Note: Descriptions are shown in the official language in which they were submitted.
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Tin Alloy Solder Coynpositions
Cross-Refes ence to Related Application
[0001] This application claims priority to US Provisional Patent Application
No.
60/679,869, filed on May 11, 2005.
Field of the In.vention
[0002] The invention relates to a lead-free and bismuth-free tin alloy that
contains
antimony and nickel or cobalt.
Backgs=ound Of Th.e Invention
[0003] The present invention relates generally to an improved solder
composition.
More specifically, the present invention relates to an improved solder
composition that
contains no lead or bismuth yet still achieves superior soldering
characteristics.
[0004] In the electronic manufacturing of printed circuit boards and the
assembly of
components thereon, the solders employed generally contain tin and lead to
provide
mechanical and electrical connections. Solders that contain tin and lead
typically yield highly
reliable connections in both automated and manual soldering and provide a
surface on printed
circuit boards extremely conducive to soldering.
[0005] Tin-lead alloys of, for example, sixty (60%) percent tin, forty (40%)
percent
lead; and sixty-three (63%) percent tin, thirty-seven (37%) percent lead have
historically been
used for most electronic soldering operations. These alloys have been selected
and are
preferred because of their low melting temperatures, mechanical strength, low
relative cost,
as well as superior wetting characteristics and electrical conductivity. '
[0006] However, the use of such tin-lead solders in the manufacture of printed
circuit
boards and assembly of components is becoming more and more problematic due to
the toxic
effects of lead exposure to worlcers and the inevitable generation of
hazardous waste. For
example, even small amounts of lead can affect the neurological development of
fetuses in
pregnant workers. Due to these enviromnental concerns, action is being talcen
to limit the
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amount of lead entering into the environment. Federal and many state
government agencies
have begun to urge the electronics industry to find alternatives to tin-lead
solders to reduce
worker lead exposure and lessen the amount of lead waste going back into the
environment.
[0007] Due td the materials used, many components and printed circuit boards
are
easily damaged by exposure to high temperatures during manufacture or
assembly. Because
of heat transfer and distribution limitations and concerns, printed circuit
boards are typically
exposed to temperatures higher than the liquidus teinperature of the alloy
employed. In
response to this concern, electronic manufacturers are exploring alternative
alloys to replace
the tin-lead alloys.
[0008] The prior art has not provided a solder composition exhibiting optimum
wetting and flow properties without toxicity. Currently federal, military and
commercial
solder specifications lack a suitable non-toxic composition. The following
prior art patents
illustrate inadequate attempts to meet these needs.
[0009] Soviet Union Patent No. 183,037, issued to A. I. Gubin et al. discloses
an
alloy containing antimony of 1 0.3%; copper 2 0.3%; silver 5* 0.3% and the
remainder
being tin and having a melting point of 225 -250 C. This alloy has a liquidus
temperature
that does not allow it to be used in electronic soldering because the
soldering temperature
required to flow the alloy would destroy the printed circuit board and many of
the
components. No feasible equipment or means currently exists to allow this
alloy to be used
for the purpose of electronic soldering or coating. Due to the high silver
content, this alloy
has an economic disadvantage in the marketplace.
[0010] U.S. Patent No. 3,503,721, issued to Lupfer, discloses a tin-silver
alloy of
96.5% tin and 3.5 0.5% silver with wetting and electrical conductivity
characteristics
marginally acceptable to suit the needs of the electronics industry. However,
this alloy has
mechanical strength weaknesses that would prohibit its use on a wide range of
electronic
printed circuit board assemblies. For example, creep strength, a measure of
flow under
pressure, and percent elongation, metal stretching before fracture, are
considerably lower than
that of the tin-lead alloys now used. Even with the common tin-lead alloys,
solder joints
stress fractures are the cause of many field failures in printed circuit
boards where vibration
or temperature variations occur. In addition, the liquidus temperature of 221
C requires that
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automated soldering be accomplished at a temperature that in many situations
would damage
the printed circuit board and/or the components. Due to the high content of
silver, the cost of
this alloy is considerably higher than tin-lead alloys. For each percentage
point of silver
added to the alloy, the price increases by approximately $0.75/lbs. (based on
a silver market
of $5.00/troy ounce).
