U.S. patent application number 14/878056 was filed with the patent office on 2016-01-28 for lead-free and antimony-free tin solder reliable at high temperatures.
This patent application is currently assigned to ALPHA METALS, INC.. The applicant listed for this patent is ALPHA METALS, INC.. Invention is credited to Ravi Bhatkal, Pritha Choudhury, Morgana De Avila Ribas, Anil Kumar, Sutapa Mukherjee, Ranjit Pandher, Siuli Sarkar, Bawa Singh.
Application Number | 20160023309 14/878056 |
Document ID | / |
Family ID | 49382536 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160023309 |
Kind Code |
A1 |
Choudhury; Pritha ; et
al. |
January 28, 2016 |
LEAD-FREE AND ANTIMONY-FREE TIN SOLDER RELIABLE AT HIGH
TEMPERATURES
Abstract
A lead-free, antimony-free tin solder which is reliable at high
temperatures and comprises from 3.5 to 4.5 wt. % of silver, 2.5 to
4 wt. % of bismuth, 0.3 to 0.8 wt. % of copper, 0.03 to 1 wt. %
nickel, 0.005 to 1 wt. % germanium, and a balance of tin, together
with any unavoidable impurities.
Inventors: |
Choudhury; Pritha;
(Bangalore, IN) ; De Avila Ribas; Morgana;
(Bangalore, IN) ; Mukherjee; Sutapa; (Bangalore,
IN) ; Kumar; Anil; (Bangalore, IN) ; Sarkar;
Siuli; (Bangalore, IN) ; Pandher; Ranjit;
(Plainsboro, NJ) ; Bhatkal; Ravi; (South
Plainfield, NJ) ; Singh; Bawa; (Voorhees,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALPHA METALS, INC. |
South Plainfield |
NJ |
US |
|
|
Assignee: |
ALPHA METALS, INC.
South Plainfield
NJ
|
Family ID: |
49382536 |
Appl. No.: |
14/878056 |
Filed: |
October 8, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14434470 |
Apr 9, 2015 |
|
|
|
PCT/GB2013/052624 |
Oct 9, 2013 |
|
|
|
14878056 |
|
|
|
|
61711277 |
Oct 9, 2012 |
|
|
|
Current U.S.
Class: |
403/272 ;
228/101; 420/561 |
Current CPC
Class: |
B23K 35/0222 20130101;
B23K 1/00 20130101; B23K 1/203 20130101; B23K 35/025 20130101; B23K
1/012 20130101; B23K 35/0244 20130101; B23K 1/085 20130101; Y10T
403/479 20150115; B23K 1/002 20130101; B23K 2103/12 20180801; B23K
35/0227 20130101; C22C 13/02 20130101; B23K 1/19 20130101; H05K
3/3463 20130101; B23K 1/0016 20130101; B23K 35/262 20130101; C22C
13/00 20130101; B23K 35/0233 20130101; B23K 1/0056 20130101; H05K
3/3457 20130101 |
International
Class: |
B23K 35/26 20060101
B23K035/26; H05K 3/34 20060101 H05K003/34; C22C 13/00 20060101
C22C013/00; B23K 1/00 20060101 B23K001/00; C22C 13/02 20060101
C22C013/02 |
Claims
1. A lead-free, antimony-free solder alloy comprising: (a) 3 to 5
wt % of silver (b) 2.5 to 5 wt % of bismuth (c) 0.3 to 2 wt % of
copper (d) at least one of the following elements up to 1 wt. % of
nickel up to 1 wt. % of titanium up to 1 wt. % of cobalt up to 3.5
wt. % of indium up to 1 wt. % of zinc up to 1 wt. % of arsenic (e)
optionally one or more of the following elements 0 to 1 wt. % of
manganese 0 to 1 wt. % of chromium 0 to 1 wt. % of germanium 0 to 1
wt. % of iron 0 to 1 wt. % of aluminum 0 to 1 wt. % of phosphorus 0
to 1 wt. % of gold 0 to 1 wt. % of gallium 0 to 1 wt. % of
tellurium 0 to 1 wt. % of selenium 0 to 1 wt. % of calcium 0 to 1
wt. % of vanadium 0 to 1 wt. % of molybdenum 0 to 1 wt. % of
platinum 0 to 1 wt. % of magnesium 0 to 1 wt. % of rare earths (f)
the balance tin, together with any unavoidable impurities; wherein
the alloy affirmatively contains said Ni in a concentration of 0.01
to 1 wt %; and wherein the alloy affirmatively contains said Ge in
a concentration of 0.005 to 1.
2. The solder alloy according to claim 1, wherein the alloy
comprises: from 3.5 to 4.5 wt. % of silver; from 2.5 to 4 wt. % of
bismuth; from 0.3 to 0.8 wt. % of copper; from 0.03 to 1 wt. %
nickel; from 0.005 to 1 wt. % germanium, the balance tin, together
with any unavoidable impurities.
3. The solder alloy according to claim 2, wherein the alloy
comprises from 2.8 to 4 wt % bismuth.
4. The solder alloy according to claim 2, wherein the alloy
comprises from 0.6 to 0.8 wt % copper.
5. The solder alloy according to claim 2, wherein the alloy
comprises from 0.03 to 0.6 wt. % nickel.
6. The solder alloy according to claim 2, wherein the alloy
comprises: from 2.8 to 4 wt. % bismuth; and from 0.1 to 0.3 wt. %
nickel.
7. The solder alloy as claimed in claim 2, wherein the alloy has
melting point of from 195 to 222.degree. C.
8. The solder alloy as claimed in claim 2 in the form of a bar, a
stick, a solid or flux cored wire, a foil or strip, or a powder or
paste (powder plus flux blend), or solder spheres for use in ball
grid array joints or chip scale packages, or other pre-formed
solder pieces, with or without a flux core or a flux coating.
