Low temperature electrical solder compositions (by weight percent) having between 45-60% Sn; 25-40% Pb; 5-15% Bi; and 0.5-2.5% In. Preferably, the solder compositions have a melting temperature of about 154.degree.-162.degree. C. The solder compositions have microstructure similarly to Sn/Pb eutectic microstructure which makes them have excellent properties like higher yield strength and better creep resistance providing long term reliability to solder joints.

TECHNICAL FIELD 
The present invention relates to an electrical solder composition 
comprising tin (Sn), lead (Pb), bismuth (Bi), and indium (In). More 
specifically, the present invention relates to an electrical solder 
composition having between 45-60% Sn; 25-40% Pb; 5-15% Bi, and 0.5-2.5% 
In. Preferably the solder has a melting point of about 
154.degree.-162.degree. C. All percentages are by weight. 
BACKGROUND OF THE INVENTION 
In electronics manufacturing, solders provide inexpensive, mass-producible 
and generally reliable interconnections to complete the electronic 
circuitry between various elements that make up an electronic assembly. 
Solder joints provide electrical interconnections and serve as the 
mechanical attachment of the electronic components to the printed circuit 
board and also serve a critical heat transfer function as well. Soldering 
make these connections at temperatures just below those that would cause 
damage to some of the elements of the assembly and substrate materials. 
Reflow soldering is the predominant soldering method for Surface Mount 
Technology (SMT) assemblies. This technique provides an opportunity to 
handle a wider range of electronic assemblies than were possible with wave 
soldering. Eutectic solder, 63% Sn/37% Pb solder (melting temperature 
being 183.degree. C.) is commonly used for reflow soldering with peak 
reflow temperature of 210.degree.-220.degree. C. In order to use low cost 
thermoplastic substrate materials and some temperature-sensitive 
components for integration of many of such components in a small area as 
perhaps attached to an automotive dashboard, it is required to reflow at 
lower peak temperatures than 220.degree. C. 
It is known that eutectic 63 Sn/37 Pb solder joints are reliable during 
thermal cycling between -40.degree. C. and 125.degree. C. Since maximum 
temperature for thermal cycling in integration applications is 105.degree. 
C. instead of 125.degree. C., a low temperature solder based on tin-lead 
eutectic and having a melting temperature 20.degree. C. lower than that of 
63 Sn/37 Pb is likely to survive thermal cycling between -40.degree. C. 
and 105.degree. C. Low temperature solder compositions such as 43 Sn/43 
Pb/14 Bi or 52 Sn/48 Bi (U.S. Pat. No. 5,573,602) can be reflowed at a 
peak temperature of 180.degree. C. But these solder compositions have wide 
pasty ranges (up to 20.degree. C.) between solidus and liquidus 
temperatures which can lead to damage of the solder joints during reflow 
soldering/cooling process. Also, such solders which might include a 
significant amount of indium to reduce the melting point greatly increase 
the cost of the solder. 
It would be desirable to provide new low temperature solders with a melting 
temperature of less than 165.degree. C. and with a narrow pasty range, 
preferably being about 5.degree. C., which can provide long term 
reliability of solder joints and which is priced to be commercially useful 
for large volume applications. The present invention provides such solder 
compositions. 
SUMMARY OF THE INVENTION 
The present invention is related to an electrical solder composition having 
between 45-60% Sn; 25-40% Pb; 5-15% Bi; and 0.5-2.5% In. Preferably the 
solders have melting temperature of about 154.degree.-162.degree. C. All 
percentages are by weight. These solders have been found to have 
microstructures very similar to Sn/Pb eutectic microstructure with some 
primary Pb-phase dendrites and Bi dissolution in Pb-phase and In 
dissolution in Sn-phase. Near eutectic microstructure of these solders 
also causes higher yield strength and better creep resistance, and thus 
provides long term reliability to the solder joints. 
Advantageously, because of the low amount of indium incorporated in the 
present invention solders, these invention solders are considerably less 
costly than currently available low temperature solders. Hence these 
solders advantageously is affordable for large volume use as necessary in 
automotive applications. 
It is also a desired feature of the present invention to provide a solder 
exhibiting favorable thermal and electrical conductivity and satisfactory 
mechanical properties. These and other advantages, features and objects of 
the present invention will become more apparent to those of ordinary skill 
in the art upon reference to the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention solder compositions include the components: 45-60 % tin, 
25-40% lead, 5-15% bismuth and 0.5-2.5% indium. Two particularly preferred 
embodiments of such compositions have the proportions 51 Sn/39 Pb/8.5 
Bi/1.5 In and 50 Sn/37 Pb/10.5 Bi/2.5 In, respectively, of such 
components. 
The present invention is directed to a low-temperature solder that has a 
narrow pasty range. Having a narrow pasty range allow for fast set up of 
the solder so that components can be made rapidly without damage to the 
solder joints. Preferably, the pasty range is about 5.degree. C. The 
invention solder also has good fatigue and creep resistance. 
The solder is especially useful to provide a durable and damage resistant 
electrical interconnect for electronic components exposed to wide 
temperature variations as is commonly found in automotive applications. A 
common automotive test for measuring the durability of a solder is to 
expose the interconnect to a large temperature variation, typically from 
-40.degree. to 105.degree. C. in applications where components are mounted 
to thermoplastic substrates in the integration systems mentioned above. As 
disclosed above, an object of the invention is to provide solders useful 
in electronic circuit integration systems in automotive vehicles. In this 
accelerated test, temperature variation is repeated more than one thousand 
times. Each cycle from hot to cold causes the substrate, electronic 
component, metal leads and the solder to expand and contract (often with 
widely different coefficients of thermal expansion). 
