.
[0123] In another embodiment of this invention, one or more
free-standing sheets 210, 211 of one or more fusible materials 210,
211 may be used in an asymmetric configuration. For example, FIG.
21 illustrates an alternative configuration for joining of SiC 212
and Ti 213. As illustrated in FIG. 21, the alternative
configuration uses free-standing sheets 210, 211 of Au--Sn solder
210, 211 as the fusible material. The sheets 210, 211 may each have
a thickness of about 25 .mu.m. The SiC 212 and Ti 213 may be
treated in substantially the same fashion as any of the
configurations set forth herein. For example, an Incusil layer 215
having a thickness of about 62 .mu.m may be pre-deposited onto the
Ti 213 and/or an Incusil layer 214 having a thickness of about 100
.mu.m may be pre-deposited onto the SiC 212. The reactive nanofoils
220 may have Incusil layers 216, 217 pre-deposited on either side.
The Incusil layers 216, 217 pre-deposited on the reactive nanofoils
may have a thickness of about 1 .mu.m.
[0124] In the configuration shown in FIG. 21, the nanofoil may
preferably deliver sufficient amounts of heat to completely melt
the free-standing Au--Sn layers 210, 211. However, melting of one
or more of the Incusil braze layers 214, 215, 216, 217 may not be
necessary, as each Au--Sn solder layer 210, 211 may adhere
sufficiently to its respective Incusil braze layers 214, 215, 216,
217 regardless of whether the braze itself melts. As discussed
below, a parametric study was conducted to determine the effect
that the thickness of the nanofoil has on the melting of the solder
layers 210, 211 and/or the melting of the one or more Incusil braze
layers 214, 215 that are pre-deposited onto the Ti 213 and SiC 212.
The thickness of the reactive nanofoil layer 220 was varied between
about 30 .mu.m and about 270 .mu.m.
[0125] Since the present configuration may require substantially
complete melting of the Au--Sn solder 210, 211, the predictive
analysis was conducted by monitoring the solder temperature at the
interface 218, 219 of each Au--Sn solder layer 210, 211 and its
respective Incusil braze layers 214, 215 which are pre-deposited on
the component Ti 213 and/or SiC 212. For each of the configurations
(e.g., where the thickness of the reactive nanofoil layer 220 was
varied), time intervals were recorded during which the solder
layers 210, 211 remained above its melting temperature at each of
interfaces 218, 219. The predicted results are shown in FIG. 22,
where the time interval during which solder layers 210, 211
remained above its melting temperature at each of the interfaces
218, 219 is plotted against the nanofoil thickness. The predicted
results reveal that a minimal nanofoil thickness of about 30 .mu.m
may be necessary in order to melt both Au--Sn solder layers 210,
211 (e.g., the Au--Sn solder layer on the Ti side and/or the SiC
side). For nanofoils 220 having a thickness of less than about 30
.mu.m, the model predicts that there may be only partial melting of
one or more Au--Sn solder layers 210, 211, and therefore a lack of
bonding between one or more of the Au--Sn solder layers 210, 211
and the one or more Incusil braze layers 214, 215.
[0126] The strength of reactively formed joints using Au--Sn solder
was determined experimentally, examples of which are set forth
herein, and the shear strength measurements were compared with
computational predictions. The analyses set forth below reveal that
the joint strength may initially increase as the duration of the
melting of the Au--Sn solder increases, and that peak strengths of
the joints may be obtained when the Au--Sn solder at the critical
interfaces is above its melting temperature for a time duration
exceeding about 0.5 ms. Based on this work, a nanofoil thickness of
about 70 .mu.m may be needed to achieve an adequate joint strength.
The computations were also used, examples of which are set forth
herein, to examine possible melting of Incusil which is
pre-deposited onto the components. The results indicate that when
the nanofoil thickness is smaller than about 200 .mu.m, the braze
layers pre-deposited onto the Ti and SiC may remain below the
Incusil's melting temperature. For thicker nanofoils, partial
melting of the Incusil in one or both of these layers 214, 215 may
occur.
