Exclusive Series, Part 7 of 8
By Drs. Jennie S. Hwang and Zhenfeng Guo, H-Technologies Group Inc., Cleveland, Ohio When a lead-free solder alloy is soldered onto the Pb-containing surface, Pb will contaminate the lead-free solder alloy through a metallurgical reaction. This represents fundamentally a secondary alloying process that is almost instantaneous under common soldering conditions. We investigated the effects of Pb contamination in lead-free Sn/Ag/Cu/Bi/In and Sn/Cu/In/Ga solder by intentionally doping the lead-free solder with a small amount of Pb. In our study, we have examined a Pb dosage that accounts for up to 0.5% of the surface coating. Experimental The experimental protocol and test procedures are same as outlined in Part 1 (Chip Scale Review, January/February 2001). Results for the SnAgBiCuIn System* Pb at 0.1%, 0.2% and 0.5% was added to the optimum lead-free solder composition (alloy 322: 82.3Sn/3Ag/2.2Bi/0.5Cu/ 12In) as representative of a Sn/Ag/Bi/ Cu/In system. The resulting solder compositions, along with their melting temperatures (Tm), yielded 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%, and are summarized in Table 1. All compositions are expressed in weight percent, unless otherwise specified. (Also included in Table 1 is the reference alloy of 63Sn/37Pb.)
Melting Temperature As seen in Table 1, the addition of small amounts of Pb caused no notable influence on the alloy melting temperatures. Strength and Plasticity Figure 1 compares the tensile stress (σ) vs. strain (ε) curves of SnAgBiCuIn alloys with and without Pb contamination. When Pb was added to alloy 322: 82.3Sn/3Ag/2.2Bi/ 0.5Cu/12In, there was no significant effect on alloy strength, nor on the plasticity. Low-Cycle Fatigue Life The presence of lead in an Sn/Ag/Cu/ Bi/In system largely reduced the low-cycle fatigue life (Nf). However, the fatigue life of the Pb-contaminated solder in the Sn/Ag/Cu/Bi/In system was still higher than that of 63Sn/37Pb (Table 1). Results for the SnCuInGa System* Pb at 0.1 %, 0.2 % and 0.5 % was added to the optimum lead-free solder composition (alloy 719: 92.8Sn/0.7Cu/6In/0.5Ga) as representative of a Sn/Cu/In/Ga system. The testing results are summarized in Table 2 with the reference alloy of 63Sn/37Pb.
Melting Temperature As seen in Table 2, the addition of Pb in small amounts produced no notable difference on the alloy melting temperatures. Strength and Plasticity Figure 2 compares the tensile stress (σ) vs. strain (ε) curves of SnCuInGa alloys with and without Pb contamination. When Pb was added to alloy 719, there was no apparent effect on alloy strength, but alloy plasticity was largely reduced. Low-Cycle Fatigue Life The presence of lead in the Sn/Cu/In/Ga system reduced the low-cycle fatigue life (Nf). Summary and Conclusion The contamination of Pb up to 0.5% in 82.3Sn/3Ag/2.2Bi/0.5Cu/12In produced no apparent effect on the alloy melting temperature. Its impact on the strength and plasticity of the Sn/Ag/Cu/Bi/In composition is also negligible. However, the Pb addition reduced the alloy fatigue life. Nevertheless, the fatigue life of the Pb-contaminated alloy in Sn/Ag/Cu/Bi/In system is still higher than that of 63Sn/37Pb. The contamination of Pb up to 0.5% in alloy 719: 92.8Sn/0.7Cu/6In/0.5Ga produced no apparent change on the alloy melting temperature. Its impact on the strength of Sn/Cu/In/Ga composition is also negligible. However, the Pb addition reduced the alloy plasticity and fatigue life. * J.S. Hwang, Environment-Friendly Electronics: Lead-Free Technology, Electrochemical Publications, Great Britain, 2001.
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