9. A soldered joint comprising an alloy as defined in claim 2.
10. A method of forming a solder joint comprising: (i) providing
two or more work pieces to be joined; (ii) providing a solder alloy
as defined in claim 2; and (iii) heating the solder alloy in the
vicinity of the work pieces to be joined.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 14/434,470 filed Apr. 9, 2015, which is a U.S. national stage
application of International Patent Application No.
PCT/GB2013/052624, filed Oct. 9, 2013, and claims the benefit of
priority of U.S. Application No. 61/711,277, filed Oct. 9, 2012,
the entire disclosures of which are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the field of
metallurgy and to an alloy and, in particular, a lead-free and
antimony-free solder alloy. The alloy is particularly, though not
exclusively, suitable for use in electronic soldering applications
such as wave soldering, surface mounting technology, hot air
leveling and ball grid arrays, land grid arrays, bottom terminated
packages, LEDs and chip scale packages.
BACKGROUND OF THE INVENTION
[0003] Wave soldering (or flow soldering) is a widely used method
of mass soldering electronic assemblies. It may be used, for
example, for through-hole circuit boards, where the board is passed
over a wave of molten solder, which laps against the bottom of the
board to wet the metal surfaces to be joined.
[0004] Another soldering technique involves printing of the solder
paste on the soldering pads on the printed circuit boards followed
by placement and sending the whole assembly through a reflow oven.
During the reflow process, the solder melts and wets the soldering
surfaces on the boards as well as the components.
[0005] Another soldering process involves immersing printed wiring
boards into molten solder in order to coat the copper terminations
with a solderable protective layer. This process is known as
hot-air leveling.
[0006] A ball grid array joint or chip scale package is assembled
typically with spheres of solder between two substrates. Arrays of
these joints are used to mount chips on circuit boards.
[0007] There are a number of requirements for a solder alloy to be
suitable for use in wave soldering, hot-air leveling processes and
ball grid arrays. First, the alloy must exhibit good wetting
characteristics in relation to a variety of substrate materials
such as copper, nickel, nickel phosphorus ("electroless nickel").
Such substrates may be coated to improve wetting, for example by
using tin alloys, gold or organic coatings (OSP). Good wetting also
enhances the ability of the molten solder to flow into a capillary
gap, and to climb up the walls of a through-plated hole in a
printed wiring board, to thereby achieve good hole filling.
[0008] Solder alloys tend to dissolve the substrate and to form an
intermetallic compound at the interface with the substrate. For
example, tin in the solder alloy may react with the substrate at
the interface to form an intermetallic compound (IMC) layer. If the
substrate is copper, then a layer of Cu.sub.6Sn.sub.5 may be
formed. Such a layer typically has a thickness of from a fraction
of a micron to a few microns. At the interface between this layer
and the copper substrate an IMC of Cu.sub.3Sn may be present. The
interface intermetallic layers will tend to grow during aging,
particularly where the service is at higher temperatures, and the
thicker intermetallic layers, together with any voids that may have
developed may further contribute to premature fracture of a
stressed joint.
[0009] Other important factors are: (i) the presence of
intermetallics in the alloy itself, which results in improved
mechanical properties; (ii) oxidation resistance, which is
important in solder spheres where deterioration during storage or
during repeated reflows may cause the soldering performance to
become less than ideal; (iii) drossing rate; and (iv) alloy
stability. These latter considerations are important for
applications where the alloy is held in a tank or bath for long
periods of time or where the formed solder joints are subjected to
high operating temperatures for long periods of time.
[0010] For environmental and health reasons, there is an increasing
demand for lead-free and antimony-free replacements for lead- and
antimony-containing conventional alloys. Many conventional solder
alloys are based around the tin-copper eutectic composition, Sn-0.7
wt. % Cu. For example, the tin-silver-copper system has been
embraced by the electronics industry as a lead-free alternative for
soldering materials. One particular alloy, the eutectic alloy
SnAg3.0Cu0.5, exhibits a superior fatigue life compared to a Sn--Pb
solder material while maintaining a relatively low melting point of
about 217 to 219.degree. C.
[0011] In some fields, such as automotive, high power electronics
and energy, including LED lighting, for example, it is desirable
for solder alloys to operate at higher temperatures, for example
150.degree. C. or higher. The SnAg3.0Cu0.5 alloy does not perform
well at such temperatures.
SUMMARY OF THE INVENTION
[0012] The present invention aims to solve at least some of the
problems associated with the prior art or to provide a commercially
acceptable alternative.
[0013] Accordingly, in a first aspect, the present invention
provides a lead-free, antimony-free solder alloy comprising:
[0014] (a) 10 wt. % or less of silver
[0015] (b) 10 wt. % or less of bismuth
[0016] (c) 3 wt. % or less of copper
[0017] (d) at least one of the following elements [0018] up to 1
wt. % of nickel [0019] up to 1 wt. % of titanium [0020] up to 1 wt.
% of cobalt [0021] up to 3.5 wt. % of indium [0022] up to 1 wt. %
of zinc [0023] up to 1 wt. % of arsenic
[0024] (e) optionally one or more of the following elements [0025]
0 to 1 wt. % of manganese [0026] 0 to 1 wt. % of chromium [0027] 0
to 1 wt. % of germanium [0028] 0 to 1 wt. % of iron [0029] 0 to 1
wt. % of aluminum [0030] 0 to 1 wt. % of phosphorus [0031] 0 to 1
wt. % of gold [0032] 0 to 1 wt. % of gallium [0033] 0 to 1 wt. % of
tellurium [0034] 0 to 1 wt. % of selenium [0035] 0 to 1 wt. % of
calcium [0036] 0 to 1 wt. % of vanadium [0037] 0 to 1 wt. % of
molybdenum [0038] 0 to 1 wt. % of platinum [0039] 0 to 1 wt. % of
magnesium [0040] 0 to 1 wt. % of rare earths
[0041] (f) the balance tin, together with any unavoidable
impurities.