[0011] U.S. Patent No. 4,778,733, issued to Lubrano et al. discloses an alloy
containing, by weight, 0.7% to 6% copper; 0.05% to 3% silver; with the
remainder being tin
with a temperature range of 440 -630 F. This alloy has a melting temperature
that is too
high to be used in a wide range of electronic soldering applications without
damaging printed
circuit boards or components. In addition, the alloy disclosed by Lubrano et
al. exhibits
iiiferior soldering performance, slow wetting times and mechanical strengths
ill-suited to
electronic assembly applications.
[0012] U.S. Patent No. 4,695,428, issued to Ballentine et al. discloses an
alloy
containing 0.5-4% antimony; 0.5-4% zinc; 0.1-3% silver; 0.1-2% copper; 88-
98.8% tin. The
zinc content in this alloy causes the alloy to oxidize quickly. This inhibits
wetting and flow,
producing high dross formation which results in extremely high defect levels.
The
productivity lost in using such a composition for mass electronic soldering
makes it an
unacceptable alternative to tin-lead solders.
[0013] U.S. Patent No. 4,758,407, issued to Ballentine et al. discloses an
alloy
containing tin, copper, nickel, silver and antimony. All of the alloy
combinations disclosed
by Ballentine et al. have liquidus temperatures in excess of those required
for electronic
assembly. The lowest disclosed liquidus temperature is 238 C, which is
unacceptable for
use in the electronics industry.
[0014] The most commonly used lead-free alloy is comprised of tin-silver-
copper.
Industry testing has proven that tin-silver-copper, lead-free solder alloys do
not offer
sufficient drop testing characteristics as compared to tin-lead solder alloys,
especially on 0.3
mm BGA devices. Common tin-silver-copper alloys, known as SAC alloys, contain
3-4%
silver and 0.5-1% copper. The main problem with these alloys in a BGA type
application is
the AgSn intermetallic plate formation as well as Kirkendal voiding that
occurs. To make
SAC alloys more stable, several elements have been added to reduce copper
erosion as well
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as limit large intermetallic plates from forming. For example, P, Ge, rare
earth metals, Sb,
Ni, and Co have been tried. In addition, the solder alloys composed of tin-
silver-copper-
antimony described in US Patent Nos. 5,352,407 and 5,405,577, issued to Seelig
et al. show
improvement versus tin-silver-copper alloys. However, this alloy shows some
iinprovements
of tin, silver, copper alloys; however there is still a need for enhanced
performance.
[0015] Since heretofore no acceptable substitute for tin-lead alloys in BGA
applications have been found, there remains a need in the electronics industry
for an alloy
composition without lead or bismuth that can achieve the physical
characteristics and
application performance of tin-lead solder alloys but without the toxic
elements.
Summary Of The Ibzveutiou
[0016] The present invention provides solder alloys with new advantages not
found in
currently available solder compositions, and overcomes many of the
disadvantages of
currently available compositions.
[0017] The invention is generally directed to novel and unique solder
compositions
with particular application in the electronic manufacturing of printed circuit
boards and the
assembly of components therein, as well as lead less component bumping arrays
and column
arrays. The solder compositions of the present invention achieve desired
physical
characteristics, such as wetting, peel strength, low melting point, physical
strength, fatigue
resistance, electrical conductivity, matrix stability, and uniform joint
strength, but without the
toxic elements found in known tin-lead solder alloys.
