U.S. patent application number 11/366523 was filed with the patent office on 2007-07-26 for lead-free solder with low copper dissolution.
Invention is credited to Brian T. Deram.
Application Number | 20070172381 11/366523 |
Document ID | / |
Family ID | 38285754 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070172381 |
Kind Code |
A1 |
Deram; Brian T. |
July 26, 2007 |
Lead-free solder with low copper dissolution
Abstract
Lead-free solder compositions suitable for joining electronic
devices to printed wiring boards, which comprises by weight 0.2 to
0.9% copper, 0.006 to 0.07% nickel, 0.03 to 0.08% bismuth, less
than 0.5% silver, less than 0.010% phosphorus, and a balance of tin
and inevitable impurities. A solder composition embodying this
invention finds particular application in automated wave-soldering
machines where conventional lead-free solders dissolve excessive
copper from printed wiring circuitry and component
terminations.
Inventors: |
Deram; Brian T.;
(Lincolnshire, IL) |
Correspondence
Address: |
HARVEY S. KAUGET;PHELPS DUNBAR, LLP
100 S. ASHLEY DRIVE
SUITE 1900
TAMPA
FL
33602-5311
US
|
Family ID: |
38285754 |
Appl. No.: |
11/366523 |
Filed: |
March 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60761400 |
Jan 23, 2006 |
|
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Current U.S.
Class: |
420/561 |
Current CPC
Class: |
C22C 13/00 20130101;
B23K 35/262 20130101 |
Class at
Publication: |
420/561 |
International
Class: |
C22C 13/02 20060101
C22C013/02 |
Claims
1. A lead-free solder composition containing consisting essentially
of 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by weight of
nickel, 0.03 to 0.08% by weight of bismuth, less than 0.5% by
weight of silver, less than 0.010% by weight of phosphorus, and the
balance of tin.
2. The lead-free solder composition according to claim 1, wherein
the copper content falls within a range of between 0.5% by weight
and 0.7% by weight.
3. The lead-free solder composition according to claim 1, wherein
the nickel content falls within a range of between 0.04% by weight
and 0.06% by weight.
4. The lead-free solder composition according to claim 1, wherein
the bismuth content falls within a range of between 0.05% by weight
and 0.07% by weight.
5. A lead-free solder composition containing consisting essentially
of 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by weight of
nickel, 0.03 to 0.08% by weight of bismuth, and the balance of
tin.
6. The lead-free solder composition according to claim 5, further
containing silver in an amount not larger than 0.5% by weight.
7. The lead-free solder composition according to claim 5, further
containing phosphorus in an amount not larger than 0.010% by
weight.
8. The lead-free solder composition according to claim 5, wherein
the copper content falls within a range of between 0.5% by weight
and 0.7% by weight.
9. The lead-free solder composition according to claim 5, wherein
the nickel content falls within a range of between 0.04% by weight
and 0.06% by weight.
10. The lead-free solder composition according to claim 5, wherein
the bismuth content falls within a range of between 0.05% by weight
and 0.07% by weight.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority from and is related to
commonly owned U.S. Provisional Patent Application Ser. No.
60/761,400 filed Jan. 23, 2006, entitled: Lead-Free Solder With Low
Copper Dissolution, this Provisional Patent Application
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to low-cost solder
compositions for bonding electronic devices or parts to printed
wiring boards (PWBs). In particular, the present invention relates
to solder compositions that prevent or minimize the dissolution of
copper into the solder during the soldering process.
BACKGROUND OF THE INVENTION
[0003] Electronic assemblies are composed of a printed wiring board
(PWB), sometimes also known as a printed circuit board (PCB), which
is constructed of an insulating board, such as glass-epoxy or
paper-epoxy, on which copper circuitry is formed on one or both
sides. Electronic components, wires, or other devices with metal
terminations are joined to the copper circuitry with solder. Though
some of the soldering of electronic assemblies is being done by
hand with soldering irons, the much larger percentage of production
is being accomplished with automated soldering machines. U.S. Pat.
No. 2,993,272 discloses one of the earliest wave soldering
processes for soldering electrical and mechanical portions of a
circuit board, a process still in present use.
[0004] Automated soldering machines, often also known as mass
soldering machines, employ a conveyor to transport a printed
circuit board assembly first across a fluxing unit that applies
soldering flux to the bottom side of the printed circuit board and
component terminations, then across a preheating unit, and then
across the solder station. The solder station consists of an iron
or steel heated pot that maintains the solder in a melted
condition, usually 50-100.degree. C. above the liquidus melting
point of the solder composition. The printed circuit assembly may
be transported across the surface of the melted solder, known as
drag soldering, or, more commonly, across a standing wave of molten
solder, the wave being generated by a pumping system contained in
the melted solder.
[0005] Solders for joining electronic parts to copper terminations
of a printed wiring board traditionally have been composed of tin
and lead of about 60% to 63% tin (Sn) and the balance lead (Pb).
More recently, international environmental regulations are
restricting the use of lead (Pb) in solders for electronic
products; so, solder technologists and manufacturing personnel have
been evaluating and using alternative solders for soldering
electronic assemblies. The process of automated soldering results
in dissolution of copper from the printed circuit board and
component terminations into the melted solder. Copper is very
soluble in melted tin, a primary component in solders used for
electronic assembly. Copper is particularly susceptible to
dissolving in lead-free solders because the lead-free solders melt
at higher temperatures than tin-lead solder and consist of more
than 95% tin. As copper dissolves into the lead-free solder, the
liquidus melting point of the solder alloy increases dramatically.
For example, the eutectic melting temperature (solidus and liquidus
points are the same) for tin containing 0.7% copper is 227.degree.