[0127] In another embodiment of this invention, the effect of the
melting duration of the solder or braze on the strength of the
resulting reactive joints has been analyzed experimentally. The
experimental investigation has been applied to configurations
having different lengths and widths for one or more of the foil,
solder layers, and components, but with fixed thicknesses for one
or more of the foil, the solder layers, and of the components.
Specifically, reactive joints between SiC and Ti-6-4 have been
formed using Incusil (braze) as the fusible material, and using
AgSnSb (solder) as the fusible material. Both small-area (0.5
in..times.0.5 in.) and large-area (4 in..times.4 in.) have been
considered, and the strength of the resulting joints experimentally
determined. In both case, a 90 .mu.m reactive foil was used. The
measured strength of the joints is shown in the table below as
function of the joint area:
TABLE-US-00004 Fusible Material Area Incusil (braze) AgSnSb
(solder) 0.5 in .times. 0.5 in 59.5 MPa 67.5 MPa 4 in .times. 4 in
0 MPa 66.9 MPa
[0128] For the present conditions, the model predictions indicate
that, irrespective of the joint area, the melting duration of the
braze is about 0.28 ms, while for the solder the melting duration
is about 5.49 ms. The larger melting duration of the solder is in
fact expected, since the latter has much lower melting temperature.
Comparison of the prediction of melting duration with measured
shear strength reveals that the larger the length and the width of
the configuration (i.e. the joining area), the larger the melting
duration needed to achieve adequate strength of the reactive joint.
This is evidenced by the fact that with Incusil as the fusible
material, the melting duration was short, and strong bonds were
obtained for the small-area joint but the joints failed when the
same protocol was applied to a large-area joint. On the other hand,
when AgSnSb as the solder material, the melting duration was larger
and similar strengths were obtained for both small-area and
large-area joints. It should be evident for one skilled in the art
to generalize these findings to other material systems and joint
areas.
[0129] In another embodiment of this invention, another asymmetric
configuration corresponding to reactive joining of Al-6101-T6 to
Al.sub.2O.sub.3 is considered in FIG. 23. In particular, the
configuration in FIG. 23 may be used to analyze the effect of the
thickness of the foil 230 on the wetting of the critical interface
between the foil 230 and the solder 231, 232, namely by quantifying
the time duration during which the solder 231, 232 is locally in a
molten state. To this end, the thickness of the foil 230 may be
systematically varied, while the remaining parameters may be held
fixed.
[0130] The model inputs include the thermophysical properties of
the foil 230, the joining layers, 231, 232, 233, 234, and of the
components 235, 236, as set forth in the following table and FIG.
13.
TABLE-US-00005 Thermal Heat Conductivity Capacity Density Material
(W/m/K) (J/kg/K) (kg/m.sup.3) Al-6101-T6 218 895 2700 Ag-Sn 33 227
7360 Incusil-ABA 70 276 9700 Al--NiV Foil 152 830 5665
Al.sub.2O.sub.3 30 88 3900
[0131] Other possible inputs may include the solidus temperature in
Incusil (T.sub.s=878K), the liquidus temperature of Incusil
(T.sub.1=988K), the heat of fusion of Incusil (H.sub.f=10,792
J/mol), the solidus temperature of Ag--Sn solder (T.sub.s=494K),
the liquidus temperature of the Ag--Sn solder (T.sub.1=494K), and
the heat of fusion of Ag--Sn solder (H.sub.f=14200 J/mol).
[0132] In the configuration shown in FIG. 23, the solder layer 231
on the Al.sub.2O.sub.3 component 235 may have a thickness of about
100 .mu.m, while the solder layer 232 on the Al-6101-T6 component
236 may have a thickness of about 75 .mu.m. The reactive multilayer
foil 230 may have about 1 .mu.m thick layers 233, 234 of Incusil
deposited on both sides of the foil 230.