[0018] The alloy compositions of the present invention include a combination
of tin,
silver, copper, antimony, and either nickel or copper, to offer a unique set
of physical
characteristics that allow it to be used as a viable alternative to tin-lead
alloys in electronic
soldering and printed circuit board coating, as well as lead less component
bumping arrays
and column arrays. The alloy of the present invention possesses physical
characteristics that
result in a stronger mechanical joint with superior fatigue resistance to tin-
lead alloys, tin-
silver alloys, or alloys containing bismuth. In addition, the melting point
temperature is
lower than any other lead-free or bismuth free alternative solder alloy.
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[0019] The preferred embodiment of the present invention has a reduced
toxicity and
a melting point of about 217 C and consisting of, in weight percent, 85-99%
tin; 0.01-4.5%
silver; 0.01-3.0% copper; and 0.002-5.0% antimony and either 0.0001-1.0%
nickel or 0.0001-
1.0% cobalt.
[0020] It is therefore an object of the present invention to provide solder
compositions
that are a viable substitute for tin-lead solder alloys.
[0021] Another object of the present invention is to provide solder alloy
coinpositions
that are well suited for the electronic manufacturing of printed circuit
boards and the
assembly of components thereon.
[0022] It is a further object of the present invention to provide solder alloy
compositions acceptable for the electronics industry that contain no lead or
bismuth.
[0023] It is yet a further object of the present invention to provide solder
alloy
compositions that are free of toxic elements and safe for the environment.
Detailed Description Of The Ifzvesztion
[0024] The preferred embodiments of the present invention are lead-free and
bismuth-
free solder coinpositions that contain tin, silver, copper, antimony, and
nickel or contain tin,
silver, copper, antiinony, and cobalt. The solder alloy compositions of the
present invention
have the physical characteristics and the application performance to
economically meet the
needs of the electronic industry and the assembly and coating of printed
circuit boards. In
particular, the alloy exhibits ideal physical characteristics yet does not
contain toxic elements
as alloys found in the prior art which could harm worlcers and the
environment.
[0025] The alloys of the present invention have advantages over the tin,
silver,
copper, antimony alloy described in the prior art. Below is an independent
comparison test
between a tin, silver, copper, antimony alloy, as described in US Patent No.
5,405,577 and a
prior art tin-lead solder alloy containing 63% tin and 37% lead. As seen
below, the
mechanical strength of this alloy is superior to known tin-lead alloys.
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63% Tin/37% Tin/Silver/Copper/Antimony Alloy
Lead
Tensile Categories
UTS(ksi) 4.92 5.73
YIELD STGTH.(ksi) 4.38 4.86
YOUNGS' MODULUS(msi) 52.8 42.40
Compression Categories
ELASTIC MOD.(msi) 3.99 4.26
YS(ksi) 4.52 4.33
STRESS 25% (ksi) 7.17 8.54
Hardness Category
ROCKWELL 15W 10.08 18.28
[0026] The alloy compositions of the present invention that exhibit the
desired
physical characteristics is comprised by weight as follows:
Metal % Composition
Tin (Sn) 85-99%
Silver (Ag) 0.01-4.5%
Copper (Cu) 0.01-3.0%
Antimony (Sb) 0.002-5.0%
Nickel (Ni) or Cobalt (Co) 0.0001-1%
[0027] In an embodiment, the solder composition comprises about 1.75% to about
2.0% silver; about 0.8% copper; about 0.5% antimony; about 0.08% nickel; and
about 96.6%
to about 96.9% tin. The melting point temperature of the composition is in the
range of about
217 C. The liquidus temperature of about 217 C coupled with superior wetting
allows the
alloy of the present invention to be used with existing mass and hand
soldering equipment
without damaging most printed circuit boards or electronic components. In
addition, this
alloy when tested in JEDEC drop tests of 1500g X 0.5 meters demonstrated twice
the fatigue
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life of the lcnown SAC alloys. By coinparison, an SAC alloy containing
antimony but
lacking nickel (e.g., having the coinposition about 1.75% to about 2.0%
silver; about 0.8%
copper; about 0.5% antimony; and about 96.6% to about 96.9% tin) demonstrated
only a 30%
increase in fatigue life.