C. Because of the superheating required for automated wave
soldering, the solder may be heated to above 300.degree. C. for
reliable soldering. During the soldering process, the melted
tin-copper solder can dissolve additional copper from the assembly
being soldered. A very small increase in copper from 0.7% to 1.0%
will raise the liquidus melting point of the tin-copper solder
alloy to 239.degree. C., while an increase in copper to only 1.5%
results in a liquidus melting point of 260.degree. C. One solution
would be to increase the solder temperature to accommodate the
increased amount of copper, but this would not be acceptable
because of the damage caused by excessive heating of the electronic
devices being soldered.
[0006] A common practice by those using automated dip or wave
soldering machines, to adjust for the increasing copper percentage
in the solder, is to use a second alloy with reduced copper
percentage for replacement additions of solder to the pot. For
example, where a solder composition of 0.7% copper in tin is the
most economical lead-free solder of choice in the automated
soldering machine, additions of solder to replace expended solder
and dilute dissolved copper may contain only from 0% to 0.5%
copper. This adjustment is not precise and can affect the quality
of the soldering process and the reliability of the soldered
electronic assembly. A significant advantage of the present
invention is that the dissolution rate of copper into the melted
solder is sufficiently low that the use of another solder alloy is
not necessary.
[0007] A solder wave is formed by pumping molten solder, contained
within a solder pot, up through a nozzle to provide a standing
wave. Usually, only one wave is employed, but dual waves are also
employed, particularly when surface mounted devices are being
soldered to the bottom side of printed circuit boards. Solder
cascades and solder jets also find application as wave soldering.
Alternatively, the solder may be maintained in an open solder pot
where an electronic assembly may be dragged across the melted
solder surface to accomplish the soldering. One of the problems
encountered with automated soldering processes is that the molten
solder oxidizes when exposed to the oxygen in the air. The oxidized
solder forms a surface oxide layer, which must be removed by a flux
before the components being soldered will wet with solder.
Particularly with wave soldering, the surface oxide layer is
continually broken by the flow of solder in the wave. This exposes
fresh solder which, in turn, is also oxidized. A mixture of oxide
and solder, thus, collects within the solder pot. This mixture is
known as dross, which must be removed and disposed. Dross
generation adds to the cost of the process due to the lost value of
the solder and the maintenance time required to remove it and
repair mechanical parts of the wave soldering apparatus damaged by
the abrasive action of the dross.
[0008] One method employed to minimize the formation of oxide on
the solder in a wave soldering machine is to cover the surface of
the melted solder with an oil. This is effective in protecting the
solder from atmospheric oxygen, but the oil degrades and must be
replaced periodically. Furthermore, the oils commonly used are
difficult to clean off of the components being soldered and can
produce a great deal of smoke at wave soldering temperatures. A
solution for this problem for over three decades is to add
phosphorus to the solder. U.S. Pat. No. 5,240,169 describes the use
of known low dross solder containing 10 to 1000 parts per million
(ppm) phosphorus.
[0009] More recently, concerns about safety and environmental
pollution caused by lead (Pb) in the solder used for assembling
electronic products has resulted in the development of
environmentally acceptable, substitute solder compositions where
the lead (Pb) has essentially been replaced with tin (Sn). There
are a variety of other metals--such as silver (Ag), copper (Cu),
antimony (Sb), zinc (Zn), indium (In), and bismuth (Bi)--that can
be added to tin (Sn) either individually or in combination to
reduce the melting temperature, improve the ductility and strength
of the solder joint, and/or improve wetting to the metal surfaces
being soldered.
[0010] Some examples are described by the patents referenced in
Table 1 below: TABLE-US-00001 TABLE 1 (Prior Art) Values Given in
Weight Percent U.S. Pat. No. Sn Ag Cu Sb Zn In Bi Ni P Other
1,239,195 Balance 0.5-1 0.5-1 -- -- -- -- -- -- -- 1,437,641 85-95
0.5-4.5 0.5-4.5 -- 0.5-9.5 -- -- -- -- -- 4,193,530 Balance -- --
-- -- 0.1-0.5 0.1-0.5 -- -- -- 4,670,217 90.0-98.5 0.5-2 -- 0.5-4.0
0.5-4.0 -- -- -- -- -- 4,758,407 92.5-96.9 0.1-0.5 3.0-5.0 -- -- --
-- 0.1-2.0 -- -- 4,879,096 88-99.35 0.05-3 0.5-6 -- -- -- 0.1-3 --
-- -- 4,929,423 Balance 0.01-1.5 0.02-1.5 -- -- -- 0.08-20 -- 0.10
Rare Earth 0-0.2 5,094,813 95.68 0.08-0.16 2.8-3.5 -- 0.2-0.5 -- --
0.08-0.16 -- -- 5,352,407 93-98 1.5-3.5 0.2-2.0 0.2-2.0 -- -- -- --
-- -- 5,817,194 Balance .ltoreq.10 .ltoreq.3 -- -- -- -- 0.5-5.0
0.05-1.5 -- 5,837,191 Balance 0.05-0.6 0.05-0.6 0.75-2 -- -- --
0.05-0.6 -- -- 5,863,493 91.5-96.5 2.0-5 0-2 -- -- -- -- 0.1-2 --
-- 5,980,822 81.4-99.6 0.1-5.0 0.1-5.5 0.1-3.0 -- -- 0.1-5.0 --
0.001-0.01 Ge 0.01-0.1 5,985,212 >75.0 -- 0.01-9.5 -- -- 0-6.0
-- -- -- Ga 0.01-5.0 6,139,979 Balance -- 0.7-2* 3.0-5.0* -- -- --
0.01-0.5 -- *at least one 6,179,935 Balance >0-4.0 <0-2.0 --
-- -- -- >0-1.0 -- Ge >0-1 6,180,055 Balance -- 0.1-2 -- --
-- -- 0.002-1 -- Ge 0-1 6,296,722 Balance -- 0.1-2 -- -- -- --
0.002-1 -- Ga 0.001-1 6,365,097 Balance .ltoreq.4 .ltoreq.2 -- --
-- .ltoreq.21 .ltoreq.0.2 -- Ge <0.1 6,440,360 Balance -- 0.8 --
-- -- -- 0.9 -- 6,488,888 Balance 0.1-3.5* 0.1-3* -- 7-10 -- --
0.01-1 0.001-1 *at least one 6,649,127 Balance -- 0.3-3 -- 0.5-10
-- 0.5-8 -- -- Ge 0.005-0.05 6,660,226 >88.0 0.5-9 0.5-2 -- --
-- -- -- -- Co 0.1-2.0 6,702,176 Balance 1.0-4.0 0.2-1.3 -- -- --
-- -- -- Co 0.02-0.06 6,843,862 88.5-93.2 3.5-4.5 0.3-1.0 -- --
2.0-6.0 -- -- 0.01 --
[0011] There are specific problems with the above listed solder
alloy compositions that make them not as desirable for soldering of
electronic assemblies as the tin-lead solder compositions they are
intended to replace.