BRIEF DESCRIPTION OF THE FIGURES
[0042] FIG. 1 shows electron microscope images of the
microstructure of Alloy A (a) as cast, and (b) after heat treatment
at 150.degree. C. Intermetallics compounds were identified by
SEM-EDS.
[0043] FIG. 2 shows electron microscope images of the
microstructure of Alloy B (a) as cast, and (b) after heat treatment
at 150.degree. C. Intermetallics compounds were identified by
SEM-EDS.
[0044] FIG. 3 shows electron microscope images of the
microstructure of Alloy C (a) as cast, and (b) after heat treatment
at 150.degree. C. Intermetallics compounds were identified by
SEM-EDS.
[0045] FIG. 4 shows electron microscope images of the
microstructure of Alloy D (a) as cast, and (b) after heat treatment
at 150.degree. C. Intermetallics compounds were identified by
SEM-EDS.
[0046] FIG. 5 shows a comparison of (a) ultimate tensile strength,
and (b) yield strength at room temperature for SnAg3.0Cu0.5 and
alloys according to the present invention.
[0047] FIG. 6 shows a comparison of (a) ultimate tensile strength,
and (b) yield strength at 150.degree. C. for SnAg3.0Cu0.5 and
alloys according to the present invention.
[0048] FIG. 7 shows a comparison of (a) creep rupture time and (b)
creep elongation at rupture measured at 150.degree. C. of for
SnAg3.0Cu0.5 and alloys according to the present invention.
[0049] FIG. 8 shows zero wetting time of SnAg3.0Cu0.5 and alloys
according to the present invention as a measure of their
solderability.
[0050] FIG. 9 shows the Weibull distribution curves describing BGA
failures during drop shock test.
[0051] FIG. 10 shows the Weibull distribution curves describing BGA
failures during thermal cycling test.
[0052] FIG. 11 shows electron microscope images of BGA
cross-sections before and after thermal cycling test.
[0053] FIG. 12 shows shear force of chip resistors components
measured before and after thermal cycling test.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0054] The present invention will now be further described. In the
following passages different aspects of the invention are defined
in more detail. Each aspect so defined may be combined with any
other aspect or aspects unless clearly indicated to the contrary.
In particular, any feature indicated as being preferred or
advantageous may be combined with any other feature or features
indicated as being preferred or advantageous.
[0055] The alloys described herein exhibit improved
high-temperature reliability and are capable of withstanding
operational temperatures of typically at least 150.degree. C. The
alloys exhibit improved mechanical properties and high-temperature
creep resistance compared to the conventional SnAg3.0Cu0.5
alloy.
[0056] The alloys are lead-free and antimony-free meaning that no
lead or antimony is added intentionally. Thus, the lead and
antimony contents are zero or at no more than accidental impurity
levels.
[0057] The alloy composition comprises 10 wt. % or less of silver,
for example from 1 to 10 wt. %. Preferably, the alloy comprises
from 2.5 to 5 wt. % silver, more preferably from 3 to 5 wt. %
silver, even more preferably from 3 to 4.5 wt. % silver, and most
preferably from 3.5 to 4 wt. % silver. The presence of silver in
the specified amount may serve to improve mechanical properties,
for example strength, through the formation of intermetallic
compounds. In addition, the presence of silver may act to decrease
copper dissolution and improve wetting and spread.
[0058] The alloy composition comprises 10 wt. % or less of bismuth,
for example from 1 to 10 wt. %. Preferably, the alloy comprises
from 2 to 6 wt. % bismuth, more preferably from 2.5 to 5 wt. %
bismuth, even more preferably from 2.7 to 4.5 wt. % bismuth, and
most preferably from 2.8 to 4 wt. % bismuth. The presence of
bismuth in the specified amount may serve to improve mechanical
properties through solid solution strengthening. Bismuth may also
act to improve creep resistance. Bismuth may also improve wetting
and spread.
[0059] The alloy composition comprises 3 wt. % or less of copper,
for example from 0.1 to 3 wt. %. Preferably, the alloy comprises
from 0.3 to 2 wt. % copper, more preferably from 0.4 to 1 wt. %
copper, even more preferably from 0.5 to 0.9 wt. % copper, and most
preferably from 0.6 to 0.8 wt. % copper. The presence of copper in
the specified amount may serve to improve mechanical properties,
for example strength, through the formation of intermetallic
compounds. In addition, the presence of copper reduces copper
dissolution and may also improve creep resistance.
[0060] The alloy composition optionally comprises 0 to 1 wt. % of
nickel, for example from 0.01 to 1 wt. %. If nickel is present, the
alloy preferably comprises from 0.03 to 0.6 wt. % nickel, more
preferably from 0.05 to 0.5 wt. % nickel, even more preferably from
0.07 to 0.4 wt. % nickel, and most preferably from 0.1 to 0.3 wt. %
nickel. The presence of nickel in the specified amount may serve to
improve mechanical properties through the formation of
intermetallic compounds with tin, which can result in precipitation
strengthening. In addition, the presence of nickel may act to
reduce the copper dissolution rate. Nickel may also increase drop
shock resistance by decreasing IMC growth at the substrate/solder
interface.
