Exclusive Series, Part 6 of 8
By Dr. Jennie S. Hwang and Dr. 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 reaction is fundamentally a secondary alloying process and is almost instantaneous under the common soldering conditions. To simulate this problem and to investigate the effects of Pb contamination in lead-free Sn/Ag/Bi/In solder, we doped the lead-free solder with a small amount of Pb. Considering the thickness of the common surface coating, we have experimented with a dosage up to 0.5%. Experimental Protocol Solder compositions were prepared by melting, in sequence, commercial solder alloys and pure metals in a heat-resistant glass beaker in an electric resistance chamber furnace. The master solder alloys of 93.5Sn/6.5Ag, 42Sn/58Bi, 50Sn/50In, Sn, and 63Sn/37Pb (in a bar form) were selected. The experimental protocol and test procedures are same as those outlined in Part 1 (Chip Scale Review, January-February 2001). The resulting solder compositions, along with their melting temperatures (Tm), yield strengths (σY), tensile strengths (σTS), Young's modulus (ε), plastic strains (εp) at fracture and fatigue lives (Nf) at a total strain range of 0.2% are summarized in the table. Results Pb at 0.1%, 0.2% and 0.5% were added to the optimum lead-free solder composition (alloy AC: 90Sn/3.3Ag/3Bi/3.7In) as representative of a Sn/Ag/Bi/In system.* Strength and Plasticity Figure 1 compares the tensile stress (σ) vs. strain (ε) curves of alloy AC2: 89.9Sn/ 3.3Ag/3Bi/3.7In/0.1Pb, alloy AC1: 89.8Sn/ 3.3Ag/3Bi/3.7In/0.2Pb, and alloy AC+: 89.5Sn/3.3Ag/3Bi/3.7In/0.5Pb with those of both alloy AC: 90Sn/3.3Ag/3Bi/3.7In and 63Sn/37Pb. As observed in Figure 1, the Pb-free solder alloy, AB, possessed a superior strength to 63Sn/37Pb. When Pb was added to alloy AB, there was no apparent effect on alloy strength, but the alloy plasticity was significantly reduced. Low-Cycle Fatigue Life The presence of lead in the Sn/Ag/Bi/In system largely reduced the low-cycle fatigue life (Nf). Figure 2 shows the low-cycle fatigue life (Nf) vs. the Pb dosage. We found that Pb at 0.1% caused an abrupt drop in fatigue life, but that it recovered somewhat when the Pb dosage was increased toward 0.5%. Discussion We expect that the Pb atoms in an Sn/Ag/Bi/In system will likely precipitate 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 cause few effects on alloy strength. With the softer Pb particles present in an Sn-matrix, plastic deformation tends to concentrate on these particles, which may cause an early fracture leading to the plasticity reduction. The fatigue cyclic strain will also be concentrated on the soft Pb inclusions, accounting for the deterioration of the 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, causing early grain boundary fracture.
Conclusion The contamination of Pb up to 0.5% in 90Sn/3.3Ag/3Bi/3.7In produced no apparent effect on the alloy melting temperature. Its impact on the strength of Sn/Ag/Bi/In composition was also negligible; however, the Pb addition reduced alloy plasticity and fatigue life. * J.S. Hwang, Environment-Friendly Electronics - Lead-Free Technology, Electrochemical Publications, Great Britain, 2001.
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