[0012] Tin without the addition of other metals is unacceptable for
soldering electronic assemblies for several reasons. First,
soldering temperatures required for using tin (Sn) with its high
melting point (232.degree. C.) may damage electronic components.
Second, the wetting ability is poor because of the high surface
tension of the melted tin. Third, the tensile strength and
ductility are unacceptably low because of the course grain
structure of the solidified metal. Adding other metals, such as
lead, silver, copper, and/or zinc will decrease the surface tension
and the melting points of the solder alloys, while increasing the
tensile strengths and ductility. U.S. Pat. No. 1,239,195 describes
the addition of copper and/or silver to tin to harden the
composition.
[0013] U.S. Pat. No. 1,437,641 describes the improvement of
mechanical properties of tin-silver by adding copper to tin-silver
solder compositions. Tin (Sn) solders containing 3.5% or less
silver, such as SnAg3.5 or SnAg3.0Cu0.5, have an acceptable melting
temperature (217 to 221.degree. C.) and higher mechanical strength
than the tin-lead solders. U.S. Pat. No. 5,863,493 describes the
addition of small amounts of copper (0-2.9%) and nickel (0.1-3%) to
SnAg3.5 solder compositions to provide resistance to grain growth
of tin-silver intermetallic compound in the tin matrix, especially
during thermal cycling of the solder joints. Nevertheless, the
ability of these solders to wet or bond to copper is impaired by
the high surface tension and slow wetting speed of tin-silver
solders on copper. Also, silver is too costly for large scale
manufacturing of electronic products, and the appearance of the
completed solder joints is dull or frosty, unlike the acceptable
appearance of tin-lead solder.
[0014] Tin-copper solders, such as the SnCu.sub.0.7 eutectic
composition, melt at an acceptable temperature (227.degree. C.) for
hand or automated soldering, but the surface tension of the solder
alloy is still high compared to the conventional tin-lead solder,
resulting in inferior wetting ability compared to that of the
tin-lead solder. The dissolved tin-copper intermetallic compound
(primarily Cu.sub.6Sn.sub.5) crystallizes and causes a grainy, dull
appearance on the solidified solder.
[0015] Tin-antimony solder, such as the well-known composition
SnSbO5, provides solder joints with acceptable tensile strength,
but the melting temperature (232-240.degree. C.) is higher than
alloys of tin and copper or silver, and therefore too high of a
melting temperature for heat-sensitive electronic components. Also,
antimony seriously reduces the ability of the solder to spread on a
copper surface because of the formation of a copper-antimony
intermediate phase between the copper and the solder alloy.
Antimony is considered the most toxic metal in this grouping of
solder compositions. As antimony has been added to tin-lead solder
to improve strength, so to is antimony added to other tin
compositions incorporating silver, copper, bismuth, and zinc, as
shown in Table 1.
[0016] U.S. Pat. No. 1,437,641 describes a well-known tin-zinc
solder alloy with acceptable melting temperature, but unacceptably
rapid oxidation and corrosion problems. U.S. Pat. No. 4,670,217
describes solder compositions for joining copper that contain up to
4% zinc added to solder compositions containing tin, silver, and
antimony. However, for automated applications, such as wave
soldering, tin alloys containing zinc (Zn) are subject to very
rapid oxidation while being pumped to generate a standing wave of
solder, resulting in a large production of dross, i.e., a mixture
of metal oxides and metal particles that float on the surface of
the melted solder. There is also much concern about the potential
galvanic corrosion of solder joints made with solders containing
zinc.
[0017] U.S. Pat. No. 4,193,530 describes the addition of small
amounts (0.1% to 0.5%) of bismuth and indium to tin metal to
improve the corrosion resistance of the tin. U.S. Pat. No.
4,879,096 describes adding 0.1% to 3% bismuth to solder
compositions containing tin, silver, and copper, to increase the
strength of the solder joints. U.S. Pat. No. 5,980,822 describes
adding 0.1% to 5.0% bismuth to reduce the solidus melting
temperature of the solder alloy that contains tin, silver, copper,
and antimony. U.S. Pat. No. 4,929,423 describes adding up to 20%
bismuth, preferably 3% to 6%, to solders containing tin, silver,
and copper. U.S. Pat. No. 6,649,127 describes the addition of
bismuth up to 8% to tin-copper solder composition containing up to
10% zinc, for the purpose of reducing the melting temperature and
improving the wetting speed when soldering to copper surfaces.