The repeated cycle of hot and cold may cause fatigue in the solder 
interconnect and weaken the attachment. If the weakening is extensive, the 
interconnect can fail by solder cracking and the component becomes 
inoperable. While not wishing to be bound by the following theory, it is 
believed that the thermal cycling causes the microstructure of the solder 
to coarsen. The coarser grain structure can lead to reduced fatigue 
strength and makes the solder more susceptible for crack formation and 
propagation under thermomechanical loading. The invention solder overcomes 
this problem it is believed by reducing the rate of grain 
growth/coarsening with bismuth and indium additions in small quantities. 
However, neither the accuracy nor understanding of this theory is 
necessary for practice of the present invention. 
Fabrication of Sample Compositions 
Solder samples were fabricated using well-established methods. High purity 
metals were used as starting materials. These included 99.99+% Sn and Pb 
shots, 99.99+% Bi and In wire bits. These ingredients were mixed in 
pre-determined proportions. Alumina crucibles were used. The ingredients 
were melted in a tube furnace, under a flowing N.sub.2 atmosphere to 
prevent oxidation. The alloy was kept in a molten state for up to 20 
minutes and stirred for homogenization. Furnace temperature was measured 
with thermocouples and recorded with a chart recorder. 
The following samples were made by mixing respective starting metals in the 
proportions shown below. 
TABLE 1 
______________________________________ 
Solder Alloys (weight percentages) 
Micro 
Alloy Hardness 
Melting 
Sample No. 
Sn Pb Bi In (KHN) Temp (.degree.C.) 
______________________________________ 
1 51 39 8.5 1.5 15.4 157/162 
2 50 37 10.5 2.5 16.4 154/159 
______________________________________ 
Samples were taken from each alloy ingot and submitted for chemical 
analysis. Results indicate that the final alloy composition is close to 
the starting composition. 
Differential Scanning Calorimetric (DSC) Analysis 
Samples were taken for each alloy ingot for DSC analysis. The melting 
temperatures of the alloys in Table 1 were determined by differential 
scanning calorimetric (DSC) analysis. The analysis was performed using a 
DuPont DSC 2910 system under a flowing N.sub.2 atmosphere at a given 
heating/cooling rate (5.degree. C./minute). The characteristic 
temperatures from the DSC curves were analyzed to determine the melting 
temperatures of the alloys, which are summarized in Table 1 above. 
Microstructural Analysis 
Microstructural analysis was performed using chemical etching, optical 
microscopy, scanning electron microscope (SEM) microprobe, and x-ray 
diffraction (XRD). Suitable etchants for the disclosed alloys include a 
solution of diluted hydrochloric acid. 
The microhardness of these alloys have been measured on a microhardness 
tester. Thirty measurements were taken for each sample using a 25 g load 
for 0.5 seconds and the average was taken for each sample. The results of 
the hardness (expressed as a Knoop hardness number or KHN) are summarized 
in Table 1 above. 
The alloys exhibit a eutectic microstructure consisting of the Pb-phase 
dendrites and Bi dissolution in Pb-phase and In dissolution in Sn-phase. 
During thermal cycling the grain growth of these phases is very limited. 
Thermal cycling is a process of aging the soldered component between 
temperature extremes (typically -40.degree. to 105.degree. C.). The 
dissolution of bismuth and indium in these phases make the solder more 
resistant to crack formation and propagation in the interconnect. This 
long term fine-grain microstructure is believed to be key to durability by 
increasing the creep resistance and the fatigue strength because the fine 
grain structure resists crack propagation and reduces the likelihood of 
fracture. Concentrations higher than 15% Bi or 2.5% In are undesirable 
because they will reduce the benefits of eutectic microstructures and will 
cause formation of coarse grain structure. Thus, another feature of the 
present solders is their stability after thermal aging. This long term 
thermal stability is essential for solder interconnects used in automotive 
applications. 
Application 
The alloys made according to the present invention were found to be readily 
manufacturable and utilized existing commercial equipment for 
manufacturing and usage. Because of the relatively low indium content, the 
alloys were low cost and suitable for production in large commercial 
quantities. 
The solders made from these compositions may be used in either a paste form 
(as in reflow soldering) or alternatively in bar solder form (as in wave 
soldering). Regular soldering processes (reflow, wave soldering and 
others) may be used with these solder compositions. In each case, the 
soldering peak temperature will generally be at least 
10.degree.-15.degree. C. or preferably 15.degree.-30.degree. C. above the 
melting temperature of the solder alloy. These solder alloys are also 
compatible with conventional flux systems such as no-clean flux. 
The most preferred alloy embodiment are shown in Table 1. The various 
embodiments of the invention alloys, especially the preferred embodiments, 
may be adapted for further improvement of mechanical and physical 
properties by the inclusion of small quantities (less than 1% by weight) 
of other elements such as Sb, Co, Si, Mg, Ca, Se, Cs, Ce, Te, Au, Ni, Cu 
and Zn. The addition of up to 2.5% Au improves ductility and strength of 
the alloy. 
While the best mode for carrying out the present invention has been 
described in detail, those familiar with the art to which the invention 
relates will recognize various alternative designs and embodiments for 
practicing the invention as defined by the following claims.