U.S. patent application number 09/771163 was filed with the patent office on 2001-07-26 for method of making a product with improved material properties by moderate heat treatment of a metal incorporating a dilute additive.
Invention is credited to Chen, Jimmy Kuo-Wei, Dozier, Thomas H., Eldridge, Benjamin N., Herman, Gayle J., Yeh, Junjye J..
Application Number | 20010009724 09/771163 |
Document ID | / |
Family ID | 46255317 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009724 |
Kind Code |
A1 |
Chen, Jimmy Kuo-Wei ; et
al. |
July 26, 2001 |
Method of making a product with improved material properties by
moderate heat treatment of a metal incorporating a dilute
additive
Abstract
Deposition of metal in a preferred shape, including coatings on
parts, or stand-alone materials, and subsequent heat treatment to
provide improved mechanical properties. In particular, the method
gives products with relatively high yield strength. The products
often have relatively high elastic modulus, and are thermally
stable, maintaining the high yield strength at temperatures
considerably above 25.degree. C. This technique involves depositing
a material in the presence of a selected additive, and then
subjecting the deposited material to a moderate heat treatment.
This moderate heat treatment differs from other commonly employed
"stress relief" heat treatments in using lower temperatures and/or
shorter times, preferably just enough to reorganize the material to
the new, desired form. Coating and heat treating a spring-shaped
substrate provides a resilient, conductive contact useful for
electronic applications.
Inventors: |
Chen, Jimmy Kuo-Wei;
(Pleasanton, CA) ; Eldridge, Benjamin N.;
(Danville, CA) ; Dozier, Thomas H.; (Livermore,
CA) ; Yeh, Junjye J.; (Livermore, CA) ;
Herman, Gayle J.; (Danville, CA) |
Correspondence
Address: |
FormFactor, Inc.
Legal Department
5666 La Ribera St.
Livermore
CA
94550
US
|
Family ID: |
46255317 |
Appl. No.: |
09/771163 |
Filed: |
January 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09771163 |
Jan 29, 2001 |
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08931923 |
Sep 17, 1997 |
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08931923 |
Sep 17, 1997 |
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08452255 |
May 26, 1995 |
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08931923 |
Sep 17, 1997 |
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08819464 |
Mar 17, 1997 |
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Current U.S.
Class: |
428/607 ;
200/266; 200/267; 205/150; 205/224; 205/236; 205/238; 205/255;
205/257; 205/260; 205/261; 205/264; 205/265; 205/266; 205/267;
205/269; 205/270; 205/271; 205/274; 205/283; 205/290; 205/291;
205/296; 205/75; 257/E21.503; 257/E21.508; 257/E21.509;
257/E21.511; 257/E21.519; 257/E21.525; 257/E21.582; 257/E23.021;
257/E23.024; 257/E23.068; 257/E23.078; 257/E25.011; 257/E25.029;
428/929; 428/933; 439/886 |
Current CPC
Class: |
H01L 2924/01033
20130101; C23C 18/50 20130101; C25D 5/22 20130101; H01L 2224/16145
20130101; H01L 2224/45144 20130101; H01L 2924/01028 20130101; H01L
2924/01079 20130101; H05K 3/20 20130101; H01L 2225/0651 20130101;
H05K 3/3421 20130101; C25D 5/617 20200801; C25D 5/619 20200801;
H01H 1/0036 20130101; H01L 23/49811 20130101; H01L 2924/01022
20130101; H05K 2201/10318 20130101; C23C 18/1692 20130101; C25D
5/08 20130101; C25D 5/611 20200801; H01L 24/81 20130101; H05K 1/141
20130101; C23C 18/36 20130101; H01L 2224/45015 20130101; H01L
2224/04042 20130101; H05K 3/4015 20130101; Y02P 70/50 20151101;
H01L 2924/01015 20130101; H01L 2224/81801 20130101; H01L 2924/01046
20130101; H01L 2924/30107 20130101; C25D 7/123 20130101; H01L
2224/05599 20130101; H05K 2201/10757 20130101; H01L 2924/01011
20130101; H01L 2924/1532 20130101; H01L 2225/06527 20130101; H05K
2201/1031 20130101; B23K 20/004 20130101; H01L 2224/06136 20130101;
H05K 2201/10946 20130101; H01L 24/72 20130101; H01L 2924/01082
20130101; H01L 2224/85399 20130101; H01L 2924/01029 20130101; H05K
2201/10909 20130101; H01L 25/0652 20130101; H01L 2924/01016
20130101; H01L 2924/01039 20130101; H01L 2224/0401 20130101; H01L
2924/01023 20130101; H05K 2201/10878 20130101; H01L 21/563
20130101; H01L 2924/01014 20130101; H01L 2924/01027 20130101; H01L
2924/01078 20130101; H01L 21/4846 20130101; H01L 25/16 20130101;
H01L 2225/06555 20130101; H01L 2924/10253 20130101; G01R 1/06761
20130101; Y10T 428/12438 20150115; C25D 21/02 20130101; H01L 24/06
20130101; H01L 2924/01012 20130101; H01L 2224/73203 20130101; H01L
2224/85201 20130101; H01L 2924/00014 20130101; H01L 24/45 20130101;
H01L 2224/13655 20130101; H01L 2924/01006 20130101; H05K 3/368
20130101; B81B 7/0006 20130101; H01L 2224/13144 20130101; H01L
2924/01045 20130101; H01L 2225/06572 20130101; B23K 2101/40
20180801; G01R 3/00 20130101; H01L 21/4853 20130101; H01L 2924/014
20130101; H01L 22/20 20130101; H01L 2224/1357 20130101; H01L
2924/01074 20130101; H05K 3/326 20130101; H01L 21/4889 20130101;
H01L 2924/00013 20130101; H01L 2924/01005 20130101; H01L 2224/85399
20130101; H01L 2924/00014 20130101; H01L 2224/13144 20130101; H01L
2924/00014 20130101; H01L 2224/13655 20130101; H01L 2924/01027
20130101; H01L 2924/00013 20130101; H01L 2224/13099 20130101; H01L
2924/00014 20130101; H01L 2224/05599 20130101; H01L 2924/10253
20130101; H01L 2924/00 20130101; H01L 2924/00014 20130101; H01L
2224/48 20130101; H01L 2224/45015 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
428/607 ;
428/933; 428/929; 439/886; 200/266; 200/267; 205/150; 205/75;
205/274; 205/271; 205/270; 205/264; 205/265; 205/291; 205/296;
205/290; 205/283; 205/266; 205/267; 205/269; 205/261; 205/236;
205/255; 205/260; 205/257; 205/238; 205/224 |
International
Class: |
B32B 015/02; C25D
001/08; C25D 005/50; C25D 007/12; C25D 003/02; C25D 003/10; C25D
003/12; C25D 003/20; C25D 003/38; C25D 003/48 |
Claims
What is claimed is:
1. A method of fabricating a resilient structure comprising the
steps of providing an elongate member, depositing a coating on the
elongate member to give a coated elongate member, the coating
comprising at least one metal and at least one additive, the
additive capable of codepositing with the at least one metal, and
heat treating the coated elongate member at a combination of time
and temperature that gives a coating with improved material
properties.