[0133] Details of the temperature distribution during the reactive
joining process are shown in FIG. 24, which depicts instantaneous
profiles across the joint due to the chemical transformation of a
foil 230 having a thickness of about 148 .mu.m at different times.
As seen in FIG. 24, thermal transport may occur in an asymmetric
fashion on either side of the foil 230, and that the thermal
gradients in solder layers 231, 232 may be weaker on the side with
the Al.sub.2O.sub.3 component 235 than on the side with the
Al-6101-T6 component 236. These phenomena may be directly traced to
the disparity between the components' 235, 236 thermal diffusivity,
which may be much higher for the Al-6101-T6 component 236 than for
the Al.sub.2O.sub.3 component 235.
[0134] The effect of the thickness of the foil 230 is analyzed in
FIGS. 25(a) and 25(b). FIG. 25(a) shows the amount of melting of
the solder layers 231, 232 and FIG. 25(b) illustrates the duration
of melting at the critical foil-solder interfaces 237, 238 and at
the solder-component interfaces 239, 190. The predictions indicate
that joining may occur for all the foil thicknesses considered,
which range between about 20 .mu.m and about 148 .mu.m. Note that
when the thickness of the foil 230 is less than about 60 .mu.m,
partial melting may occur in both solder layers 231, 232. For foil
thicknesses between about 60 .mu.m and about 100 .mu.m, complete
melting may occur of the solder layer 231 lying on the side of the
Al.sub.2O.sub.3 component 235, while the solder layer 232 on the
side of the Al-6101-T6 component 236 may partially melt. For foil
230 having a thickness larger than about 100 .mu.m, both solder
layers 231, 232 may completely melt. In the latter regime, the
results indicate that the local melting duration of the solder
layers 231, 232 may increase substantially linearly with increasing
thickness of the foil 230. Consistent with the results in FIG. 24,
FIGS. 25a and 25b also indicate that there may be more complete and
uniform melting on the side of the Al.sub.2O.sub.3 component 235
than on the side of the Al-6101-T6 component 236. In particular,
the duration of melting at the solder-foil interface 237 on the
Al.sub.2O.sub.3 side may be approximately equal to the duration of
melting at the solder-component interface 239 also on
Al.sub.2O.sub.3 side, as shown in FIG. 25b. On the other hand,
these melting durations may differ substantially on the Al side, as
shown in interfaces 238, 190 in FIG. 25a. Combined, the results in
FIGS. 24, 25a, and 25b demonstrate that the thermal diffusivity of
the solder and the components may be critical to duration and
uniformity of the melting, and hence to joint strength.
Consequently, the design of reactive joining applications should
carefully account for these parameters.
[0135] In another embodiment of this invention, a reactive joining
configuration may be used that involves multiple fusible-material
layers that are chemically distinct. One particular configuration
is set forth in FIG. 26. FIG. 26 shows an asymmetric configuration
in which two fusible materials 262, 263 are employed, where the
fusible material 262 with higher melting temperature Ti may be used
on the side with the component 260 having a lower thermal
conductivity k1, while the fusible material 263 with lower melting
temperature may be used on the side with the more conductive
component 261 having a higher relative thermal conductivity k2.
Examples of such arrangement include the joining of SiC and Ti,
where a lower melting temperature braze such as Incusil is
pre-deposited onto the more conductive SiC, while a higher melting
temperature braze such as Gapasil or TiCuNi is used on the less
conductive Ti component. Such arrangements offer the possibility of
designing for thermal transport during the reaction, chemical
compatibility between individual braze or solder layers for the
adjoining components, as well as thermophysical properties of the
reactive joint. It should be obvious for someone skilled in the art
how to generalize the present embodiment to a variety of other
configurations.
[0136] III. Optimizing Reactive Foil Joints
[0137] As shown in Part I, above, the mechanical properties of a
brazed or soldered joint are enhanced by refining the
microstructure of brazing or solder layer in the formed joint.
