The fatigue resistance of lead-tin eutectic solder is increased by doping the solder with about 0.1 to 0.8 weight % of a dopant selected from cadmium, indium antimony and mixtures thereof. The doped eutectic solder exhibits increased resistance to thermally or mechanically induced cyclic stress and strain. As a result, the fatigue resistance of the solder joint is increased. Combination of dopants, such as indium and cadmium, in combined amounts of less than 0.5 weight % are especially effective in increasing the fatigue resistance of the lead-tin eutectic solder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to lead-tin eutectic solder. More 
particularly, the present invention relates to improving the fatigue 
resistance of lead-tin eutectic solder. 
2. Description of Related Art 
Eutectic and near-eutectic lead-tin solder alloys are used to provide 
solder joints in a wide variety of electronic devices. In addition to 
providing electrical connections, the solder joint provides a vital 
mechanical link between electronic devices and connectors. 
During operation, many electrical devices are subjected to vibration and 
continual changes in temperature. Many times, the coefficient of thermal 
expansion of the various materials at and around the solder joint are 
different. As a result, the continual changes in temperature cause the 
solder joint to be continually subjected to varying degrees of stress and 
strain. The solder joint may also undergo continual stress due to 
vibrations and other forces exerted against the joint. 
It would be desirable to provide solder joints which are structurally 
strong and resist fatigue due to mechanical or thermal stress and strain. 
Such fatigue resistant solder would be especially well-suited for use in 
electronic equipment which is subjected to extreme thermal fluctuations 
and mechanical duress. Further, fatigue resistant solder would be 
desirable for use in electronic devices where a long service life is 
required. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved lead-tin eutectic 
solder is provided which is more resistant to fatigue and breakage than 
conventional lead-tin eutectic solder. The present invention is based upon 
the discovery that the addition of about 1.0 to 0.8 weight % of a dopant, 
such as cadmium, indium or antimony, increases the fatigue resistance of 
lead-tin eutectic solder. 
As a feature of the present invention, it was discovered that optimum 
increases in fatigue resistance for the lead-tin eutectic solder was 
achieved by adding between about 0.1 and 0.8 weight % of the dopant. In 
addition, it was discovered that further increases in fatigue resistance 
can be achieved by adding a mixture of dopants, such as indium and 
cadmium. Large increases in fatigue resistance are obtained when the 
lead-tin eutectic is doped with about 0.2 weight % cadmium and about 0.2 
weight % indium. 
As other features of the present invention, the solder is provided in the 
form of a powder and in the form of a paste. 
As a further feature of the present invention, a method is disclosed 
wherein the doped lead-tin eutectic solder is used to bond two metal 
surfaces together. This method is especially well-suited for soldering the 
whole spectrum of electronic connectors together. 
The present invention also provides a method of improving the fatigue 
resistance of a lead-tin solder joint by forming the solder joint from a 
lead-tin eutectic solder comprising the above-discussed dopant. 
The improved lead-tin eutectic solder in accordance with the present 
invention is an improvement over existing lead-tin eutectic solder since 
it provides the same degree of electrical conductivity and connection as 
the conventional eutectic solder while at the same time providing 
increased resistance to fatigue and joint fracture in the solder joint. 
The above-discussed and many other features and attendant advantages of the 
present invention will become better understood by reference to the 
following description of the preferred embodiments. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is based upon the discovery that lead-tin eutectic 
solder can be doped with small amounts of specific dopants in order to 
increase the resistance of the solder to fatigue and fracture caused by 
continual stress and strain at the solder joint. The lead-tin eutectic 
solder in accordance with the present invention includes conventional 
eutectic solder which is free of silver and gold and which is doped with 
about 1.0 to 0.8 weight % of the dopants cadmium, indium, or antimony or 
mixtures thereof. 
The conventional lead-tin eutectic solder which is doped is the well-known 
and widely used eutectic solder material which contains 63 weight % tin 
and 37 weight % lead. The present invention is also applicable to 
near-eutectic Iead-tin solders wherein the weight percent of tin and lead 
are about 3 weight % higher or lower than the 63/37 weight % eutectic 
mixture. As used herein the term "eutectic" is intended to include "near 
eutectic" compositions, unless otherwise specified. 