[0028] In another embodiment, the solder composition comprises about 0.5% to
about
1.75% silver; about 0% to about 0.5% copper; about 0.002% to about 0.2%
antimony; about
0.08% to about 0.04% nickel; and about 97.5% to about 99.4% tin.
[0029] In another embodiment, the solder composition comprises about 1.0% to
about
1.75% silver; about 0.8% copper; about 1.0% antimony; about 0.008% cobalt; and
about
96.44% to about 97.2% tin. This alloy also showed an improved fatigue life
over SAC alloys
in JEDEC drop tests of 1500g X 0.5 meters.
[0030] In another embodiment, the solder composition comprises about 0.02% to
about 1.0% silver; about 0.2% to about 0.8% copper; about 0.2% to about 0.8%
antimony,
about 0.008% to about 0.4% cobalt, and about 97% to about 99.6% tin.
[0031] The present solder compositions may comprise about 85% to about 87%
tin;
about 87% to about 89% tin; about 89% to about 91% tin; about 91% to about 93%
tin; about
93% to about 95% tin; about 95% to about 97% tin; or about 97% to about 99%
tin, or a
combination of two or more of the above ranges (e.g., from about 95% to about
99% tin).
[0032] The present solder compositions may comprise about 0.01% to about 0.05%
silver; about 0.05% to about 0.1% silver; about 0.1% to about 0.5% silver;
about 0.5% to
about 1.0% silver; about 1.0% to about 2.0% silver; about 2.0% to about 3.0%
silver; about
3.0% to about 4.0% silver; or about 4.0% to about 4.5% silver, or a
combination of two or
more of the above ranges (e.g., from about 1.0% to about 3% silver).
[0033] The present solder compositions may comprise about 0.01% to about 0.05%
copper; about 0.05% to about 0.1% copper; about 0.1% to about 0.5% copper;
about 0.5% to
about 1.0% copper; about 1.0% to about 2.0% copper; or about 2.0% to about
3.0% copper,
or a combination of two or more of the above ranges (e.g., from about 0.1 % to
about 1%
copper).
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[0034] The present solder compositions may comprise about 0.002% to about
0.005%
antimony; about 0.005% to about 0.01 1o antimony; about 0.01% to about 0.05%
antimony;
about 0.05% to about 0.1% antimony; about 0.1% to about 0.5% antimony; or
about 0.5% to
about 1.0% antimony; about 1.0% to about 2.0% antimony; about 2.0% to about
5.0%
antimony; or a combination of two or more of the above ranges (e.g., from
about 0.1 % to
about 1% antimony).
[0035] The present solder compositions may comprise about 0.002% to about
0.005%
nickel; about 0.005% to about 0.01% nickel; about 0.01% to about 0.05% nickel;
about
0.05% to about 0.1% niclcel; about 0.1% to about 0.5% nickel; or about 0.5% to
about 1.0%
nickel, or a combination of two or more of the above ranges (e.g., from about
0.01% to about
0.1% nickel).
[0036] The present solder compositions may comprise about 0.002% to about
0.005%
cobalt; about 0.005% to about 0.01% cobalt; about 0.01% to about 0.05% cobalt;
about
0.05% to about 0.1% cobalt; about 0.1% to about 0.5% cobalt; or about 0.5% to
about 1.0%
cobalt, or a combination of two or more of the above ranges (e.g., from about
0.01% to about
0.1 % cobalt).
[0037] Not to be limited to any particular theory, the coinbination of
antimony with
nickel or cobalt may inhibit the SAC alloy from dissolving copper and forming
large
intermetallic platelets, thereby yielding a more stable matrix over time, and
providing better
stability and more uniform joint strength.
[0038] The alloys of the invention exhibit excellent wetting and melting
temperatures,
as well as superior physical strength, electrical conductivity, and
thermocycling fatigue, for
exainple. As a result of these excellent physical characteristics, the solder
alloy compositions
of the present invention may be successfully substituted for the known tin-
lead alloys
currently used for electronics assembly and printed circuit board manufacture,
as well as lead
less component buinping arrays and column arrays. Most capital equipment used
in
electronic soldering can enlploy these compositions. The low melting
temperature is low
enough not to cause heating damage to the board or components therein.