2. The method of claim 1, wherein the elongate member comprises a
wire skeleton.
3. The method of claim 2, wherein the wire skeleton is attached to
a pad on a semiconductor.
4. The method of claim 2, wherein the wire skeleton is attached to
a pad on a carrier.
5. The method of claim 1, wherein the elongate member comprises a
sacrificial substrate.
6. The method of claim 1, wherein the elongate member comprises a
sacrificial substrate coated with a seed layer of material to
promote plating.
7. The method of claim 1, wherein the elongate member comprises a
metal skeleton.
8. The method of claim 1, wherein the coating is formed by
electroplating.
9. The method of claim 8, wherein the electroplating is performed
in a bath which includes the at least one metal and the at least
one additive.
10. The method of claim 1 further comprising, before said coating
step, preparing said desired substrate for electroplating.
11. The method of claim 1, wherein the method of depositing the
coating is selected from the group consisting of electroplating,
chemical vapor deposition (CVD), physical vapor deposition (PVD),
electrolytic or electroless aqueous solution plating of metals, and
any process that causes deposition of materials through
decomposition or reaction of gaseous, liquid or solid
precursors.
12. The method of claim 1 wherein the coating is sufficiently thick
that the increased yield strength imparts resiliency to the coated
substrate.
13. The method of claim 1 wherein said at least one metal comprises
a metal selected from the group consisting of nickel, cobalt, iron,
rhodium, palladium, tungsten, copper, chromium, titanium, aluminum,
gold and platinum.
14. The method of claim 1 wherein said at least one metal comprises
a metal selected from the group consisting of nickel, cobalt and
iron.
15. The method of claim 1 further comprising coating with a
material including at least two metals, said two metals selected
from the group consisting of Ni--Co, Co--Mn, Ni--Mn, Pd--Au,
Pd--Co, W--Co, Ti--N and Ti--W.
16. The method of claim 1, wherein the coating comprises an
alloy.
17. The method of claim 10, wherein the coating comprises a Ni--Co
alloy.
18. The method of claim 1 further comprising coating with a
material including at least three metals, said three metals
selected from the group consisting of Ni--Co--Mn and Ni--W--B.
19. The method of claim 1 wherein the at least one additive is a
relatively minor component.
20. The method of claim 1 wherein the at least one additive or a
derivative of the at least one additive is capable of codepositing
with the at least one metal and capable of coexisting with the at
least one metal upon moderate heat treating to organize the
structure of the coating to provide an increase in yield strength
of the coated substrate.
21. The method of claim 1, wherein the at least one additive
comprises a sulfur-containing compound.
22. The method of claim 1, wherein said at least one additive is
selected from the group consisting of saccharin,
napthalene-tri-sulfonic acid (NTSA), 2-butyne-1,4-diol, and
thiourea.
23. The method of claim 1 further comprising coating in the
presence of a material selected from the group consisting of NiCl,
NiBr, a Class 1 brightener and a Class 2 brightener.
24. The method of claim 1, wherein the coating undergoes an
exothermic transformation in microstructure from a less organized
to a more organized state, an exothermic transformation which be
readily detected using differential scanning calorimetry and has a
peak temperature, and wherein the temperature of heat treating is
between about 0.degree. C. above and about 100.degree. C. above the
peak temperature.
25. The method of claim 1, wherein the coating comprises an
amorphous material before the heat treating step.
26. The method of claim 25, wherein the heat treating step causes a
significant although not necessarily complete transformation in the
coating from the amorphous material to an ordered material.
27. The method of claim 1, wherein a significant but not
necessarily complete portion of the coating is an ordered material
after the heat treating step.
28. The method of claim 1, wherein the coated substrate comprises
an electrical interconnection.
29. The method of claim 1, further comprising coating the elongate
member such that the coated and annealed substrate is attached to a
larger structure wherein the coated substrate comprises a resilient
electrical contact.
30. The method of claim 1 further comprising fully enveloping the
elongate member during the coating step.
31. The method of claim 1 further comprising removing the
substrate, in whole or in part, after the coating step and before
or after the heat treating step.
32. The method of claim 1, wherein the elongate member comprises a
material selected from the group consisting of gold, silicon,
aluminum, and titanium-tungsten.
33. The method of claim 1, wherein the coated substrate has higher
yield strength after heat treating than before heat treating.
34. The method of claim 33, wherein the heat treating is at a
combination of time and temperature to give a final, coated
substrate with a yield strength near the maximum for that coating,
such that significant further heat treating will reduce the yield
strength significantly from that maximum.
35. The method of claim 1, wherein the improved material property
of the coating comprises increased yield strength compared with the
yield strength of the coating before heat treating.
36. The method of claim 1, wherein the improved material property
of the coating comprises increased elastic modulus compared with
the elastic modulus of the coating before heat treating.
37. The method of claim 1, wherein the improved material property
of the coating comprises increased temperature stability under load
at temperatures above 100.degree. C. compared with the temperature
stability of the coating before heat treating.
38. The method of claim 1 wherein the elongate member is less
resilient before heat treatment than the coating after heat
treatment.
39. A method of fabricating a resilient structure comprising the
steps of providing an elongate member, depositing a coating on the
elongate member to give a coated elongate member, the coating
comprising at least one metal and at least one additive, the
additive capable of codepositing with the at least one metal, the
metal comprising a metal selected from the group consisting of
nickel and cobalt, and the additive selected from the group
consisting of saccharin and 2-butyne-1,4-diol, and heat treating
the coated elongate member at a combination of time and temperature
that increases the yield strength of the coating.
40. The method of claim 39, wherein the elongate member comprises a
wire skeleton.
41. The method of claim 39, wherein the elongate member comprises a
metal skeleton.
42. The method of claim 39 further comprising using a plating bath
to deposit said coating wherein said additive is saccharin at a
concentration in the plating bath of more than about 20 mg/L.
43. The method of claim 39 further comprising using a plating bath
to deposit said coating wherein said additive is 2-butyne-1,4-diol
at a concentration in the plating bath of more than about 5
mg/L.
44. The method of claim 39, wherein the coating undergoes an
exothermic transformation in microstructure from a less organized
to a more organized state, an exothermic transformation which be
readily detected using differential scanning calorimetry and has a
peak temperature, and wherein the temperature of heat treating is
between about 0.degree. C. above and about 100.degree. C. above the
peak temperature.