[0018] These described solders are designed for copper pipe
plumbing applications. However, for soldering electronic components
to a printed circuit board, solder alloys that contain more than
about 2% to 5% bismuth (Bi) are incompatible with lead (Pb) that
may be contained on the electronic component terminations,
resulting in potential cracked solder joints. Nevertheless, bismuth
can be added in small amounts to certain lead-free solder alloy
compositions to improve the wetting ability and slightly reduce the
melting temperature of the solder. As much as 1% bismuth is soluble
in solid tin. The much lower surface tension of bismuth compared to
tin would help wetting.
[0019] Tin alloys containing indium (In) have the same high cost
problem as those with silver, even though indium additions are able
to improve the wetting ability of the solder. Though U.S. Pat. No.
6,843,862 describes lead-free solders containing 4% indium as
dissolving the copper substrate metal at a reduced rate compared to
lead-free solders that do not contain indium, there is no
significant difference when bismuth is added to these alloys. U.S.
Pat. No. 5,985,212 also describes the addition of gallium (Ga) to
lead-free solders containing tin, copper, and indium with the
purpose of increasing strength and reducing the melting point of
the resulting solder composition.
[0020] Tin-nickel compositions with as little as 4 weight % nickel
(Ni) melt above 400.degree. C., which is too high of a temperature
for electronic soldering applications. However, the addition of
nickel in smaller amounts (0.1% to 2.0%) to tin alloys containing
3% to 5% copper is described in U.S. Pat. No. 4,758,407 as
improving wettability and increasing strength of the solder
composition. Nevertheless, while acceptable for plumbing
applications, the specified solder compositions have high liquidus
temperatures exceeding 600.degree. F. (315.degree. C.), which is
exceedingly high for soldering of electronic assemblies. Subsequent
patents, as shown in Table 1, have incorporated nickel as an
additive to lead-free solders also to improve the solderability and
reduce the melting point of the specified solder compositions. U.S.
Pat. No. 5,863,493 teaches that tin-silver solder alloys experience
grain growth coarsening during thermal cycling, resulting in
decreased creep and fatigue resistance, and the additions of nickel
and copper to the tin-silver solder composition improves both
properties.
[0021] Alternatively, U.S. Pat. Nos. 6,660,226 and 6,702,116
describe the addition of cobalt (Co) to lead-free solders to
prevent leaching by the lead-free solder of certain transition
metals, such as copper, silver, gold, palladium, platinum, nickel,
and zinc. Yet another U.S. Pat. No. 6,702,176 describes the
addition of cobalt to a tin-based, lead-free solder to prevent
leaching of copper metal being soldered.
[0022] U.S. Pat. No. 5,980,822 describes the addition of germanium
(Ge) in combination with phosphorus (P) to prevent the formation of
metal oxide and improve the thermal fatigue of the solder.
Subsequent patents, as shown in Table 1, have incorporated
germanium (Ge) with nickel (Ni) to enhance the wettability and
tensile strength of a variety of lead-free solders containing tin,
silver, copper, and/or zinc. The use of germanium in price
competitive solder compositions is precluded because of the very
high cost of germanium.
[0023] Phosphorus is commonly added to bulk molten metal, such as
copper and solder, to remove oxides. The addition of phosphorus to
tin solder alloys is recognized by those skilled in the art as a
standard method for deoxidizing metals during the manufacture of
solder or while using the solder for joining electronic assemblies.
Because the Gibb's Free Energy of Formation of phosphorus oxide
(P.sub.2O.sub.5) is much lower than that of tin, copper, bismuth,
nickel, silver, or lead oxides, the oxygen affinity of phosphorus
is higher. So, phosphorus oxide preferentially forms on the melted
solder surface during the solder processing. The resulting
phosphorus oxide (specific gravity 2.4) floats on the surface of
the solder (specific gravity 7.3). U.S. Pat. No. 6,488,888
describes adding phosphorus to reduce drossing or oxide formation
resulting from using tin-zinc solders. U.S. Pat. No. 5,817,194
describes the addition of phosphorus to soldering/brazing material
in the range of 0.05% to 1.5% to act as a fluxing agent to improve
the wettability of the solder/brazing material to stainless steel.
Also stated is that phosphorus addition must exceed 0.05% to
exhibit fluxing properties, and that the nickel addition must be
greater than 0.05% to be effective.
[0024] Therefore, there is a need for improving lead-free solder
compositions.
[0025] Nothing in the prior art provides the benefits attendant
with the present invention.
[0026] Therefore, it is an object of the present invention to
provide an improvement which overcomes the inadequacies of the
prior art devices and which is a significant contribution to the
advancement of the solder art.
[0027] It is another object of the present invention to form solder
compositions that will result in smooth, shiny solder connections
equivalent to those obtained when using tin-lead solder.
[0028] Another object of the present invention is to provide
lead-free solder compositions for use in joining electronic
components and other parts to printed wiring boards.
[0029] Yet another object of the present invention is to provide a
lead-free solder composition containing 0.2 to 0.9% by weight of
copper, 0.006 to 0.07% by weight of nickel, 0.03 to 0.08% by weight
of bismuth, less than 0.5% by weight of silver, less than 0.010% by
weight of phosphorus, and the balance of tin.
[0030] Still yet another object of the present invention is to
provide a solder paste composition comprising a powder containing
0.2 to 0.9% by weight of copper, 0.006 to 0.07% by weight of
nickel, 0.03 to 0.08% by weight of bismuth, less than 0.5% by
weight of silver, less than 0.010% by weight of phosphorus, and the
balance of tin; a rosin-based resin; an activating agent, and a
solvent.
[0031] Another object of the present invention is to provide a
soldered article comprising a workpiece containing a transition
metal conductor capable of readily diffusing into melted tin; and a
lead-free solder composition containing 0.2 to 0.9% by weight of
copper, 0.006 to 0.07% by weight of nickel, 0.03 to 0.08% by weight
of bismuth, less than 0.5% by weight of silver, less than 0.010% by
weight of phosphorus, and the balance of tin, the lead-free solder
composition bonded to the workpiece so as to be electrically and
mechanically bonded to the transition metal conductor.
