Exclusive Series, Part 5 of 8
By Dr. Jennie S. Hwang and Dr. Zhenfeng Guo, H-Technologies Group Inc., Cleveland, Ohio One of the common surface coatings on component leads and the PWB pads is Sn/Pb solder coating. As the industry begins to convert to lead-free solders, there is uncertainty about the compatibility between the lead-free solder joint and the Sn/Pb coating. The uncertainty is present because coating may still be used on the surface of the component leads and PWB pads during the initial period of lead-free implementation. Lead-Free Alloy When a lead-free solder alloy is soldered on the Pb-containing surface, Pb will contaminate the lead-free solder alloy through a metallurgical reaction. This reaction is fundamentally a secondary alloying process and is almost immediate under the common soldering conditions. To simulate this problem, this study investigated the effects of Pb contamination in lead-free Sn/Ag/Cu/In solder by intentionally doping the lead-free solder with a small dosage of Pb. Considering the thickness of the common surface coating, a dosage up to 0.5% was examined. Experimental Solder compositions were prepared by melting, in sequence, commercial solder alloys and pure metals in a heat-resistent glass beaker in an electric resistance chamber furnace. The master solder alloys of 99.3Sn/0.7Cu, 95Sn/5Cu, 93.5Sn/6.5Ag, 50Sn/50In and 63Sn/37Pb, all in bar form were used. The experimental protocol and test procedures were as outlined in Part 1 (Chip Scale Review, January/February 2001). Results Pb at 0.1 %, 0.2 % and 0.5 % was added to the optimum lead-free solder composition (Alloy 370: 88.5Sn/3Ag/0.5Cu/8In) as representative of a Sn/Ag/Cu/In system. The resulting solder compositions, along with their melting temperatures (Tm), yield strengths (σy), tensile strengths (σTS), Young's modulus (E), plastic strains (εp) at fracture and fatigue lives (Nf) at a total strain range of 0.2% are summarized in the table. All compositions are expressed in weight percent, unless otherwise specified. The reference alloy, 63Sn/37Pb, is also included in the table. Melting Temperature As shown in the table, the Pb addition in small amounts produced no noticeable effects on the alloy melting temperatures.
Strength and Plasticity Figure 1 compares the tensile stress (σ) vs. strain (ε) curves of alloy 3702: 88.4Sn/ 3Ag/0.5Cu/8In/0.1Pb, alloy 3701: 88.3Sn/ 3Ag/0.5Cu/8In/0.2Pb, and alloy 370+: 88Sn/3Ag/0.5Cu/8In/0.5Pb with alloy 370: 88.5Sn/3Ag/0.5Cu/8In and 63Sn/37Pb. The tensile flow of solder alloys typically consists of an elastic region, a strain hardening region, a stress-recovery region and a cracking region. Strain hardening continued until necking occurred at the maximum load or the tensile strength (σTS). Necking was caused by an inhomogeneous plastic deformation. This occurred somewhere along the gauge length and is associated with strain localization. Stress-recovery mechanisms are believed to be dominant in the region after necking and before abrupt fracture for high temperature plastic deformation. When Pb is added to alloy 370: 88.5Sn/ 3Ag/0.5Cu/8In, there was no apparent effect on alloy strength, nor on plasticity. Overall, the strength and plasticity of the Pb-contaminated solders in Sn/Ag/Cu/In system still surpassed those of 63Sn/37Pb. Low-Cycle Fatigue Life The presence of lead in the Sn/Ag/Cu/In system slightly reduced the low-cycle fatigue life (Nf). Figure 2 shows the low- cycle fatigue life (Nf) vs. Pb dosage. We found that Pb at 0.1% caused a drop in fatigue life, and that the fatigue life somewhat recovered when the Pb dosage increased towards 0.5%. However, the fatigue life of the Pb-contaminated solder in the Sn/Ag/Cu/In system was still significantly higher than that of 63Sn/37Pb, as was the strength. Discussion The Pb atoms in the Sn/Ag/Cu/In system will likely precipitate out as second-phase particles. Since the strength is affected by the second phase following the mixture rule, such a small amount of soft Pb particles causes few effects on alloy strength. Because of the relatively ductile nature of the Sn-based matrix in the Sn/Ag/Cu/In system, the effect of the second phase was not as pronounced as in a brittle matrix. This may explain the negligible effect of Pb particles on the alloy plasticity, as reflected by the test results in the table. With the presence of the softer Pb particles in the Sn matrix, the fatigue cyclic strain is expected to concentrate on the soft Pb inclusions, accounting for the deterioration of fatigue life. Additionally, the distribution of Pb second phase in microstructure should also play a role in the fatigue mechanism. We postulate that a small amount of Pb precipitates tend to reside preferably at the grain boundaries, contributing to early grain boundary fracture. Summary and Conclusion The contamination of Pb up to 0.5% in 88.5Sn/3Ag/0.5Cu/8In produces no apparent effect on the alloy melting temperature. The impact of this contamination on the strength and plasticity of the Sn/Ag/ Cu/In composition is also negligible. However, the Pb addition has reduced the alloy fatigue life. Nevertheless, the fatigue life of the Pb-contaminated alloy in Sn/Ag/Cu/In system is still significantly superior to 63Sn/37Pb. In other words, this Sn/Ag/Cu/In system has enough capacity to absorb contaminants such as Pb without experiencing alarming performance deterioration. * J.S. Hwang, Environment-Friendly ElectronicsÐLead-Free Technology, Electrochemical Publications, Great Britain, 2001.
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