45. The method of claim 39, further comprising coating the elongate
member such that the coated and annealed substrate is attached to a
larger structure wherein the coated substrate comprises a resilient
electrical contact.
46. A resilient structure manufactured by the method comprising the
steps of providing an elongate member, depositing a coating on the
elongate member to give a coated elongate member, the coating
including at least one metal and at least one additive, and heat
treating the coated elongate member at a combination of time and
temperature that increases the yield strength of the coating.
47. The resilient product of claim 46, wherein the elongate member
comprises a wire skeleton.
48. The resilient product of claim 46, wherein the elongate member
comprises a sacrificial substrate.
49. The resilient product of claim 46, wherein the elongate member
comprises a sacrificial substrate coated with a seed layer of
material to promote plating.
50. The resilient product of claim 46, wherein the elongate member
comprises a metal skeleton.
51. The resilient product of claim 46, wherein the coating is
formed by electroplating.
52. The resilient product of claim 51, wherein the electroplating
is performed in a bath which includes the at least one metal and
the at least one additive.
53. The resilient product of claim 46 wherein said at least one
metal comprises a metal selected from the group consisting of
nickel, cobalt, iron, rhodium, palladium, tungsten, copper,
chromium, titanium, aluminum, gold and platinum.
54. The resilient product of claim 46 wherein said at least one
metal comprises a metal selected from the group consisting of
nickel, cobalt and iron.
55. The resilient product of claim 46 further comprising coating
with a material including at least two metals, said two metals
selected from the group consisting of Ni--Co, Co--Mn, Ni--Mn,
Pd--Au, Pd--Co, W--Co, Ti--N and Ti--W.
56. The resilient product of claim 55, wherein the coating
comprises a Ni--Co alloy.
57. The resilient product of claim 46 further comprising coating
with a material including at least three metals.
58. The resilient product of claim 46 wherein the at least one
additive is a relatively minor component.
59. The resilient product of claim 46 wherein the at least one
additive or a derivative of the at least one additive is capable of
codepositing with the at least one metal and capable of coexisting
with the at least one metal upon moderate heat treating to organize
the structure of the coating to provide an increase in yield
strength of the coated substrate.
60. The resilient product of claim 46, wherein the at least one
additive comprises a sulfur-containing compound.
61. The resilient product of claim 46, wherein said at least one
additive is selected from the group consisting of saccharin,
napthalene-tri-sulfonic acid (NTSA), 2-butyne-1,4-diol, and
thiourea.
62. The resilient product of claim 46, wherein the coating is an
ordered material after the heat treating step.
63. The resilient product of claim 46, further comprising coating
the elongate member such that the coated and annealed substrate is
attached to a larger structure wherein the coated substrate
comprises a resilient electrical contact.
64. The resilient product of claim 46, wherein the coated substrate
has higher yield strength after heat treating than before heat
treating.
65. The resilient product of claim 64, wherein the heat treating is
at a combination of time and temperature to give a final, coated
substrate with a yield strength near the maximum for that coating,
such that significant further heat treating will reduce the yield
strength significantly from that maximum.
66. The resilient product of claim 46, wherein the coated substrate
has higher temperature stability after heat treating than before
heat treating.
67. A method of fabricating a resilient structure comprising the
steps of providing an elongate member, depositing a metastable
coating on the elongate member to give a coated elongate member,
the metastable coating comprising at least one metal and at least
one additive, the additive capable of codepositing with the at
least one metal, and heat treating the coated elongate member at a
combination of time and temperature to initiate a transition in the
metastable coating to give a stable coating.
68. The method of claim 67 wherein said stable coating has a yield
strength greater than that of the metastable coating.
69. The method of claim 67 wherein said stable coating has an
elastic modulus greater than that of the metastable coating.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This patent application is a continuation-in-part of
copending U.S. patent application Ser. No. 08/452,255, filed on May
26, 1995. This application also is a continuation-in-part of
copending U.S. patent application Ser. No. 08/819,464, filed on
Mar. 17, 1997.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] This invention relates to depositing a material into a
selected shape then modifying the initial material to provide
desirable mechanical properties. Inclusion of small amounts of an
additive allows selective heat treatment to give a material with
improved material properties when compared to the properties of the
deposited material. In particular, a shaped, soft material may be
coated with the new material and heat treated to give a shaped,
hardened coating, particularly in the form of a conductive
spring.
[0004] 2. Description of Related Art
[0005] The concept of applying a coating to impart desirable
mechanical properties is used in many fields, from semiconductors
to automobiles. For example, micro-electromechanical structures,
microelectronic packaging, and magnetic storage media all employ
such coatings. A variety of processes from sputtering to
electroforming to chemical vapor deposition are widely used to
fabricate such coatings. However, the mechanical properties of many
of these coatings are not completely stable, especially at elevated
temperatures. This is especially true for deposition processes that
result in a non-equilibrium structure. Thus parts with these
coatings have a fundamental problem in applications which require
stable mechanical properties under load, particularly at elevated
temperatures.
[0006] Annealing, or heating a material at a significantly elevated
temperature for an extended period of time, is generally recognized
as a way to bring a structure more into equilibrium. Annealing is
often used to relieve brittleness. Brittleness often results from
various material forming processes. For example, in forming a wire,
it is common to extrude the material through a die, which involves
various compression and deformation processes. The wire as extruded
has the desired shape, but examination of the microstructure of the
material reveals large amounts of internal stresses as internal
stress fields. If these internal stress fields are high, the
material might be considered to be brittle, and will break under
moderate applied stresses. Heat treating such a wire will allow the
material to reorganize and relieve these internal stress
fields.
[0007] Heat treatment also is used to redistribute components
within a system. For example, it is common in semiconductor
processing to apply a dopant such as boron or phosphorous on the
surface of a silicon substrate. Heating, or annealing, this product
allows redistribution of the dopant atoms within the silicon
structure as the dopant atoms diffuse within the base material.
[0008] Annealing of metal coatings such as nickel (Ni) also is
common in many plating operations. It is quite common to
electroplate nickel on a substrate, then anneal at, for example,
700.degree. C. for one to two hours. This is generally to relieve
essentially all stress in the coating, so the annealing is
continued for a relatively long time and/or at a relatively high
temperature. In traditional applications, nickel is plated
relatively quickly, which gives a relatively disordered initial
structure, which in turn provides many sources for residual stress
fields. Annealing allows the material to reach an equilibrium
structure, which is much more stable.
[0009] Note that a typical annealing heat treatment involves both
time and temperature and one skilled in the art can balance higher
temperatures against shorter times or vice versa.
[0010] Another traditional process is the preparation of thin films
for hard disk or other recording surfaces. A thin film of material
such as NiP is deposited on a substrate, then annealed to give a
hard material.
[0011] As semiconductor technology advances and the density of
devices on chips increases, increased demands are placed on
electrical interconnections in microelectronics packaging and
microelectronics diagnostics. The mechanical properties of such
interconnections are important in achieving reliable packaging and
diagnostic solutions.