[0032] Yet another object of the present invention is to provide a
lead-free solder composition containing 0.2 to 0.9% by weight of
copper, 0.006 to 0.07% by weight of nickel, 0.03 to 0.08% by weight
of bismuth, and the balance of tin.
[0033] The foregoing has outlined some of the pertinent objects of
the present invention. These objects should be construed to be
merely illustrative of some of the more prominent features and
applications of the intended invention. Many other beneficial
results can be attained by applying the disclosed invention in a
different manner or modifying the invention within the scope of the
disclosure. Accordingly, other objects and a fuller understanding
of the invention may be had by referring to the summary of the
invention and the detailed description of the preferred embodiment
in addition to the scope of the invention defined by the
claims.
SUMMARY OF THE INVENTION
[0034] According to the present invention, the melted solder
compositions dissolve copper from electronic components, printed
wiring boards and wires at a rate slower than that experienced in
the same applications with 63% tin, 37% lead solder alloy.
Additionally, the need to attempt to balance the solder composition
in a solder pot by adding another solder composition with reduced
copper content is eliminated. Still further, the solder
compositions of the present invention are low cost, exhibit good
wetting properties, have acceptably low melting temperatures, and
result in the formation of reflective solder joints with a shiny
appearance similar to that experienced in the same applications
with 63% tin, 37% lead solder alloy.
[0035] It was discovered that a very small amount of bismuth added
to tin-based solder containing copper and nickel can significantly
reduce the dissolution rate of copper into the solder when compared
to tin-lead and tin-copper solder alloys. Further, the synergistic
effect of adding nickel and bismuth improves the cosmetic
appearance of the solder joint.
[0036] A feature of the present invention is to provide a lead-free
solder composition containing copper, nickel, bismuth, silver,
phosphorus, tin and inevitable impurities. The range of copper in
the lead-free solder composition can be between 0.2 to 0.9% by
weight or, in a preferred embodiment, between 0.5% and 0.7% by
weight. The range of nickel in the lead-free solder composition can
be between 0.006 to 0.07% by weight or, in a preferred embodiment,
between 0.04% by weight and 0.06% by weight. The range of bismuth
in the lead-free solder composition can be between 0.03 to 0.08% by
weight or, in a preferred embodiment, between 0.05% by weight and
0.07% by weight. The range of silver in the lead-free solder
composition is less than 0.5% by weight. The range of phosphorus in
the lead-free solder composition is less than 0.010% by weight. The
balance of the lead-free solder composition is of tin.
[0037] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder bar where the
solder bar can be used in electronic assembly solder machines.
[0038] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder ingot where the
solder ingot can be used in electronic assembly.
[0039] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder wire where the
solder wire can be used in electronic assembly.
[0040] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder chip where the
solder chip can be used in electronic assembly.
[0041] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder ribbon where the
solder ribbon can be used in electronic assembly.
[0042] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder powder where the
solder powder can be used in electronic assembly.
[0043] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be employed in hot air leveling of
printed circuit boards.
[0044] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be employed in assembling surface
mounted printed circuit boards.
[0045] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be employed in the solder coating of
printed circuit boards.
[0046] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be employed in roll tinning of circuit
boards.
[0047] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be employed in surface mount assembly of
electronic components onto a printed circuit board.
[0048] The lead-free solder composition of the present invention
containing 0.2 to 0.9% by weight of copper, 0.006 to 0.07% by
weight of nickel, 0.03 to 0.08% by weight of bismuth, less than
0.5% by weight of silver, less than 0.010% by weight of phosphorus,
and the balance of tin can be formed into a solder preform where
the solder preform can be used in electronic assembly. The solder
preform can be fluxed or unfluxed.
[0049] Another feature of the present invention is to provide a
solder paste composition comprising a powder containing 0.2 to 0.9%
by weight of copper, 0.006 to 0.07% by weight of nickel, 0.03 to
0.08% by weight of bismuth, less than 0.5% by weight of silver,
less than 0.010% by weight of phosphorus, and the balance of tin; a
rosin-based resin; an activating agent, and a solvent.
[0050] Yet another feature of the present invention is to provide a
soldered article comprising a workpiece containing a transition
metal conductor capable of readily diffusing into melted tin; and a
lead-free solder composition containing 0.2 to 0.9% by weight of
copper, 0.006 to 0.07% by weight of nickel, 0.03 to 0.08% by weight
of bismuth, less than 0.5% by weight of silver, less than 0.010% by
weight of phosphorus, and the balance of tin. The lead-free solder
composition is bonded to the workpiece so as to be electrically and
mechanically bonded to the transition metal conductor. The
transition metal conductor is selected from the group consisting of
copper, silver, nickel, gold, palladium, platinum, zinc and an
alloy thereof.
[0051] Still yet another feature of the present invention is to
provide a lead-free solder composition containing copper, nickel,
bismuth, tin and inevitable impurities. The range of copper in the
lead-free solder composition can be between 0.2 to 0.9% by weight
or, in a preferred embodiment, between 0.5% and 0.7% by weight. The
range of nickel in the lead-free solder composition can be between
0.006 to 0.07% by weight or, in a preferred embodiment, between
0.04% by weight and 0.06% by weight. The range of bismuth in the
lead-free solder composition can be between 0.03 to 0.08% by weight
or, in a preferred embodiment, between 0.05% by weight and 0.07% by
weight. The balance of the lead-free solder composition of the
present invention is of tin. In addition, silver can be added to
the lead-free solder composition in an amount no greater than 0.5%
by weight. In addition, phosphorus can be added to the lead-free
solder composition in an amount no greater than 0.010% by
weight.