[0061] The alloy composition optionally comprises 0 to 1 wt. % of
titanium, for example from 0.005 to 1 wt. %. If titanium is
present, the alloy preferably comprises from 0.005 to 0.5 wt. %
titanium, more preferably from 0.007 to 0.1 wt. % titanium, even
more preferably from 0.008 to 0.06 wt. % titanium, and most
preferably 0.01 to 0.05 wt. % titanium. The presence of titanium in
the specified amount may serve to improve strength and interfacial
reactions. Titanium may also improve drop shock performance.
[0062] The alloy composition optionally comprises 0 to 1 wt. % of
cobalt, for example from 0.01 to 1 wt. %. If cobalt is present, the
alloy preferably comprises from 0.01 to 0.6 wt. % cobalt, more
preferably from 0.02 to 0.5 wt. % cobalt, even more preferably from
0.03 to 0.4 wt. % cobalt, and most preferably 0.04 to 0.3 wt. %
cobalt. The presence of cobalt may act to lower the copper
dissolution rate. Cobalt may also slow the rate of IMC formation at
the substrate/solder interface, and increase drop-shock
resistance.
[0063] The alloy composition optionally comprises 0 to 3.5 wt. % of
indium, for example from 0.01 to 3.5 wt. %. If indium is present,
the alloy preferably comprises from 0.05 to 3.5 wt. % indium, more
preferably from 0.1 to 3.5 wt. % indium. The presence of indium may
act to improve mechanical properties through solid solution
strengthening.
[0064] The alloy composition optionally comprises 0 to 1 wt. % of
zinc, for example from 0.01 to 1 wt. %. If zinc is present, the
alloy preferably comprises from 0.03 to 0.6 wt. % zinc, more
preferably from 0.05 to 0.5 wt. % zinc, even more preferably from
0.07 to 0.4 wt. % zinc, and most preferably 0.1 to 0.3 wt. % zinc.
The presence of zinc may act to improve mechanical properties
through solid solution strengthening. Zinc may also act to slow IMC
growth and reduce void formation.
[0065] The alloy composition optionally comprises 0 to 1 wt. % of
arsenic, for example from 0.01 to 1 wt. %. If arsenic is present,
the alloy preferably comprises from 0.03 to 0.6 wt. % arsenic, more
preferably from 0.05 to 0.5 wt. % arsenic, even more preferably
from 0.07 to 0.4 wt. % arsenic, and most preferably 0.1 to 0.3 wt.
% arsenic. The presence of arsenic may act to improve mechanical
properties through particle dispersion.
[0066] The alloy may optionally also contain one or more of 0.005
to 1 wt. % of manganese, 0.005 to 1 wt. % of chromium, 0.005 to 1
wt. % of germanium, 0.005 to 1 wt. % of iron, 0.005 to 1 wt. % of
aluminum, 0.005 to 1 wt. % of phosphor, 0.005 to 1 wt. % of gold,
0.005 to 1 wt. % of gallium, 0.005 to 1 wt. % of tellurium, 0.005
to 1 wt. % of selenium, 0.005 to 1 wt. % of calcium, 0.005 to 1 wt.
% of vanadium, 0.005 to 1 wt. % of molybdenum, 0.005 to 1 wt. % of
platinum, 0.005 to 1 wt. % of magnesium and/or 0.005 to 1 wt. % of
rare earth element(s).
[0067] Rare earths may act to improve spread and wettability.
Cerium has been found to be particularly effective in this regard.
Aluminium, calcium, gallium, germanium, magnesium, phosphorus and
vanadium may act as deoxidizers and may also improve wettability
and solder joint strength. Other elemental additions, such as gold,
chromium, iron, manganese, molybdenum, platinum, selenium and
tellurium may act to improve strength and interfacial
reactions.
[0068] The term rare earth element as used herein refers to one or
more elements selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb and Lu.
[0069] The alloy will typically comprise at least 88 wt. % tin,
more typically at least 90 wt. % tin, still more typically at least
91 wt. % tin.
[0070] In a further aspect, there is provided an alloy comprising
from 3 to 5 wt. % silver, from 2 to 5 wt. % bismuth, from 0.3 to
1.5 wt. % copper, from 0.05 to 0.4 wt. % nickel, optionally 0.008
to 0.06 wt. % titanium, optionally 0.005 to 0.2 of a rare earth
element (preferably cerium), optionally 3 to 4 wt. % of indium,
optionally up to 1 wt. % germanium, optionally up to 1 wt. %
manganese, optionally 0.01 to 0.1 wt. % cobalt, and the balance tin
together with unavoidable impurities.
[0071] In one embodiment, there is provided an alloy comprising 3
to 4.5 wt. % silver, 3 to 4.5 wt. % bismuth, 0.5 to 1.5 wt. %
copper, 0.05 to 0.25 wt. % nickel, and the balance tin together
with unavoidable impurities. Such an alloy has a melting range of
from 207.2 to 215.9.degree. C., which is lower than the near
eutectic temperature of the conventional SnAg3.0Cu0.5 alloy. Such
an alloy has a hardness that is about twice the magnitude of the
hardness of SnAg3.0Cu0.5. In one specific example of this
embodiment, the alloy comprises approximately 3.63 wt. % silver,
3.92 wt. % bismuth, 0.76 wt. % copper, 0.18 wt. % nickel, and the
balance tin together with unavoidable impurities.
[0072] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 3 to 4.5 wt. % bismuth, 0.5 to 1.5 wt. %
copper, 0.05 to 0.25 wt. % nickel, 0.005 to 0.05 wt. % of a rare
earth element, for example cerium, and the balance tin together
with unavoidable impurities. Such an alloy has a melting range of
from 208.8 to 219.4.degree. C. and a hardness that is about twice
the magnitude of the hardness of SnAg3.0Cu0.5. In one specific
example of this embodiment, the alloy comprises approximately 3.81
wt. % silver, 3.94 wt. % bismuth, 0.8 wt. % copper, 0.25 wt. %
nickel, 0.04 wt. % cerium, and the balance tin together with
unavoidable impurities.