[0012] For example, it is typically desirable for such
interconnections to have some resiliency. Currently, commonly-used
technologies in microelectronics packaging exhibit little or no
resiliency. Typical packaging includes wirebonding, tape automated
bonding (TAB), solder bump technology, pin-in-hole solder, pin
brazing, and surface-mount solder. While "pogo" pins used in
microelectronics diagnostics are designed to have a resilient
mechanical structure, their substantial inductance inhibits the use
of high frequency signals by the diagnostics system.
[0013] Other resilient structures useful in microelectronics
include a class of structures known as microelectronic mechanical
structures or MEMS. A number of researchers have fabricated small
structures such as horizontal beams positioned with other
electronic components to make devices such as relays. A variety of
gears and other mechanical structures have been prepared.
[0014] Before the present invention, there was perceived a need to
form strong, resilient microstructures but there was no technology
that would allow this. Forming microstructures directly from a
resilient material is in general quite difficult, if not
impossible, in that a resilient material resists specific shaping
methods. For example, tungsten needle in convention probe cards can
be bent at a 90.degree. angle, positioned, then cut to length, but
subtler shaping is extremely difficult.
[0015] Before the present invention, it was not possible to plate a
coating on a substrate of small (tens to hundreds of microns) or
even large (millimeters, centimeters, or larger) minimum feature
size to provide resilient characteristics, particularly where it
was desirable to have a structure with good mechanical yield
strength. This limitation was particularly troublesome when the
device was intended for use at moderately elevated temperatures,
temperatures in excess of 100.degree. C., 85.degree. or even
50.degree. C. Conventional coatings could not be used to create
durable, strong spring structures due to the thermal instability of
the resulting coated products. The lack of structures with useful
mechanical properties made it extremely difficult to build devices
with large numbers of small springs, devices such as a probe
card.
[0016] Early work in formable microstructures showed that a soft
material such as gold could be shaped readily, then plated to give
a hard coating and a resilient structure. See U.S. Pat. No.
5,476,211, issued Dec. 19, 1995, assigned to FormFactor, Inc,
entitled "Method of Manufacturing Electrical Contacts, Using a
Sacrificial Member." The work that lead to the present invention
showed that use of improved materials, and subsequent heat
treatment could provide a strong, resilient final product. The use
of these same or similar materials together with appropriate heat
treatment can provide a resilient structure in a wide variety of
applications.
[0017] Plating techniques in general are well known. See, for
example, U.S. Pat. No. 4,439,284, "Composition Control of
Electrodeposited Nickel-Cobalt Alloys." However, the selection of
plating materials and the heat treatment conditions disclosed
herein have not been disclosed in the past, in the '284 patent or
elsewhere.
[0018] One skilled in the art will recognize other applications in
which a material with high yield strength would be beneficial. This
is particularly true for base materials with an arbitrary and
possibly complex shape where retention of that shape is important,
or where the base material does not have a sufficiently high yield
strength. In particular, when making various spring structures of
equivalent geometry and scale, an increase in elastic modulus will
increase the spring value proportionally. For a fine-pitch
interconnect, achieving greater spring value in a fixed volume is
beneficial.
SUMMARY OF THE INVENTION
[0019] The new invention solves the traditional problem of older
materials that do not provide stable mechanical properties under
load, particularly at elevated temperatures. The general technique
allows fabrication of coatings on parts, or stand-alone coatings,
with improved mechanical properties, even at elevated operating
temperatures. In particular, the method gives products with a
relatively high yield strength, relatively high elastic modulus and
improved temperature stability, that is, resistance to deformation
under load at elevated temperatures. This technique involves
depositing a coating in the presence of additives, and then
subjecting the coating to a moderate heat treatment. This moderate
heat treatment differs from other commonly employed "stress relief"
heat treatments in using a combination of significantly less
temperature and/or times. Traditional annealing heat treatment
warms the material to a temperature and for a time to relieve
essentially all stress after a subsequent cooling. The new
technique takes the as-coated material to a more moderate
temperature and/or for a shorter time, preferably just enough to
reorganize the material to the new, desired form. Mechanical
properties improve after the treatment. Yield stress actually
increases after the new heat treatment, while the yield stress
generally decreases after a typical stress relief heat treatment.
See, generally, R. J. Walter, Plating & Surface Finishing,
October 1986, pp. 48-53; A. J. Dill, Plating, November 1974, pp.
1001-1004; and A. W. Thompson and H. J. Saxton, Metallurgical
Transactions, Vol. 4, June 1973, pp. 1599-1605.
[0020] Many materials are suitable for use with the new technique
but one particularly preferred system includes nickel or a
nickel/cobalt alloy, with small amounts of a sulfur-containing
additive such as saccharin. This material is electroplated onto a
substrate, then heat treated under moderate conditions.
[0021] A preferred method of deposition is electroplating, but
other useful deposition processes include chemical vapor deposition
(CVD), physical vapor deposition (PVD), electrolytic or electroless
aqueous solution plating of metals, and any process that causes
deposition of materials through decomposition or reaction of
gaseous, liquid or solid precursors.
[0022] The new technique of fabricating coatings with stable
mechanical properties lends itself to fabricating resilient
structures, structures that are critical in many applications such
as microelectronic interconnections. In addition to the stable
mechanical properties, many of the new coatings provide a structure
with high electrical conductivity. Thus, by using this invention, a
low-inductance, resilient interconnect can be fabricated.
[0023] In one particularly preferred embodiment, the coating is
applied to a microspring, such as that described in U.S. Pat. No.
5,476,211, entitled "Method of Manufacturing Electrical Contacts,
Using a Sacrificial Member". This technique has been developed in
connection with work on such microsprings, particularly for coating
wires of some 1-2 mils (about 25-50 microns) thickness with a
coating of some 1-2 mils (25-50 microns). In general, the technique
is useful in thin film coatings of 200 or more Angstroms thick and
for coatings with thick films of millimeter or even centimeter
dimensions. One skilled in the art will appreciate that the
teachings of this invention are applicable to a wide variety of
structures.
[0024] This and other objects and advantages of the invention, as
well as the details of an illustrative embodiment, will be more
fully understood from the following specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 illustrates a flow chart of a method of manufacturing
a coated wire according to a preferred embodiment of this
invention.
[0026] FIG. 2A illustrates a cross-sectional view of a skeleton
attached to a base.
[0027] FIG. 2B illustrates a cross-sectional view of an as-coated
wire comprising a non-heat-treated coating covering the
skeleton.
[0028] FIG. 2C illustrates a cross-sectional view of a heat-treated
coated wire comprising a heat-treated coating covering the skeleton
according to a preferred embodiment of this invention.