Military-qualified lead-tin eutectic solders which are used in the 
electronics industry, such as for soldering printed wiring boards, 
typically contain impurities such as antimony, bismuth, copper, iron, 
zinc, aluminum, arsenic, and other miscellaneous elements, each at levels 
well below 0.08 weight percent, except for antimony which may be present 
at up to 0.50 weight percent and bismuth which may be present at 0.25 
weight percent. The specific values for these impurities are set forth in 
the "Federal Specification, Solder: Tin Alloy, Tin-Lead Alloy, and Lead 
Alloy," Fed. Spec. QQ-S-571E for composition Sn 63 in Table V therein. 
Solders used for commercial purposes may contain other impurities also. 
However, in accordance with the present invention silver and gold are not 
included in the solder composition. 
The preferred amount of dopant added to the lead-tin eutectic solder is 
between about 0.1 and 0.8 weight % of dopant. The preferred dopants are 
cadmium and indium. The dopants may be added individually to the solder or 
they may be added in combination. Further, it was discovered that doping 
the lead-tin eutectic solder with both indium and cadmium at a total 
doping level of less than about 0.5 weight % provides even further 
increases in fatigue resistance. When using a combination of dopants, the 
dopants may be added in equal or unequal amounts. 
The dopants in accordance with the present invention are incorporated into 
the solder by any of the well known processes for doping lead-tin solders. 
Preferably, the dopants in granular form are added to the solder 
components in granular form and the solder pre-mix is heated to a 
sufficient temperature (e.g. 230.degree. to 250.degree. C.) to form a 
liquid. The solder and dopants are maintained as a liquid for a sufficient 
time to insure uniform distribution of the dopants throughout the solder 
and the formation of a uniform alloy. Optionally the granular dopant and 
the eutectic lead-tin solder may be melted to alloy the dopant. The doped 
solder may then be immediately used or solidified and stored for future 
use. Other procedures for doping the solder are possible provided that the 
dopants are uniformly distributed throughout the solder mixture. 
For use in soldering, the doped lead-tin alloy of the present invention may 
be provided in various forms such as an ingot, a billet, wire, a preform, 
a powder or a paste. The molten doped lead-tin alloy described above may 
be solidified to form an ingot. The ingot may later be melted in a 
non-oxidizing atmosphere, such as nitrogen, formed into a billet, which is 
then drawn to a wire, which may subsequently be stamped or formed into a 
preform having a desired geometric shape, such as a disk, washer, bar, or 
sphere, using known methods. The preform may then be placed between the 
two surfaces to be soldered and heat is applied to effect soldering, as is 
known in the art. 
To form the powder, an ingot of the doped lead-tin alloy formed as 
described above is melted in a non-oxidizing atmosphere and then formed 
through gas jets having orifices of a given diameter, to form droplets of 
liquid alloy which subsequently solidify into spherical particles. 
Alternatively, the melted alloy may be subjected to a rotating blade to 
form liquid droplets which subsequently solidify into powder particles. 
The particle size of the powder is optimized for the intended use of the 
powder. Typical particle sizes are within the range of about 10 to 75 
micrometers, and multiple particle sizes may be used in single powder 
formulation. The powder is stored in an oxygen-free environment to prevent 
oxidation and is subsequently used to form a paste as described below. 
Each particle in the powder is composed of the above-described doped 
lead-tin alloy. 
To form the paste, the powder formed as described above is mixed with a 
flux, in order to make the solder dispensible. The flux comprises an 
activator (i.e. an oxide-removal or tarnish-removal agent), a vehicle 
(i.e. a solvent), and a rheology modifier or thinner, to adjust the 
viscosity. A typical paste formulation may comprise about 80-90 weight 
percent powder and about 20-10 weight percent flux. The powder and flux 
are mechanically mixed to form a homogenous mixture. The viscosity of the 
paste is optimized for the intended use of the paste, such as by modifying 
powder size and powder distribution. The paste is then used for soldering 
employing known methods. 
As is known in the art, one or both of the metal surfaces to be soldered 
may optionally be plated with the solder material, such as by known 
electrolytic processes, prior to the solder operations in order to improve 
the adhesion of the solder to the metal surface. For example, the copper 
pads on a printed wiring board may be plated with the solder material and 
then the solder of the present invention is applied to the plated surface 
to form a solder joint using known techniques. The metal surface is 
preferably plated with the doped lead-tin eutectic surface of the present 
invention, but may alternatively be plated with an undoped, commercially 
available solder. Example 12 presents data showing the improved results 
obtained by using the doped solder of the present invention on a plated 
metal surface. 