[0052] The foregoing has outlined rather broadly the more pertinent
and important features of the present invention in order that the
detailed description of the invention that follows may be better
understood so that the present contribution to the art can be more
fully appreciated. Additional features of the invention will be
described hereinafter which form the subject of the claims of the
invention. It should be appreciated by those skilled in the art
that the conception and the specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art
that such equivalent constructions do not depart from the spirit
and scope of the invention as set forth in the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0053] The solder alloy compositions of the present invention are
essentially free of potentially toxic metals including antimony,
arsenic, cadmium, cobalt, gallium, mercury, and thallium. The term
"essentially free" is used in the context to mean that if any of
these metals are present in the composition, the included
concentration is so low that the expected health or environmental
effects are insignificant.
[0054] According to one preferred embodiment of the present
invention, the solder compositions comprise, as essential
ingredients, from about 0.2% to about 0.9% by weight copper (Cu),
from about 0.006% to about 0.07% by weight nickel (Ni), from about
0.03% to about 0.08% by weight bismuth (Bi), less than about 0.5%
by weight silver (Ag) and the balance tin (Sn), together with
incidental impurities. Optionally, to reduce drossing in automatic
soldering machines, phosphorus (P) may be added from about 0.001%
to about 0.010%.
[0055] The alloy compositions of the present invention may be
prepared by techniques known in the art by melting the tin metal
and then adding the remaining elements while mixing until all added
elements are dissolved into the tin. The alloy compositions can
then be cast into billets or continuous wire, and subsequently
manufactured into ingots, bars, wire, or other predetermined
shapes. Though primarily intended for use as bar or solid wire form
in automatic wave or dip soldering machines, the alloy compositions
can also be manufactured as solid or flux cored wire solder for
hand soldering.
[0056] The following formulation examples and tests performed are
intended to enable those skilled in the art to apply the principles
of this invention in practical embodiments, but are not intended to
limit the scope of the invention. TABLE-US-00002 TABLE 2 Solder
Alloy Compositions (weight percentages) Solder Melting Time
(seconds) to Sample Temp. dissolve copper at Number Sn Pb Ag Cu Bi
Ni P (.degree. C.) 300.degree. C. 1 Balance 37 -- -- -- -- -- 183
954.3 2 Balance 37 -- -- -- -- 0.010 183 945.6 3 Balance -- 3.5 --
-- -- -- 221 421.2 4 Balance -- 3.5 -- -- -- 0.010 221 405.6 5
Balance -- 3.5 0.7 -- -- -- 217 468.6 6 Balance -- 3.5 0.7 -- --
0.010 217 445.3 7 Balance -- 3.0 0.5 -- -- -- 217-219 444.3 8
Balance -- 0.9 0.7 -- -- -- 217-225 488.7 9 Balance -- 0.4 0.7 --
-- -- 219-225 602.9 10 Balance -- 0.4 0.7 -- -- 0.010 219-225 596.7
11 Balance -- 0.3 0.7 0.1 -- 0.012 219-225 609.5 12 Balance -- 0.3
0.7 -- 0.036 0.007 220-225 628.8 13 Balance -- 0.03 0.7 -- 0.041
0.010 227 833.3 14 Balance -- 0.001 0.7 -- 0.005 -- 227 635.3 15
Balance -- 0.001 0.7 -- 0.057 0.010 227 850.8 16 Balance -- 0.001
0.7 0.06 0.005 -- 227 750.4 17 Balance -- 0.46 0.7 0.054 0.05 0.008
219-225 1187.9 18 Balance -- 0.1 0.7 0.054 0.05 0.008 227 1196.7 19
Balance -- 0.08 0.7 0.054 0.05 0.008 227 1205.4 20 Balance -- 0.04
0.7 0.06 0.05 0.010 227 1274.6 21 Balance -- 0.001 0.7 0.06 0.06
0.010 227 1347.5
[0057] Samples were taken from each alloy melt and submitted for
analysis using a spark emission spectrograph. Individual solder
alloys were cast into small ingots for testing of properties.
[0058] The eutectic composition for tin-lead solder is Sn61.9Pb38.1
weight % (Sn70.9Pb29.1 atomic %), melting at 183.degree. C., but
the convention in the solder industry is to refer to the eutectic
composition as either 63/37 or Sn63Pb37 (weight %). The eutectic
composition for tin-silver is Sn96.5Ag3.5 weight % (Sn96.2Ag3.8
atomic %), melting at 221.degree. C. The eutectic composition for
tin-copper is Sn99.3Cu0.7 weight % (Sn98.7Cu1.3 atomic %) melting
at 227.degree. C. The melting points of these standard industry
solder alloys and the other alloys shown in Table 1 were verified
by measurement with a differential scanning calorimeter (DSC).
[0059] The choice of solder compositions is very limited for use as
alternative alloys to the tin-lead solders that are no longer
acceptable for assembly of electronic products. More recently,
lead-free solders have been used for automated soldering, including
dip, wave, and reflow soldering techniques, as well as for hand
soldering applications. The commonly acceptable lead-free solders
contain more than 95% tin in combination primarily with silver
and/or copper. The higher tin percentage and higher melting
temperature of the lead-free solder alloys result in an increase in
the rate of copper dissolution during soldering. Consequently, the
small copper traces on a printed wiring board, small copper
electrical wires, or the coatings on component terminations may
completely dissolve into the solder, rendering the soldered product
useless.
[0060] To determine the comparative rate of copper dissolution for
the solder compositions listed in Table 2, each solder alloy
composition was heated in a temperature controlled solder pot that
maintained the temperature of the solder at 300.degree.