[0029] FIG. 3A illustrates a cross-sectional view of an alternate
embodiment of an alternate plating substrate, here a spring contact
element residing on a sacrificial substrate.
[0030] FIG. 3B illustrates a perspective view of the spring contact
element of FIG. 3A, omitting a showing of the sacrificial
substrate.
[0031] FIG. 3C illustrates a cross-sectional view of another
embodiment of a spring contact element mounted to another
component.
[0032] FIG. 4 illustrates a graph of a differential scanning
calorimetry measurement of coated material.
[0033] FIG. 5A illustrates a graph of an x-ray diffraction pattern
from a sample of as-coated Ni--Co coating material.
[0034] FIG. 5B illustrates a graph of an x-ray diffraction pattern
from a sample of heat-treated Ni--Co coating material.
[0035] FIG. 6 illustrates a graph of stress versus strain data for
an as-coated wire and a heat-treated wire.
[0036] FIG. 7 illustrates a graph of elastic modulus versus
saccharin concentration for as-coated and heat-treated wires.
[0037] FIG. 8 illustrates a graph of wire curvature before and
after heat treatments at various times and temperatures
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] This invention may start with an uncoated part of
practically any shape. Generally speaking, the materials of this
invention can be coated onto any suitable base, then heat treated
under moderate conditions as described to give a useful product.
The base may or may not be retained, depending on the specific
application. The deposition of coated material organizes into a
desirable structure very early in the coating process, probably
within the first few hundred Angstroms of material deposited, and
coatings millimeters or even centimeters thick exhibit properties
that, when heat treated, provide materials with the improved
mechanical properties discussed herein.
[0039] When compared to other potential states for the material
under study, specific improvements in mechanical properties of the
heat treated material may include increased (preferably
approximately maximal) yield strength, increased (preferably
approximately maximal) elastic modulus, and improved temperature
stability. Other potential states include the material as coated
(before heat treatment) and the material after extensive annealing
(e.g. for stress relief).
[0040] One skilled in the art can follow the teachings of this
invention to selectively control material properties. For example,
the heat treatment can be selected to give minimal ductility, or
can be selected to give selectively more ductility. In like manner,
one can select heat treatment to give less than the maximal yield
strength, or less than the maximal elastic modulus. One might, for
example, wish to balance various material properties or accommodate
other process constraints but still follow the teachings of this
invention to make a material with properties improved over those of
the material as coated without heat treatment.
[0041] One particularly preferred shape for a base is an elongate
member such as a skeleton or a falsework. One preferred skeleton is
a wire, which in turn may be supported or secured to a substrate or
base. Another preferred skeleton is a beam that is secured to but
oriented to stand off from a substrate or base. It is particularly
preferable to use an elongate member that can be formed into a
spring shape. The elongate member does not need to be resilient,
and in fact can be quite flexible to facilitate shaping into an
arbitrary or desired form.
[0042] The basic material of this invention also can be deposited
in other ways. For example, a desired shape can be defined on a
substrate such as silicon wafer by application of various materials
as is well known in the art. Such materials can include photoresist
for some applications. A shape can be defined by patterning,
etching, and the like, then prepared for electroplating by
depositing a thin seed layer, for instance a layer containing
copper metal. The basic material can be plated onto that seed layer
and heat treated as described below. Some or even all of the
underlying materials may be susceptible to selective removal,
leaving the deposited material of this invention partially or
completely free of the original substrate. The selective removal
may be effected before or after the heat treating step, as
desired.
[0043] FIG. 1 shows a flow chart of a method of manufacturing a
coated wire according to one preferred embodiment of this
invention.
[0044] First, a microscopic wire skeleton 204 (see FIG. 2A) is
formed 102 on a base 202 (see FIG. 2A). For example, the wire
skeleton 204 may be made of gold wire, and the formation 102 may be
done using a wirebonding machine. The base 202, for example, may
comprise a semiconductor substrate. Many other bases 202 are, of
course, possible, such as one comprising a ceramic, plastic, or
metal substrate.
[0045] Second, a coating 206 (see FIG. 2B) is deposited 104 onto
the wire skeleton 204. The coating 206, for example, may be an
approximately 50-50 (atomic percentage) nickel-cobalt (Ni--Co)
alloy deposited in a plating bath that includes saccharin as an
additive. The concentration of saccharin in the plating bath is
discussed below in reference to FIG. 8. The bath may be changed to
vary the Ni/Co ratio, or to include a different additive instead of
saccharin. Many variations of the plating bath will work.
[0046] In general, coatings based on nickel, cobalt or iron (Ni,
Co, Fe) are expected to give generally similar results. Preferred
alloys include Ni--Co, Co--Mn, Ni--Mn, and various ternary alloys
such as Ni--Co--Mn. Alternative coating materials include Ni--W--B,
and Rh. Other possible coatings include Pd, Pd--Au, Pd--Co, W,
W--Co, Ti--N, Cu, Cr, Ti, Ti--W, Al, Au, and Pt. Alternative
additives include napthalene-tri-sulfonic acid (NTSA),
2-butyne-1,4-diol, and thiourea. Other possible additives include
NiCl, NiBr, as well as general Class 1 and Class 2 brighteners. All
of these coating materials and additives are well known in the
plating art.
[0047] Third, the as-coated wire 208 (see FIG. 2B) is submitted to
heat treatment 106 at a temperature above a transformation
temperature of the coating 206 so that a heat-treated coated wire
212 (see FIG. 2C) is formed. The heat treatment 106 of a Ni--Co
alloy, for example, may be done at 350.degree. C. for ten minutes
or at 300.degree. C. for sixty minutes. The time and temperature
ranges at which the heat treatment 106 should occur (i.e., the heat
treatment window) are described below in reference to FIG. 8. Of
course for different alloy additive systems, different heat
treatment schedules may be required.
[0048] FIG. 2A is a cross-sectional view of a wire skeleton 204
attached to a base 202. The skeleton 204 may be formed to be in
various shapes, such as the one shown, in order to make useful
resilient parts. One useful shape is that of a spring, or a
springable shape. Different shapes will be useful depending on the
application intended for the resilient part. Alternatively, the
skeleton 204 may be a straight wire.
[0049] FIG. 2B is a cross-sectional view of an as-coated wire 208
comprising a coating 206 over the wire 204. The coating 206 is of a
thickness sufficient, in relation to the thickness of the skeleton
204, to significantly impact the mechanical properties of the
as-coated wire 208. As described below in reference to FIG. 5A, the
as-coated coating 206 has an amorphous or nanocrystalline atomic
configuration.