[0073] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 2 to 4 wt. % bismuth, 0.5 to 1.5 wt. %
copper, 0.05 to 0.25 wt. % nickel, 0.005 to 0.05 wt. % titanium,
and the balance tin together with unavoidable impurities. Such an
alloy has a melting range of from 210.4 to 215.9.degree. C. and a
hardness that is about twice the magnitude of the hardness of
SnAg3.0Cu0.5. In one specific example of this embodiment, the alloy
comprises approximately 3.8 wt. % silver, 2.98 wt. % bismuth, 0.7
wt. % copper, 0.1 wt. % nickel, 0.01 wt. % titanium, and the
balance tin together with unavoidable impurities.
[0074] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 3 to 5 wt. % bismuth, 0.4 to 1.5 wt. %
copper, 0.1 to 0.3 wt. % nickel, 0.01 to 0.2 wt. % of a rare earth
element(s) (preferably cerium), and the balance tin together with
unavoidable impurities. Such an alloy has a melting range of from
209.0 to 220.4.degree. C. In one specific example of this
embodiment, the alloy comprises approximately 3.85 wt. % silver,
3.93 wt. % bismuth, 0.68 wt. % copper, 0.22 wt. % nickel, 0.08 wt.
% cerium, and the balance tin together with unavoidable
impurities.
[0075] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 3 to 5 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.1 wt. % of titanium,
and the balance tin together with unavoidable impurities. Such an
alloy has a melting range of from 209.3 to 220.6.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.86 wt. % silver, 3.99 wt. % bismuth, 0.63 wt. %
copper, 0.16 wt. % nickel, 0.043 wt. % titanium, and the balance
tin together with unavoidable impurities.
[0076] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 3 to 5 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.1 wt. % of cobalt, and
the balance tin together with unavoidable impurities. Such an alloy
has a melting range of from 209.1 to 216.1.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.82 wt. % silver, 3.96 wt. % bismuth, 0.6 wt. %
copper, 0.16 wt. % nickel, 0.042 wt. % cobalt, and the balance tin
together with unavoidable impurities.
[0077] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 2 to 4 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.25 wt. % nickel, 0.001 to 0.01 wt. % of
manganese, and the balance tin together with unavoidable
impurities. Such an alloy has a melting range of from 209.2 to
216.8.degree. C. In one specific example of this embodiment, the
alloy comprises approximately 3.9 wt. % silver, 3 wt. % bismuth,
0.6 wt. % copper, 0.12 wt. % nickel, 0.006 wt. % Mn, and the
balance tin together with unavoidable impurities.
[0078] In another embodiment, there is provided an alloy comprising
3 to 4.5 wt. % silver, 2 to 4 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.001 to 0.01 wt. % of germanium,
and the balance tin together with unavoidable impurities. Such an
alloy has a melting range of from 208.2 to 218.6.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.85 wt. % silver, 3.93 wt. % bismuth, 0.63 wt. %
copper, 0.15 wt. % nickel, 0.006 wt. % germanium, and the balance
tin together with unavoidable impurities.
[0079] In another embodiment, there is provided an alloy comprising
4 to 5 wt. % silver, 3 to 5 wt. % bismuth, 0.3 to 1.2 wt. % copper,
0.05 to 0.3 wt. % nickel, 3 to 4 wt. % of indium, and the balance
tin together with unavoidable impurities. Such an alloy has a
melting range of from 195.6 to 210.7.degree. C. In one specific
example of this embodiment, the alloy comprises approximately 4.24
wt. % silver, 3.99 wt. % bismuth, 0.63 wt. % copper, 0.18 wt. %
nickel, 3.22 wt. % indium, and the balance tin together with
unavoidable impurities.
[0080] In another embodiment, there is provided an alloy comprising
3.5 to 5 wt. % silver, 2 to 5 wt. % bismuth, 0.4 to 1.3 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.1 wt. % of cerium, and
the balance tin together with unavoidable impurities. Such an alloy
has a melting range of from 209.8 to 217.0.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.91 wt. % silver, 2.9 wt. % bismuth, 0.72 wt. %
copper, 0.2 wt. % nickel, 0.04 wt. % cerium, and the balance tin
together with unavoidable impurities.
[0081] In another embodiment, there is provided an alloy comprising
3.5 to 5 wt. % silver, 2 to 5 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.08 wt. % lanthanum, and
the balance tin together with unavoidable impurities. Such an alloy
has a melting range of from 210.96 to 220.8.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.87 wt. % silver, 3.02 wt. % bismuth, 0.61 wt. %
copper, 0.14 wt. % nickel, 0.038% lanthanum, and the balance tin
together with unavoidable impurities.
[0082] In another embodiment, there is provided an alloy comprising
3.5 to 5 wt. % silver, 3 to 5 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.08 wt. % neodymium, and
the balance tin together with unavoidable impurities. Such an alloy
has a melting range of from 207.8 to 219.5.degree. C. In one
specific example of this embodiment, the alloy comprises
approximately 3.86% silver, 3.99% bismuth, 0.64% copper, 0.14%
nickel, 0.044% neodymium, and the balance tin together with
unavoidable impurities.