C..+-.5.degree. C. One end of a copper wire measuring 0.6 mm
diameter and 25 mm long was suspended vertically from a holder over
the solder pot. The suspended lower end of the copper wire was
dipped into a mildly activated rosin soldering flux, Kester #186,
to a depth of 10 mm. The solder pot was raised mechanically at a
speed of 2 mm/second by means of an electric elevator until 5 mm of
the wire was immersed into the solder, immediately followed by
starting the timer. The end of the test was determined by observing
the number of seconds required for the immersed 5 mm of the copper
wire to dissolve into the melted solder. The test results are shown
in Table 2 for each solder alloy composition.
[0061] As shown in the above Table 2, the rate of dissolution of
copper into the known lead-free solder alloy compositions increases
dramatically compared to the electronics industry standard tin-lead
solder. Solder samples 3 to 8, containing silver as the main
element added to the tin, were found to dissolve copper at about
two times the rate compared to tin-lead solder samples 1 and 2.
This is the case even with the addition of copper to the test
compositions, as shown with samples 5 to 8. Only when the silver is
reduced to less than 0.5% as in samples 9 and 10 does the rate of
dissolution of copper from the substrate surface become appreciably
slower. As one objective of the present invention is to reduce the
cost of the solder composition, experiments continued with the very
low silver content with solder compositions essentially consisting
of the tin-copper eutectic base alloy.
[0062] The addition of bismuth in sample 11 results in only minimal
improvement in the rate of copper dissolution. The solubility of
copper in bismuth is about 0.15 weight % at 270.degree. C. and is
expected to have minimal effect on the rate of copper dissolution.
However, bismuth additions to tin solder alloys are known to
improve the wetting ability of the solder because of the low
surface tension property of bismuth.
[0063] The addition of nickel to the solder compositions without
the addition of bismuth in samples 12 to 15 also shows some
reduction in the rate of copper dissolution. Sample 14 is an
example demonstrating the minimum nickel that has any effect.
However, sample 15 with the higher amount of nickel resulted in
approaching the low rate of copper dissolution experienced with the
conventional tin-lead solder. Copper-nickel alloys are a
metallurgical example of an isomorphous binary system in which only
a single type of crystal structure is observed for all ratios of
the components. Copper and nickel combine to form only a single
liquid phase and a single solid phase. Therefore, copper and nickel
dissolve in each other in all percentages to form a solid solution.
During soldering of a copper surface with a tin-copper solder
containing nickel, the tin in the solder composition will dissolve
some copper at the surface of the copper substrate, and, because
the nickel-copper solid solution melts above 1000.degree. C., a
nickel-copper compound is formed as a barrier on the copper to
prevent additional dissolution of the copper.
[0064] As shown for solder sample 16, and especially samples 17 to
21, the combination of bismuth and nickel acts synergistically to
greatly reduce the copper dissolution rate. The solubility of
bismuth in tin at 25.degree. C. is about 1.2 weight %, so the
bismuth addition less than 1 weight % to the tin-copper composition
is not expected to result in any crystallization problem as might
be experienced with higher amounts of bismuth. However, the
solubility of copper in bismuth is only about 0.15 weight % at
270.degree. C., the normal wave solder temperature for soldering
electronic assemblies, which allows the nickel to form the
nickel-copper compound on the copper substrate surface. Bismuth
additions also have the effect of reducing the surface tension of
the solder alloy composition.
[0065] The electronics industry bases its inspection quality
standards on the appearance of solder joints. Compared to the
normally bright, smooth, and shiny appearance of tin-lead solder
joints, the known lead-free solders by their crystalline nature
solidify with a frosty or dull surface caused by precipitation of
tin-silver or tin-copper intermetallics during solidification of
the tin alloys. The specific gravities of these intermetallic
crystals results in their rising to the surface of the solder to
make the surface frosty or grainy. This visible grainy surface is
also a sign that the grainy structure also may exist in the solder
composition matrix, a potential mechanism for cracking of the
solder joints over time with thermal cycling of the electronic
assembly.
[0066] For this test, a deoxidized copper coupon of dimensions 50
mm.times.50 mm.times.0.3 mm was prepared by polishing the copper
with #1500 abrasive paper, washing the copper coupon with alcohol,
and then heating the copper coupon in a furnace at 150.degree. C.
for one hour. Precisely 1.0 gram of the solder sample was placed on
the copper coupon, and then 100 micro liters of mildly activated
rosin soldering flux (Kester #186) was placed with a micropipette
onto the solder sample. The copper coupon was then placed onto a
hotplate with temperature controlled at 270.degree. C..+-.5.degree.
C. When the solder melted and spread out onto the copper coupon,
the coupon was removed, allowed to cool to room temperature
(25.degree. C.), and the rosin flux residue removed with alcohol.
The cosmetic appearance of the solidified solder is recorded in
Table 3. TABLE-US-00003 TABLE 3 Cosmetic Appearance of the
Solidified Solder Surface Shine of Texture of Solder Solder Surface
Solder Surface Sample Quality Quality Number Rating Observation
Rating Observation 1 1 Shiny, reflective 1 Smooth 2 1 Shiny,
reflective 1 Smooth 3 4 Dull, 100% frosty 4 Crystalline, granular 4
4 Dull, 100% frosty 4 Crystalline, granular 5 4 Dull, 100% frosty 4
Crystalline, granular 6 4 Dull, 100% frosty 4 Crystalline, granular
7 4 Dull, 90% frosty 4 Crystalline, granular 8 3.5 Dull, 70% frosty
4 Crystalline, granular 9 3.5 Dull, 70% frosty 4 Crystalline,
granular 10 3.5 Dull, 66% frosty 4 Crystalline, granular 11 3.5
Dull, 60% frosty 3.5 Crystalline, some dewetting 12 3 Dull, 50%
frosty 3 Crystalline, granular 13 1 Shiny, reflective 1 Smooth 14
1.5 Shiny, reflective 2 Small rough spot 15 1 Shiny, reflective 1
Smooth 16 1.5 Shiny, reflective 2 Small rough spot 17 2 Dull, 10%
frosty 2 Crystalline, granular 18 1 Shiny, reflective 1 Smooth 19 1
Shiny, reflective 1 Smooth 20 1 Shiny, reflective 1 Smooth 21 1
Shiny, reflective 1 Smooth
[0067] The quality ratings in Table 3 are based on the overall
shine and texture of the solder surface. The shine varies from
completely reflective which is a rating of 1, down to completely
dull or frosty which is a rating of 4. The amount of frosty or dull
appearance of the solder surface becomes a spot or spots of
increasing size as the rating goes from 1 to 4. The solder with the
best rating of 1 is equivalent in shininess and smoothness to that
experienced with the standard tin-lead solder Sn63Pb37. A rating of
4 is completely frosty or grainy looking and not acceptable for
quality inspection and reliability. The texture column rating is
the observed appearance of the frosty area.