[0050] FIG. 2C is a cross-sectional view of a heat-treated, coated
wire 212 comprising a heat-treated coating 210 over the wire
skeleton 204. The heat-treated, coated wire 212 is formed by
heating 106 the as-coated wire 208 at a temperature greater than a
transformation temperature of the non-heat-treated coating 206,
preferably for a relatively short time. As described below in
reference to FIG. 5B, the heat-treated coating 210 has a
crystalline or ordered atomic configuration. Furthermore, as
demonstrated by the test results described below in reference to
FIG. 8, the heat-treated, coated wire 212 is resilient and, after
being subject to simulated operating conditions, maintains its
resiliency much better than the as-coated wire 208.
[0051] A representative wire may have a diameter of 1 to 1.5 mils
(25 to 38 microns), with a coating thickness on the order of 1.5
mils (38 microns) for a total diameter of about 4 mils (100
microns). Coating thicknesses as small as 200 to 500 Angstroms show
the properties discussed here. Coatings can be relatively thick,
certainly millimeters and probably on the order of centimeters or
greater and still show the properties discussed here.
[0052] A useful coating can be deposited using electroplating. A
typical plating bath and methodology is as follows. This
illustrative, preferred implementation provides an alloy with
improved hardness and mechanical properties with approximately a
minimum of codeposited sulfur. A preferred grain refiner is sodium
benzosulfimide (C.sub.7H.sub.5NO.sub.3S- ), also known as sodium
saccharin. This and other grain refiners are well known to one
skilled in the art. Although a sulfur containing material is used
in this preferred embodiment, this does not appear to be an
absolute requirement. For example, 2-butyne-1,4diol has been shown
to be effective in practicing the present invention. The additive
should promote formation of the desired coating structure, as
discussed below.
[0053] Electroplating is well understood by one skilled in the art.
When the conditions of the outlined electrolyte composition,
electrodes, associated current densities, deposit thickness, and
specified apparatus are employed, there is no evidence of Ni
sulfide precipitation in grain boundaries at the heat treatment
temperatures suggested in this disclosure. This absence of
precipitation promotes prevention of grain boundary formation and
embrittlement, which in turn can lead to premature product
failure.
[0054] Successful product performance follows from high yield
strength accompanied by suitable ductility. Experience has shown
that "banded" (or lamellar) coating structures lead to favorable
product performance. The addition of grain refining additives; such
as napthalene-tri-sulfonic acid (NTSA), NDSA, para-toluene
sulfonamide or (preferably) sodium saccharin, will produce this
"banding" to further enhance yield strength and desired spring back
characteristics in the product. These additives in general should
not alloy with but should co-deposit with the primary metal or
metals being deposited. Thin deposits of certain alloys, including
the codeposition of cobalt with nickel, may not show visible
"banding" but nevertheless provide significant yield strength.
[0055] Alloy Deposition: Although the following solution make-up,
control and operating conditions are specific to this preferred
embodiment, one skilled in the art of alloy electroplating can
produce comparable electrodeposits with superior properties for
other applications. Recognized factors which influence the
composition of an alloy in electrodeposition include electrolyte
Ni/Co ratio in solution; current density, electrolyte agitation,
pH, temperature; boric acid and total metal concentration.
1 Preferred Deposit Properties: As Plated (not heat treated)
Composition: 60% .+-. 2% Nickel by wt., 40% .+-. 2% Cobalt
Hardness: .about.550 Knoop Ultimate Strength: .sigma.u265 min ksi
(ksi = thousand psi) Yield Strength: .sigma.y 160 min ksi Modulus:
E22 .+-. 2(Msi) (min. value) (Msi = million psi) Elongation: 4.5%
.+-. .5%
[0056] Deposit Appearance: The deposit was smooth and continuous,
with high spectral reflectance. Conventional Hull Cell Panel tests
showed this appearance over a wide range of plating current
density.
[0057] Typical Solution Make-Up--One preferred implementation of
this invention to produce a product with high resiliency is shown
in Table 1 below. Plating is carried out in a conventional plating
tank, such as a laminar flow polypropylene tank. Plating times and
conditions are as needed to give satisfactory coatings.
2TABLE 1 TYPICAL MAKE-UP CHEMICAL TYPICAL MAKE-UP 1) Ni Sulfamate
1) 100 gm/L 2) Boric Acid 2) 38 gm/L 3) Ni Bromide 3) 3-5 ml/L
(@18% conc) 4) Co Sulfamate 4) 8.3 gm/L (8.3 gm Co) 5) Sodium
Lauryl Sulfate 5) 25 dynes/cm 6) Na Saccharin 6) 100 mg/L
[0058] In a typical process of this invention, these materials can
be replaced in whole or in part by the alternative materials in
Table 2, below.
3TABLE 2 ALTERNATIVE MATERIALS OR COMBINATIONS TYPICAL ALTERNATIVE
CHEMICALS 1) Ni Sulfamate 1a) Ni Sulfate 1b) Ni Acetate 1c) Ni
Fluoborate 1d) Ni Chloride 1e) Ni Sulfate-Chloride 1f) Ni
Pyrophosphate 2) Boric Acid 2) Citric Acid 3) Ni Bromide 3a) Ni
Chloride 3b) Magnesium Chloride 4) Co Sulfamate 4a) Co Sulfate 4b)
Co Chloride 4c) Co Fluoborate 5) Wetting Agent 5) A commercial
wetter designed for plating applications 6) Class 1 Brighteners 6a)
Para-toluene sulfonamide 6b) Sodium naphthalene trisulfonic acid
6c) Naphthalene disulfonic acid or a combination of 6a-6c above. 7)
Leveling Agents 7a) Coumarin 7b) Quinoline 7c) 2-Butyne-1,4-diol or
a combination of Class 1 brighteners and Leveling agent.
[0059] The coating material can be deposited on a wire which has
been shaped in the form of a spring. After appropriate heating, the
heat-treated spring substantially maintained its resiliency after
being subject to simulated operating conditions. Such a
characteristic is very desirable for springs, especially for those
used in sockets, interposers, probes, on semiconductors, or in
semiconductor packaging, whenever the maintenance of resiliency is
important to maintaining pressure contacts. This same
characteristic is also beneficial for purely mechanical components
of any shape or geometry subject to loads at elevated temperatures
for extended periods.
[0060] The preferred embodiment described herein is based on a
coating over a wire, which may or may not be shaped to a desired
form. The teachings of this invention are useful for coating other
shapes as well. In particular, the object to be coated may be
removed from the new coating before or after the heat treatment
step. See, for example, co-pending U.S. patent application Ser. No.
08/802,054 entitled "Microelectronic Contact Structure and Method
of Making Same," commonly assigned with the present invention to
FormFactor, Inc., now of Livermore, Calif. This and the
corresponding PCT patent application, Serial Number 97/08271, filed
May 15, 1997, disclose forming a structure on a substrate, then
removing the structure from the substrate to yield a free-standing
part. See, for example, FIGS. 3A, 3B and 3C (FIGS. 3A, 3B and 4E,
respectively, of the reference applications), which show a
structure being formed on a substrate (FIG. 3A), that same
structure without the substrate (FIG. 3B), and a structure attached
to a different substrate (FIG. 3C). Using the teachings of the
present invention, one skilled in the art can coat the structures
of FIGS. 3A, 3B and 3C at an appropriate time to prepare a coating
as taught in the present disclosure. For example, spring contact
element 460 (FIG. 3C) might be joined to electronic component 470
then coated and heat treated. Alternatively, contact structure 300
(FIGS. 3A, 3B) might be coated and heat treated before removing
sacrificial substrate 252.