Specifically it is desirable to reduce the lamellar spacing from
the micrometer range commonly produced by furnace or torch heating
to the nanometer range (less than about 100 nm), preferably less
than about 50 nm and even more preferably less than about 10 nm. As
further noted in Part I, the lamellar spacing .lamda. is related to
the cooling rate Rc by the relation.
.lamda.=K/R.sub.c/R.sub.c.sup.1/2
[0138] The thermal modeling technique described and illustrated in
Part II can be used to model the heat flow in the reactive foil
formation of a joint and thereby provide the cooling rate R.sub.c.
The thermal modeling technique computes the temperature at each
cell center and time step. The cooling/heating rate at each cell
center and time step can be computed by deriving the temporal rate
of change of the predicted temperature (numerically deriving the
first derivative of the temperature with respect to time). Thus for
each combination of the relevant parameters, including foil
thickness, foil heat of reaction, reaction instantaneous and
average velocity, and applied pressure, the model provides a
prediction of the cooling rate distribution throughout the entire
solder/braze region and throughout the joining procedure (including
when the solder/braze solidifier). The model can thus be used to
determine which combination of the above parameters results in the
desired cooling rate gradient and resulting lamellar spacing.
[0139] The lamellar spacing in a solder or braze material is
dependant on the cooling rate during the formation of reactive
joints, which varies with the geometries and properties of the
foils and the components being joined. It is expected that a higher
cooling rate will result in a finer microstructure for the solder
or braze material that is being used.
[0140] Numerical predictions of heat transfer during the reactive
joining process show that the cooling rate of the solder or braze
material can be controlled by varying the geometries and properties
of the foil, the solder or braze, and the component. For example,
when joining thin components (1 mm) with very low thermal
conductivity, such as Silicone, using a very thick reactive foils
(1 mm) and low melting point solders (InSn), the cooling rate in
the solder layer at the solidification temperature is estimated to
be as low as 5.degree. C./second. In contrast, when very thermal
conductive components, such as diamond, are joined using a 60 .mu.m
thick reactive foils and Incusil braze layers instead of a low
melting temperature solder, the cooling rate in the braze layer at
the solidification temperature can be as high as
5.times.10.sup.6.degree. C./second. Relevant physical properties of
the components, reactive foils and solder materials are listed in
Table 1.
[0141] In general, there are several ways to increase the cooling
rate in the solder or braze layer, thus to obtain refined
microstructure of the solder or braze material, and to improve the
performance of the reactive joints. For the geometries and
properties of the components, higher thermal conductivity, lower
density, lower heat capacity, and a larger thickness will result in
a higher cooling rate in the solder or braze layer. For the
reactive foils, using thinner foils will generate less heat and
thus will increase the cooling rate across the joint. In addition,
using foils with lower heat capacity, lower density and higher heat
of reaction (J/g) will also increase the cooling rate in the solder
or braze layer at its solidification temperature. For the solder or
braze layer, using a solder or braze with higher melting point,
higher thermal conductivity, lower heat capacity and lower density
will generally yield a higher cooling rate.
TABLE-US-00006 TABLE 1 Physical properties of components and solder
or braze materials. Relevant parameters also include the solidus
and liquidus temperatures of Incusil, respectively T.sub.s = 878 K
and T.sub.l = 988 K, the Incusil heat of fusion H.sub.f = 10792
J/mol, the solidus and liquidus temperatures of the InSn solder,
respectively T.sub.s = T.sub.l = 391 K and the InSn heat of fusion
H.sub.f = 5200 J/mol. Thermal Heat Density conductivity capacity
(g/cm.sup.3) (W/mK) (J/kgK) Silicone 1.35 0.23 2100 Diamond 3.51
3050 520 InSn solder 7.3 34 270 Incusil braze 9.7 70 276
[0142] It is understood that the above-described embodiments are
illustrative of only a few of the many possible specific
embodiments, which can represent applications of the invention.
Numerous and varied other arrangements can be made by those skilled
in the art without departing from the spirit and scope of the
invention.
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