[0068] During the formation of the solder joint on copper substrate
surfaces, the tin in the solder composition will dissolve some
copper from the surface. The tin in the solder can readily dissolve
copper with the formation of a low-melting temperature (221.degree.
C.) tin-copper eutectic composition. The microstructure consists of
the copper-tin intermetallic compound Cu.sub.6Sn.sub.5 needles
contained along the grain boundaries of the solidified tin. The
solid solubility of copper in tin at the solidification point
227.degree. C. is very low (about 0.006 weight %).
[0069] When soldering with tin-lead solder (samples 1, 2), as the
copper tin intermetallic Cu.sub.6Sn.sub.5 forms between the solder
and the copper surface, the residual lead (Pb) in the solder
composition forms a barrier to prevent further copper dissolution
by the tin. Most electronic circuitry and component termination
metallization is designed with copper dissolution considerations.
However, as printed wiring circuitry becomes more fine-lined or
when very small copper wires are being soldered, the dissolution of
the copper becomes more problematic.
[0070] When soldering with lead-free solders, such as tin-silver
solder compositions (samples 3, 4), the combination of the higher
melting point of the solder alloy and much higher tin content
compared to that of the tin-lead compositions, results in much more
rapid dissolution of the copper surface. Upon solidification of the
tin-silver solder, the silver is present as tin-silver
intermetallic platelets Ag.sub.3Sn contained in the tin matrix.
Reducing the amount of silver improves the appearance of the
solidified solder. There is no residual metal barrier formed, as
with tin-lead solders, so copper continues to dissolve into the
solder, and will also migrate under solid state diffusion after the
solder solidifies, resulting in brittle, failed solder joints.
[0071] Adding copper to the tin-silver alloys (samples 5, 6, 7)
reduces the melting point and slightly reduces the rate of
dissolution of the copper surface being soldered. Additionally,
reducing the silver content (samples 8, 9, 10) results in further
reduction in the rate of copper dissolution from the substrate. The
addition of bismuth (sample 11) has little improved effect on the
rate of copper dissolution, but the addition of nickel (samples 12,
13, 14, 15), even in small amounts, reduces the rate of copper
dissolution.
[0072] In addition to the nickel participating in the reduction of
the dissolution of the copper substrate, the remaining nickel
contained in the solder composition will solidify in the tin matrix
with intermetallic compound Ni.sub.3Sn containing some copper in a
solid solution. The tin-nickel-copper compound precipitates along
the grain boundaries in the tin crystals, thereby reducing the size
and subsequent growth of the tin-copper crystals.
[0073] Sample 16 exhibits the effect of adding a small amount of
bismuth to the sample 14 with the result being a significant
reduction in the rate of copper dissolution, but not as good as
sample 15 with the larger amount of nickel content. Adding bismuth
or nickel to the solder compositions containing 0.3 weight % silver
slightly improves the appearance (samples 11 to 15), and the
combined addition of bismuth and nickel (samples 16 to 21) further
improves the appearance.
[0074] A further observation during this melting test is that the
additions of bismuth and nickel to the primarily tin-copper
compositions had very little affect, if any, on the wetting or
spreading properties of the solder compositions on the copper
substrate. All of the test samples wet out and spread on the copper
test coupon, indicating the low copper dissolution rate of the
solder composition of the present invention does not affect the
soldering ability of the alloy.
[0075] Phosphorus content in solder compositions up to 0.010 weight
% (100 parts per million) is known to reduce drossing (oxide
formation) on the surface of the solder in automated wave or dip
soldering machines, but over 0.010 weight % is known to cause
dewetting (pulling back of the solder) on the copper surface, as
experienced with sample 11. Phosphorus content in tin-lead or
tin-copper solder compositions only slightly increases the rate of
dissolution of the copper substrate without affecting the wetting
properties of the solder or the appearance of the solder joint.
Phosphorus content in alloys containing more than about 0.3 weight
% silver causes a slight increase in the rate of dissolution of the
copper substrate.
[0076] The most remarkable discovery was the synergistic effect of
adding both bismuth and nickel to the tin-copper alloy to reduce
the rate of dissolution of the copper surface by more than half,
while also improving the cosmetic appearance of the solder joints.
There is also a significant reduction in the copper dissolution by
the solder compositions of the present invention compared with the
conventional tin-lead solder alloy. Additionally, as the weight
percentage of silver is reduced from about 0.5% to about 0.001%,
the reduction in the rate of copper dissolution is further
improved.
[0077] The present disclosure includes that contained in the
appended claims, as well as that of the foregoing description.
Although this invention has been described in its preferred form
with a certain degree of particularity, it is understood that the
present disclosure of the preferred form has been made only by way
of example and that numerous changes in the details of construction
and the combination and arrangement of parts may be resorted to
without departing from the spirit and scope of the invention.
[0078] Now that the invention has been described,
* * * * *