[0061] FIGS. 3A and 3B illustrate another one of many possible
embodiments for a contact structure 300 fabricated by the
techniques described in the referenced applications. Sacrificial
substrate 252 is used for preparing contact structure 300. A
somewhat truncated-pyramidal joining feature (stud) 310 is
fabricated as an attachment feature at the base portion 302 of the
contact structure 300. The remaining portions of the contact
structure 300 are a central main body portion 306, a contact end
portion 304, and a feature 308, here a contact point. W1 and W2 are
widths at the respective ends of the contact structure.
[0062] FIG. 3C illustrates an alternate embodiment of the described
invention wherein a spring contact element 460 is mounted to an
electronic component 470 via a stud 472 extending from a surface of
the electronic component 470. The base end 462 of the spring
contact element 460 is suitably brazed to the stud 472.
[0063] In addition, one skilled in the art-could select materials
suitable as a substrate for coating, apply the teachings of this
invention to apply such a coating, and subsequently remove the
substrate to leave a product formed of the coating itself. In a
particularly preferred embodiment, a contact structure such as 300
can be built up on a suitable substrate such that the coating forms
the bulk of the material of contact structure 300. An alternative
substrate might be a conductive layer over a material of a desired
shape which might in its own right be readily removable. Such an
implementation might start with a shape in wax, photoresist, or
other material, then apply a thin conductive layer to promote
plating, plate a coating as disclosed in this invention, then heat
treat to give a product with the desired shape and properties.
[0064] FIG. 4 is a graph of a differential scanning calorimetry
measurement on as-coated coating 206. The measurement starts with
material as coated initially, without heating, and increases its
temperature by 10.degree. C. per minute from room temperature
(about 30.degree. C.) to 500.degree. C. while measuring heat
flow.
[0065] The (inverse) peak 602 in the graph centered at about
266.degree. C. indicates that an exothermic transformation occurs
in the material, particularly when the temperature is in the range
indicated by the width of the peak 602. This exothermic
transformation is deduced to be the transformation that changes the
microstructure and atomic configuration from that of the as-coated
coating 206 to that of the heat-treated coating 210. Since the peak
602 appears to begin above roughly 200.degree. C., a heat treatment
106 above about 200.degree. C. should be able to cause such a
transformation when Ni--Co alloy is used as the coating material.
However, it is generally preferred to select a temperature near or
somewhat above the peak temperature. In the illustrated example,
the peak occurs at about 266.degree. C. and useful, preferred heat
treatment conditions included 300.degree. C. for 60 minutes or
350.degree. C. for ten minutes. In general, one skilled in the art
will recognize that a range of heating temperatures will give the
desired effect. In general, a useful temperature range is within 0
and 150.degree. C. above the peak transition temperature and a
particularly useful temperature range is within 0 and 100.degree.
C. above the peak transition temperature.
[0066] A product that has been only partially transformed may be
useful as well. One skilled in the art can follow the teachings of
this invention to obtain a desired amount of transformation. In
general, useful increases in material properties are seen if a
substantial portion of the as-coated material undergoes a
transformation to a more ordered state. One particularly preferred
implementation involves heat treating a coated material at
300.degree. C. for fifteen minutes.
[0067] FIG. 5A is a graph of an x-ray diffraction pattern from a
sample of as-coated coating 206. The significant breadth of the two
x-ray diffraction peaks shown in FIG. 5 indicates a relatively
small average grain size of about 16 nanometers in the material.
(The average grain size was determined using the Debye-Scherrer
formula which is well known in the art and gives a lower limit for
the average grain size.) With an average grain size of 16
nanometers, the as-coated coating 206 may be characterized as a
nanocrystalline or an amorphous material.
[0068] FIG. 5B is a graph of an x-ray diffraction pattern from a
sample of heat-treated coating 210 according to a preferred
embodiment of this invention. In this case, the heat treatment 106
applied was 10 minutes at 330.degree. C. The narrowness of the two
peaks shown in FIG. 5B indicates a relatively large average grain
size of about 78 nanometers in the material. (Again, the
Debye-Scherrer formula was used.) With an average grain size of 78
nanometers, the heat-treated coating 210 may be characterized as a
crystalline or an ordered material.
[0069] Thus, as shown by FIGS. 5A and 5B, the coating material
undergoes a transformation during heat treatment 106 from
nanocrystalline or amorphous to crystalline or ordered.
[0070] FIG. 6 is a graph of stress versus elongation (strain) data
for an as-coated wire 208 and a heat-treated wire 212. These wires
208 and 212 tested here are straight, instead of being shaped in
the form of a spring, so that the elongation (strain) measurement
has meaning as a mechanical property of the material unrelated to
the shape of the wires 208 and 212.
[0071] The data in FIG. 6 shows that the heat-treated wire 212 has
superior mechanical properties compared to the as-coated wire 208.
The heat-treated curve 402 shows a higher yield stress (defined as
the stress for a 0.2% strain) and higher elastic modulus compared
to the as-coated curve 404. Thus the heat treated wire is elastic
over a much larger range of applied stress. This indicates a higher
resiliency of the heat treated springs, and likely is due to a
fundamental materials improvement. Since most mechanical components
are designed to operate in the elastic regime (i.e. maintain their
shape), a more resilient material is more mechanically stable.
Resilient material in the heat treated springs dramatically
improves the stability of the mechanical properties at elevated
temperatures under load.
[0072] When compared to other potential states for the material
under study, specific improvements in mechanical properties of the
heat treated material may include increased (preferably
approximately maximal) yield strength, increased (preferably
approximately maximal) elastic modulus, and improved temperature
stability. Other potential states include the material as coated
(before heat treatment) and the material after extensive annealing
(e.g. for stress relief). The improved temperature stability is
evidenced by increased resistance to deformation under load at
elevated temperatures where the temperature may be somewhat above
25.degree. C., including 85-100.degree. C. and even 300.degree. C.
or more. This shows that the material is in a state more stable
than the original material as coated.
[0073] FIG. 7 is a graph of elastic modulus versus saccharin
concentration for as-coated ("virgin" or "NHT" in FIG. 7) wires 208
and heat-treated ("heat treated" or "HT" in FIG. 7) wires 212. The
graph shows that the elastic modulus for the heat-treated wires 212
is substantially greater than the elastic modulus for the as-coated
wires 208 for all non-zero (in particular, 20 mg/L or more)
saccharin concentrations tested. When saccharin was used as an
additive in the plating bath according to the formulation of Table
1, the heat-treated wires 212 typically had an elastic modulus of
about 32 Msi while the as-coated wires 208 typically had an elastic
modulus of about 24 Msi. Msi represents "mega" or million psi.