[0083] In another embodiment, there is provided an alloy comprising
3.5 to 5 wt. % silver, 3 to 5 wt. % bismuth, 0.3 to 1.2 wt. %
copper, 0.05 to 0.3 wt. % nickel, 0.01 to 0.08 wt. % cobalt, and
the balance tin together with unavoidable impurities. Such an alloy
has a melting range of from 209 to 217.degree. C. In one specific
example of this embodiment, the alloy comprises approximately 3.94%
silver, 3.92% bismuth, 0.7% copper, 0.12% nickel, 0.023% cobalt,
and the balance tin together with unavoidable impurities.
[0084] It will be appreciated that the alloys described herein may
contain unavoidable impurities, although, in total, these are
unlikely to exceed 1 wt. % of the composition. Preferably, the
alloys contain unavoidable impurities in an amount of not more than
0.5 wt. % of the composition, more preferably not more than 0.3 wt.
% of the composition, still more preferably not more than 0.1 wt. %
of the composition, still more preferably not more than 0.05 wt. %
of the composition, and most preferably not more than 0.02 wt. % of
the composition.
[0085] The alloys described herein may consist essentially of the
recited elements. It will therefore be appreciated that in addition
to those elements that are mandatory (i.e. Sn, Ag, Bi, Cu and at
least one of Ni, Ti, Co, In, Zn and/or As) other non-specified
elements may be present in the composition provided that the
essential characteristics of the composition are not materially
affected by their presence.
[0086] In one embodiment, the alloy exhibits a relatively low
melting point, typically from about 195 to about 222.degree. C.
(more typically about 209 to about 218.degree. C.). This is
advantageous because it enables a reflow peak temperature of from
about 230 to about 240.degree. C.
[0087] In another embodiment, the alloy exhibits a thermal
conductivity and/or an electrical conductivity which is/are higher
or equivalent to the conventional SnAg3.0Cu0.5 alloy. This is
advantageous in energy-related applications such as, for example,
light-emitting diodes (LED), solar and power electronics.
[0088] The alloys of the present invention may be in the form of,
for example, a bar, a stick, a solid or flux cored wire, a foil or
strip, a film, a preform, or a powder or paste (powder plus flux
blend), or solder spheres for use in ball grid array joints, or a
pre-formed solder piece or a reflowed or solidified solder joint,
or pre-applied on any solderable material such as a copper ribbon
for photovoltaic applications or a printed circuit board of any
type.
[0089] In another aspect, the present invention provides a method
of forming a solder joint comprising:
[0090] providing two or more work pieces to be joined;
[0091] (ii) providing a solder alloy as defined in any of claims 1
to 10; and
[0092] (iii) heating the solder alloy in the vicinity of the work
pieces to be joined.
[0093] In another aspect, the present invention provides the use of
an alloy, as herein described, in a soldering method. Such
soldering methods include, but are not restricted to, wave
soldering, Surface Mount Technology (SMT) soldering, die attach
soldering, thermal interface soldering, hand soldering, laser and
RF induction soldering, and rework soldering, lamination, for
example.
[0094] The present invention will now be described further with
reference to the following non-limiting examples.
Example 1
Alloy A
[0095] Alloy A comprises 3.63 wt % silver, 3.92 wt % bismuth, 0.76
wt % copper, 0.18 wt % nickel, and the balance tin together with
unavoidable impurities.
[0096] A cross-section of this alloy, as cast, reveals a
microstructure containing Bi.sub.2Sn, Ag.sub.3Sn and
Cu.sub.6Sn.sub.5 (see FIG. 1(a)). The Ag.sub.3Sn is dispersed in
the tin matrix, but also appears as needle-shaped precipitates.
Other intermetallics Sn--Bi and Sn--Cu precipitates are
non-homogeneously distributed in the matrix. After heat treatment
at approximately 150.degree. C. for about 200 hours, significant
reduction of needle-shape Ag.sub.3Sn is observed, revealing a more
homogeneous microstructure. Also, after the heat-treatment, the
microstructure shows a more homogeneous distribution of the
precipitates in the Sn-matrix and the presence of Ni, Cu--Sn
precipitates (see FIG. 1(b)).
[0097] Such a microstructure, i.e. a more homogeneous matrix and
the presence of finely distributed intermetallics precipitates,
suggests that both solid solution and precipitation hardening are
responsible for alloy strengthening and improved mechanical
properties. The phenomenon of creep is expected to be reduced by
such a microstructure.
[0098] Alloy A has a melting range of 207.2 to 215.9.degree. C.; a
coefficient of thermal expansion CTE (.mu.m/mK) (30-100.degree. C.)
of 19.6; and a Vickers Hardness (HV-1) of 31. For comparison
purposes, the conventional alloy, SnAg3.0Cu0.5, has a melting range
of 216.6 to 219.7.degree. C.; a coefficient of thermal expansion
CTE (.mu.m/mK) (30-100.degree. C.) of 22.4; and a Vickers Hardness
(HV-0.5) of 15.
Example 2
Alloy B
[0099] Alloy B comprises 3.81 wt % silver, 3.94 wt % bismuth, 0.8
wt % copper, 0.25 wt % nickel, 0.04 wt % cerium, and the balance
tin together with unavoidable impurities. Alloy B also reveals a
microstructure containing Bi.sub.2Sn, Ag.sub.3Sn and
Cu.sub.6Sn.sub.5 (see FIG. 2(a)). Similar to Alloy A, Ag.sub.3Sn is
dispersed in the Sn matrix, but also appears as needle-shaped
precipitates, and Sn--Cu precipitates are non-homogeneously
distributed in the matrix. After a heat-treatment at approximately
150.degree. C. for about 200 hours, the eutectic Ag--Sn can clearly
be seen and a significant reduction of needle-shaped Ag.sub.3Sn is
also observed, showing a more homogeneous microstructure (see FIG.
2(b)). As with Alloy A, Ni, Cu--Sn precipitates are identified in
the matrix after the heat-treatment. Such precipitates have been
identified by X-ray diffraction analysis as NiSn.sub.2
precipitates.