The doped lead-tin solder in accordance with the present invention is used 
in the same manner as conventional lead-tin solder. The doped solder is 
well-suited for connecting wires, pins and other electrical 
interconnectors together. The preferred use for the doped solder is in 
providing joints which are subjected to continual thermal or mechanical 
stress and strain. However, the doped solder in accordance with the 
present invention may be used to replace lead-tin solder wherever a 
strong, solid and fatigue resistant solder joint is required.

Examples of practice are as follows. 
EXAMPLE 1 
This example describes the formation and testing of a high purity (99.999%) 
lead-tin eutectic solder containing 62.0 weight % tin and 37.0 weight % 
lead doped with 1% cadmium. Doping was carried out by dry mixing a 
sufficient amount of high purity (99.999%) cadmium solid with appropriate 
amounts of high purity (99.999%) lead and tin granules to provide a 1.0 
weight % cadmium level in the solder mix. The cadmium was then intimately 
alloyed in the molten eutectic solder at about 250.degree. C. for a period 
of at least 30 minutes in a controlled argon atmosphere to insure uniform 
distribution of the dopant. The cadmium doped eutectic solder displayed 
solder characteristics which are equivalent to the undoped eutectic 
solder. A dog-bone shaped tensile specimen was fabricated and mechanically 
tested using a testing machine obtained from Instron of Canton, Mass., at 
room temperature. The cadmium doped specimen was loaded into the machine. 
A cyclic sawtooth stress waveform at 0.001 Hertz (Hz) was applied with a 
peak tensile stress at about 110% of the generic yield of the lead-tin 
solder at room temperature. The cadmium doped dog-bone specimen underwent 
35 cycles prior to failure. An identical dog-bone shaped specimen was 
prepared from undoped high purity (99.999%) lead-tin eutectic solder. The 
undoped eutectic solder lasted only two cycles prior to failure when 
subjected to the same tensile fatigue conditions. 
EXAMPLE 1 
High purity (99.999%) lead-tin eutectic solder was doped with 0.8 weight 
cadmium in the same manner as described in Example 1, except that the 
cadmium was alloyed in the molten eutectic whose surface was always fully 
covered by a layer of rosin mildly activated (RMA) solder flux to prevent 
oxidation. Alloying was performed at a temperature of about 250.degree. C. 
over a period of about 8 hours in a flowing nitrogen atmosphere. The 
addition of the 0.8 weight % cadmium dopant did not alter the soldering 
characteristics of the eutectic solder. Torsion test specimens made of 
cylindrical copper rods joined by the solder were prepared. These 
specimens were subjected to torsional (shear) fatigue testing of the 
solder joint with the Instron testing machine at room temperature. The 
cycling frequency was 0.01 Hz and the plastic strain range per cycle 
applied on the solder was approximately 10%. The average cycles-to-failure 
for the 0.8% cadmium doped eutectic solder was 34. The average 
cycles-to-failure for an identical torsion test specimen of undoped 
high-purity lead-tin eutectic solder was 19. 
EXAMPLE 3 
High-purity lead-tin eutectic solder was doped with 0.4 weight % cadmium in 
the same manner as Example 2. Torsion test specimens were also prepared 
and subjected to testing in the same manner as Example 2. The specimens 
failed after an average of 32 cycles. 
EXAMPLE 4 
High-purity lead-tin eutectic solder was doped with 0.2 weight % cadmium 
following the procedure set forth in Example 2. Torsion test specimens 
were also prepared and subjected to testing under the same conditions as 
Example 2. The average cycles-to-failure for the specimens was 34. 
EXAMPLE 5 
A doped high-purity lead-tin eutectic solder dog-bone specimen was prepared 
in the same manner as Example 1 except that 1% indium was substituted for 
cadmium. The tensile testing conditions to establish cycles-to-failure 
were the same as in Example 1. The indium doped sample lasted five cycles 
before failing. 
EXAMPLE 6 
A doped high-purity lead-tin eutectic solder dog-bone specimen was prepared 
and tested in the same manner as Examples 1 and 5, except that antimony 
was used as the % dopant. The cycles-to-failure for the antimony doped 
lead-tin eutectic solder was 4. 
EXAMPLE 7 
High-purity lead-tin eutectic solder was doped with 0.1 weight % cadmium 
and 0.1 weight % indium in the same manner as Examples 2, 3 and 4. The 
same torsional fatigue testing procedure as set forth in Example 2 was 
used. The average cycles-to-failure for this combined cadmium-indium doped 
eutectic solder was 53. 