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[0039] The alloy compositions of the present invention are well suited for
many
different applications. The alloys may be employed in the coating of circuit
boards and
printed circuit board manufacture by use of "hot-air leveling" or "roll-
tinning". These
processes improve solderability on the circuit board. Also, the alloys may be
used in the
assembly of electronic components on printed circuit boards when using a
wavesoldering
machine. The alloys are also well suited for formation into various shapes and
sizes, such as
bars, ingots, wire, chips, ribbons, powder, preform and can be used with a
core of flux.
Therefore, the alloys of the present invention may be used for assembly of
electronic
components using solder wire and a heating device to hand solder the
components to the
board.
[0040] In the application of coating printed circuit boards, the compositions
of the
present invention have superior wetting characteristics and improved
productivity. Tin-lead
alloys of the prior art are easily contaminated by copper from the PC boards
that are dipped
into a bath during processing. Since the compositions of the present invention
contain
copper, minor increases in the copper content do not readily affect
performance of the
compositions. In addition, these new compositions will not absorb copper as
quickly as prior
art tin-lead solders. As a result, these new alloys can remain functional much
longer than
prior art tin-lead alloys to reduce overall solder consumption drastically and
reduce outlay by
manufacturers. Moreover, the solderability of the coated board is extended
because the
intermetallics are distributed evenly throughout the grain boundary of the
composition. The
result is a higher quality printed circuit board that cannot be achieved by
the use of prior art
solder compositions.
[0041] In surfacemount assembly or wavesoldering of components to printed
circuit
boards, the compositions of the present invention can employ the same hot
temperatures, pre-
heat temperatures, and process parameters as prior art tin-lead solders now
currently in use.
The nominal composition is very close to a eutectic alloy which exhibits
physical
characteristics important to high speed, low defect soldering. Since the
solder alloys of the
invention are less easily contaminated than tin-lead alloys, an increased
usable life of the
solder bath results. Further, solder joints formed by wavesoldering yield
higher joint
strengths and excellent electrical conductivity with even distribution of
intermetallics
througllout the solder joint.
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[0042] The solder alloy compositions of the present invention may also be used
in the
assembly of electronic coinponents using solder wire in a heating device to
hand solder the
components to the board. Such a method requires a composition that wets and
spreads
quickly at about 235 to about 260 C. The composition of the present
invention can be easily
formed into a cored wire solder and used easily and successfully in hand
soldering.
[0043] Overall, the alloy compositions of the present invention enjoy a
combination
of a sufficiently low melting temperature for electronic applications,
superior wetting
characteristics, and superior mechanical strength to make it an excellent
alternative to tin-lead
alloys for the needs of the electronic industry for manufacture of printed
circuit boards and
the assembly of components onto the boards. The superior solderability and
wetting
characteristics yield even pad thicknesses and low copper solubility to
provide a tremendous
advantage in the solder coating of printed circuit boards, such as by hot air
leveling.
Incorporation by Reference
[0044] The contents of all cited references (including literature references,
patents,
patent applications, and websites) that may be cited throughout this
application are hereby
expressly incorporated by reference. The practice of the present invention
will employ,
unless otherwise indicated, techniques for the production and use of alloys,
which are well
known in the art.
Equivalents
[0045] It will be appreciated by those skilled in the art that various changes
and
modifications can be made to the disclosed embodiments without departing from
the spirit or
essential characteristics thereof. All such modifications and changes are
intended to be
covered by the appended claims. The foregoing embodiments are therefore to be
considered
in all respects illustrative rather than limiting of the invention described
herein. Scope of the
invention is thus indicated by the appended claims rather than by the
foregoing description,
and all changes that come within the meaning and range of equivalency of the
claims are
therefore intended to be embraced herein.