[0100] Alloy B has a melting range of 208.8 to 219.4.degree. C.; a
coefficient of thermal expansion CTE (.mu.m/mK) (30-100.degree. C.)
of 22.8; and a Vickers Hardness (HV-1) of 28.
Example 3
Alloy C
[0101] Alloy C comprises 3.8 wt % silver, 2.98 wt % bismuth, 0.7 wt
% copper, 0.1 wt % nickel, 0.01 wt % titanium, and the balance tin
together with unavoidable impurities. The as cast microstructure
(FIG. 3(a)) consists of large concentration of finer Ag.sub.3Sn
precipitates dispersed along the grain boundaries, which is
expected to prevent grain boundary sliding during creep and thus
improving creep resistance of the alloy. Significant growth of the
precipitates is observed after aging at 150.degree. C. for about
200 hours (FIG. 3(b)).
[0102] Alloy C has a melting range of 210.4 to 215.9.degree. C.; a
coefficient of thermal expansion CTE (.mu.m/mK) (30-100.degree. C.)
of 23.8; and a Vickers Hardness (HV-1) of 28.
Example 4
Alloy D
[0103] Alloy D comprises 3.85% silver, 3.93% bismuth, 0.68% copper,
0.22% nickel, 0.078% cerium, and the balance tin together with
unavoidable impurities. This alloy microstructure (FIG. 4(a))
reveals long needle-shaped Ag.sub.3Sn along with Cu.sub.6Sn.sub.5
precipitates.
[0104] Alloy D has a melting range of 209.0 to 220.4.degree. C.; a
coefficient of thermal expansion CTE (.mu.m/mK) (30-100.degree. C.)
of 22; and a Vickers Hardness (HV-1) of 29.
Example 5
Alloy E
[0105] Alloy E comprises 3.86% silver, 3.99% bismuth, 0.63% copper,
0.16% nickel, 0.043% titanium, and the balance tin together with
unavoidable impurities. It has a melting range of 209.3 to
220.6.degree. C.; and Vickers Hardness (HV-1) of 30.
Example 6
Alloy F
[0106] Alloy F comprises 3.82% silver, 3.96% bismuth, 0.6% copper,
0.16% nickel, 0.042% cobalt, and the balance tin together with
unavoidable impurities. It has a melting range of 209.1 to
216.1.degree. C.; and a coefficient of thermal expansion CTE
(.mu.m/mK) (30-100.degree. C.) of 22.4.
Example 7
Alloy G
[0107] Alloy G comprises 3.9% silver, 3% bismuth, 0.6% copper,
0.12% nickel, 0.006% manganese, and the balance tin together with
unavoidable impurities. It has a melting range of 209.2 to
216.8.degree. C.; and a Vickers Hardness (HV-1) of 28.
Example 8
Alloy H
[0108] Alloy H comprises 3.83% silver, 3.93% bismuth, 0.63% copper,
0.15% nickel, 0.006% germanium, and the balance tin together with
unavoidable impurities. It has a melting range of 208.2 to
218.6.degree. C.; a coefficient of thermal expansion CTE (.mu.m/mK)
(30-100.degree. C.) of 21.7; and a Vickers Hardness (HV-1) of
29.
Example 9
Alloy I
[0109] Alloy I comprises 4.20% silver, 3.99% bismuth, 0.63% copper,
0.18% nickel, 3.22% indium, and the balance tin together with
unavoidable impurities. It has a melting range of 195.6 to
210.7.degree. C.
Example 10
Alloy J
[0110] Alloy J comprises 3.91% silver, 2.9% bismuth, 0.72% copper,
0.2% nickel, 0.04% cerium, and the balance tin together with
unavoidable impurities. It has a melting range of 209.8 to
217.0.degree. C.; a coefficient of thermal expansion CTE (.mu.m/mK)
(30-100.degree. C.) of 22.7; and a Vickers Hardness (HV-1) of
27.
Example 11
Alloy K
[0111] Alloy K comprises 3.87% silver, 3.02% bismuth, 0.61% copper,
0.14% nickel, 0.038% lanthanum, and the balance tin together with
unavoidable impurities. It has a melting range of 210.96 to
220.8.degree. C.; and a Vickers Hardness (HV-1) of 29.
Example 12
Alloy L
[0112] Alloy L comprises 3.86% silver, 3.99% bismuth, 0.64% copper,
0.14% nickel, 0.044% neodymium, and the balance tin together with
unavoidable impurities. It has a melting range of 207.8 to
219.5.degree. C.; and a Vickers Hardness (HV-1) of 29.
Example 13
Alloy M
[0113] Alloy M comprises 3.94% silver, 3.92% bismuth, 0.7% copper,
0.12% nickel, 0.023% cobalt, and the balance tin together with
unavoidable impurities. It has a melting range of 209 to
217.degree. C.; and a coefficient of thermal expansion CTE
(.mu.m/mK) (30-100.degree. C.) of 22.6.
[0114] Table 1 shows the solidus and liquidus temperatures of
SnAg3.0Cu0.5 and Alloys A-M. Solidus temperatures are lower than
the near eutectic temperature of the conventional SnAg3.0Cu0.5
alloy for all Alloys A-M. Liquidus temperatures of Alloys A-M and
conventional SnAg3.0Cu0.5 alloy are nearly the same.
TABLE-US-00001 TABLE 1 Solidus Temperatures and Liquidus
Temperatures of SnAg3.0Cu0.5 and Alloys A-M. Solidus Liquidus
Temperature Temperature Alloys (.degree. C.) (.degree. C.)