EXAMPLE 8 
High-purity lead-tin eutectic solder was doped with 0.2 weight % cadmium 
and 0.2 weight % indium in the same manner as in Examples 2, 3, 4, and 7. 
The same torsional fatigue testing procedure as set forth in Example 2 was 
used. The average cycles-to-failure for the combined cadmium-indium doped 
eutectic solder was 120. 
EXAMPLE 9 
Commercial-purity lead-tin eutectic solder which was free of silver and 
gold (Kester Sn63, QQ-S-571 obtained from Kester Solder of Des Plaines, 
Ill.) was doped with 0.1 weight % cadmium and 0.1 weight % indium. 
Alloying was conducted in the same manner as in Examples 2, 3, 4, 7 and 8. 
The same torsional fatigue testing procedure as set forth in Example 2 was 
used. The average cycles-to-failure for the combined cadmium-indium doped 
eutectic solder was 71. The average cycles-to-failure for an identical 
torsion test specimen of undoped commercial-purity eutectic solder (Kester 
Sn63, QQ-S-571) was 18. 
EXAMPLE 10 
The same commercial-purity lead-tin eutectic solder as used in Example 9 
was doped with 0.2 weight % cadmium and 0.2 weight % indium. Alloying was 
conducted in the same manner as in Examples 2, 3, 4, 7, 8, and 9. The same 
torsional fatigue testing procedure as set forth in Example 2 was used. 
The average cycles-to-failure for this combined cadmium-indium doped 
commercial-purity eutectic solder was 68. 
EXAMPLE 11 
The same commercial-purity lead-tin eutectic solder as used in Example 9 
was doped with 0.4 weight % cadmium and 0.4 weight % indium. Alloying was 
conducted in the same manner as in Examples 2, 3, 4, 7, 8, 9, and 10. The 
same torsional fatigue testing procedure as set forth in Example 2 was 
used. The average cycles-to-failure for this combined cadmium-indium doped 
commercial-purity eutectic solder was 101. 
EXAMPLE 12 
This example presents comparative data on the thermal fatigue resistance of 
the solder of the present invention and of a conventional solder when used 
to solder metal surfaces which have been previously plated with lead-tin 
solder. 
A solder paste was prepared in accordance with the present invention 
containing 0.4 weight percent cadmium and 0.4 weight percent indium. A 
control paste was prepared having essentially the same composition, except 
that the solder contained no cadmium or indium. 
Surface-mount assemblies were fabricated with both solder pastes in the 
same manner and consisted of 68-pin ceramic packages (0.95 inch by 0.95 
inch or 2.4 cm by 2.4 cm) surface mounted to polyimide-glass printed 
wiring boards. The copper pads on the printed wiring boards were initially 
solder plated with Sn-Pb eutectic alloy to a thickness of about 0.3 mils 
or 0.0008 cm before fusing. The solder joints in the assemblies were 
subsequently produced by vapor phase reflow processing using a peak 
temperature of about 216.degree. C. 
Assemblies consisting of joints fabricated from the solder paste of the 
present invention and joints fabricated from the control solder were 
tested under the same thermal cycling conditions: -55.degree. C. to 
125.degree. C. and back to -55.degree. C., 5.degree. C./minute transition 
rate, 30 minutes dwell as 125.degree. C. and 15 minutes dwell at 
-55.degree. C. Comparison between the thermal fatigue failure data (i.e. 
visible solder cracks) obtained from both types of solder joints revealed 
a significantly higher fatigue resistance of the solder thermal cycles, 
544 joints of each solder type were examined. The failure population of 
the solder joints formed from the solder paste of the present invention 
was only 0.14 times that of the transitional eutectic solder joints of the 
control. This a seven-fold improvement in the thermal fatigue resistance 
which is achieved by the present invention. 
As is apparent from the above examples, doping of lead-tin eutectic solders 
in accordance with the present invention substantially increases the 
resistance of the solder to fatigue failure. Further, the combination of 
cadmium and indium to provide a total dopant level of less than 0.5 weight 
% provides an even further increase in fatigue resistance which is not 
obtained when a single dopant is used. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations and 
modifications may be made within the scope of the present invention. 
Accordingly, the present invention is not limited to the specific 
embodiments as illustrated herein, but is only limited by the following 
claims.