[0074] FIG. 8 is a graph of the wire curvature under 0.5 inch
Mandrel tests on various wires coated with Ni--Co from a bath
including saccharin. The Mandrel test consists of wrapping a
straight plated wire around a mandrel of a given diameter to
produce a fixed strain. (The mandrel material is selected to have
the same thermal expansion coefficient as the wire to avoid
additional stresses induced by differences in thermal expansion
coefficient.) The wire is then attached to the mandrel at both ends
to maintain the strain. The mandrel-wire assembly can then be
exposed to any time/temperature combination. This test is a
convenient way to test material properties under load at elevated
temperature as a function of time, an extremely useful means of
mimicking different operating conditions. The amount of plastic
deformation that occurs for a given test condition is reflected by
changes in the resulting wire curvature, where curvature is defined
as: 1 curvature = 100 * [ radius outer - edge - of - wire radius
wire - center - 1 ]
[0075] FIG. 8 shows the effect of different heat treatment
temperatures on wire curvature. A set of as-plated wires had their
curvatures measured at room temperature. They were then subjected
to a 7 minute heat treatment at different temperatures, cooled to
room temperature, and again measured for curvature. They were
subjected to a two minute Mandrel test at room temperature, and
again measured for curvature. They were then subjected to a 13 hour
Mandrel test at 85.degree. C., cooled to room temperature, and
again measured for curvature. Finally, they were subjected to an
additional 24 hour Mandrel test at 85.degree. C. (for a total
duration of 37 hours at 85.degree. C.), cooled to room temperature,
and measured for curvature. As seen in FIG. 8, the plastic
deformation associated with mechanical loading for two minutes at
room temperature are nominally the same for the wires heat treated
at various temperatures. However, when mechanically loaded at
elevated temperatures, the wires heat treated at higher
temperatures (.ltoreq. 400.degree. C.) show smaller plastic
deformation. This is another indication of the improved mechanical
properties after heat treatment. The mechanical properties are
essentially constant for coated wires heat treated at T.gtoreq.
350.degree. C., consistent with the results of FIG. 4 (see
corresponding text, above).
[0076] The description above has discussed in detail certain Ni and
Ni/Co systems with a variety of potential additives, particularly
saccharin. However, the general principles can be used to plate a
wide variety of systems, with a variety of additives, then heat
treated under moderate conditions to give a product with the
desired properties.
[0077] Potential metal systems are listed in detail above.
Additives used to date for the most part include sulfur, but
2-butyne-1,4-diol has been used to give the desired transformation.
In one preferred embodiment, a bath concentration of more than or
about 5 mg/L 2-butyne-1,4-diol was useful. Although the physical
properties are not fully understood, the general theory is that the
material as deposited has a nonequilibrium, nanocrystalline
structure. The additive is present in relatively low concentration
and is dispersed throughout the coating as deposited. If the coated
material is heated for some time, the crystal structure reorganizes
to give larger crystals. The additive diffuses and primary metal
organizes in a way that is affected by the additive molecules,
perhaps as a crystal directly incorporating the additive, perhaps
as grains that accommodate the additive molecules, or perhaps in
some as-yet-not-understood structure. Further heat treatment leads
to a different organization where the base material organizes into
large structures, excluding the additive and/or the additive
collects as a precipitate, segregated from the primary metal. This
is the structure that results after traditional annealing (which
also generally does not include the additives of the present
invention).
[0078] To ascertain a useful amount of additive and useful heat
treatment conditions is not difficult. While predictions can be
made by studying the diffusion rate of a selected additive in the
base metal system, a few experiments will outline the primary
parameters very quickly. The temperature range where the
intermediate heat treatment is most likely to occur can be selected
by coating the desired metal system onto a suitable substrate, then
performing differential scanning calorimetry (DSC) as described
above. The transition temperature is readily identifiable, and the
peak of that transition temperature is a good starting point for
subsequent experiments. For this initial experiment, the amount of
additive is not very important as the additive has little or no
effect on this transition temperature.
[0079] A useful amount of additive can be identified by preparing a
test product using varying amounts of additive, for example, 2.5%,
1%, 0.1% and 0.01% on a molar basis, then heat treatment for a
brief time, for example 5, 10 or 20 minutes, at or near the
temperature identified above. The tensile strength of the resulting
heat treated product will show which conditions give the desired
mechanical properties. A particularly useful test structure is a
traditional dog-bone suitable for testing in a traditional tensile
strength tester. Alternatively, a coated wire will provide useful
tensile information. With initial information about effective
amounts of additive, different time and temperature conditions can
be evaluated to quickly settle on a useful set of conditions.
[0080] For a given amount of additive, upon heat treatment in
varying amounts, the yield stress generally will increase to a
maximum, but then decrease. In general, the maximum yield stress
will be found at a relatively narrow band of heat treatment
conditions (balancing time and temperature). This point may not
give the desired ductility properties. In general, heat treatment
beyond the point of this maximum yield stress condition will
increase ductility, and a modest increase in heat treatment will
give a part which has close to the maximal yield stress together
with a desired amount of ductility. Continued heat treatment
ultimately will decrease the yield stress, generally decreasing the
resiliency of the treated coating. This continued heat treatment in
general will increase ductility. It is well within the skill in the
art to identity heat treatment conditions to impart a desired
degree of yield stress and a desired degree of ductility in a given
coating system, subject to the limitations of that coating
system.
[0081] One skilled in the art can use the principles of
experimental design to identify key components and values with
relative ease. This field has been the subject of considerable
academic interest. For example, the library at the University of
California, Berkeley, listed some 287 references for experimental
design in the electronic card catalog in September 1997. See
www.lib.berkeley.edu, or, particularly
www.lib.berkeley.edu/ENGI/about.html. In particular, the study of
factorial experimental designs or fractional factorials may be
useful. The Berkeley collection lists some fourteen relevant
references. Of particular interest might be the basic references
"Statistics For Experimenters, An Introduction to Design, Data
Analysis, and Model Building," George E. P. Box, Wiley, N.Y. (1978)
and "Empirical Model-Building and Response Surfaces," George E. P.
Box and Norman R. Draper, Wiley, N.Y. (1987).
[0082] A general description of the device and method of using the
present invention as well as a preferred embodiment of the present
invention has been set forth above. One skilled in the art will
recognize and be able to practice many changes in many aspects of
the device and method described above, including variations which
fall within the teachings of this invention. The spirit and scope
of the invention should be limited only as set forth in the claims
that follow.
* * * * *
References