SnAg3.0Cu0.5 216.6 219.7 A 207.2 215.9 B 208.8 219.4 C 210.4 215.9
D 209.0 220.4 E 209.3 220.6 F 209.1 216.1 G 209.2 216.8 H 208.2
218.6 I 195.6 210.7 J 209.8 217.0 K 211.0 220.8 L 207.8 219.5 M
209.0 217.0
[0115] FIG. 5 shows a comparison of (a) ultimate tensile strength,
and (b) yield strength at room temperature for SnAg3.0Cu0.5 and
alloys according to the present invention (see ASTM E8/E8M-09 for
test methods of tensile measurements). The tensile properties at
room temperature show a significant improvement. In particular, the
ultimate tensile strengths at room temperature for Alloys A, B, C,
D, E, F, I, J, K and L are between 60% and 110% higher than that of
SnAg3.0Cu0.5. The yield strength shows similar increase in strength
of these alloys, showing between 40% and 81% improvement over
SnAg3.0Cu0.5.
[0116] FIG. 6 shows a comparison of (a) ultimate tensile strength,
and (b) yield strength at 150.degree. C. for SnAg3.0Cu0.5 and
alloys according to the present invention (see ASTM E8/E8M-09 for
test methods of tensile measurements). The ultimate tensile
strength and yield strength decrease at 150.degree. C. However, the
superior properties of Alloys A, B and C over SnAg3.0Cu0.5 remain.
Both properties show about 30 to 43% improvement when compared to
SnAg3.0Cu0.5.
[0117] Testing of the creep properties evaluates the change in
deformation (elastic and plastic) over a relatively long time. In
the case of high temperature creep, the phenomena of microstructure
strengthening alternates with the stress relief caused due to
microstructure annealing.
[0118] FIG. 7 shows a comparison of (a) creep rupture time and (b)
creep elongation at rupture measured at 150.degree. C. of
SnAg3.0Cu0.5 and alloys according to the present invention (see
ASTM E139 for test methods of creep measurements). The alloys of
the present invention have significantly higher creep strength,
which is given by the creep rupture time and creep total plastic
strain, than SnAg3.0Cu0.5. For example, the creep strength at
150.degree. C. of Alloy C is 141% higher than of SnAg3.0Cu0.5.
Similar trend was observed for the creep elongation at rupture,
which for Alloy C is 76% than SnAg3.0Cu0.5.
[0119] FIG. 8 shows zero wetting time of SnAg3.0Cu0.5 and new
alloys as a measure of their solderability and wettability (see JIS
Z 3198-4 for test method of wetting balance measurements). Wetting
properties of alloys according to the present invention are
comparable to conventional SnAg3.0Cu0.5 alloy.
[0120] Intermetallics formation due to alloying addition in the
alloys according to the present invention resulted in additional
strength of the bulk alloy and the solder joint. So far, this has
been exemplified here through tensile, hardness and creep
measurements. Next, drop shock and thermal cycling performance of
the alloys according to the present invention are compared to
standard SnAg3.0Cu0.5.
[0121] FIG. 9 shows the Weibull distribution curves describing BGA
failures during drop shock test (see JESD22-B111 for test method of
drop shock testing). Alloys A, B and C have about 37%, 23% and 44%
drop shock improvement of their characteristic life (i.e., at 63%
failures level) over SnAg3.0Cu0.5.
[0122] FIG. 10 shows the Weibull distribution curves describing BGA
failures during thermal cycling test. Thermal cycle profile used
was -40.degree. C. to +150.degree. C. with 30 minutes dwell time at
each temperature (see IPC-9701 for test method of thermal cycling
measurements). This test was carried out for a total of 2000 cycles
to evaluate thermal-mechanical fatigue resistance of the new
alloys. The reference alloy is represented by a circle, alloy A by
a square and alloy C by a diamond symbol. Before the completion of
2000 cycles, 100% of SnAg3.0Cu0.5 BGA and solder paste assemblies
has failed. However, 32% and 40%, respectively, of Alloy A and C
BGA and solder paste assemblies have survived the thermal cycling
test. Overall, a considerable improvement over SnAg3.0Cu0.5 of the
characteristic life (i.e., at 63% failures level) was observed for
Alloy C.
[0123] FIG. 11 shows electron microscope images of BGA
cross-sections before and after thermal cycling test. Crack
initiation in SnAg3.0Cu0.5 was observed after 500 thermal cycles.
For the Alloys A and C cracks were observed only after 1000 thermal
cycles. After 1500 cycles, extensive cracks were observed in
component using SnAg3.0Cu0.5 BGA and solder paste assembly.
[0124] FIG. 12 shows shear force of chip resistors components
measured before and after thermal cycling test (see JIS Z3198-7 for
test methods of shear force measurements). After 1000 thermal
cycles, the force necessary to shear a 1206 chip resistor bonded to
the PCB using Alloy A or C is 70% higher than using SnAg3.0Cu0.5
alloy. These results corroborate the superior thermal cycling
performance of the new alloys.
[0125] Accordingly, the alloy compositions exhibit improved
room-temperature and also elevated temperature mechanical
properties compared to the conventional alloy, SnAg3.0Cu0.5. These
alloy compositions have also demonstrated solderability and
wettability comparable to SnAg3.0Cu0.5. Additionally, these alloy
compositions have shown improved drop shock resistance and superior
thermal-mechanical reliability compared to conventional
SnAg3.0Cu0.5 alloy.
[0126] The foregoing detailed description has been provided by way
of explanation and illustration, and is not intended to limit the
scope of the appended claims. Many variations in the presently
preferred embodiments illustrated herein will be apparent to one of
ordinary skill in the art, and remain within the scope of the
appended claims and their equivalents.
* * * * *