Exclusive Series, Part 4 of 8
By Dr. Jennie S. Hwang and Dr. Zhenfeng Guo, H-Technologies Group Inc., Cleveland, Ohio Acommon surface coating on component leads and PWB pads is Sn/Pb solder. 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 instantaneous under common soldering conditions. To simulate this condition, we investigated the effects of Pb contamination in lead-free Sn/Ag/Cu/Bi solder by intentionally doping the lead-free solder with a small dose of Pb. Considering the thickness of the common surface coating, we have examined a dosage up to 0.5%. Experimental Protocol We prepared solder compositions by melting, in sequence, commercial solder alloys and pure metals in a Pyrex glass beaker in an electric resistance chamber furnace. The master solder alloys of 99.3Sn/0.7Cu, 95Sn/5Cu, 93.5Sn/6.5Ag, 42Sn/58Bi, and 63Sn/37Pb (in a bar form) were used. The experimental protocol and test procedures were the same as outlined in Part 1 of this series (January-February issue). Pb at 0.1 %, 0.2 % and 0.5 % was added to the optimum lead-free solder composition (Alloy 367: 93.3Sn/3.1Ag/ 3.1Bi/0.5Cu) as representative of a Sn/Ag/ Cu/Bi 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. Also included in the table is the reference alloy of 63Sn/37Pb. Melting Temperature As seen in the table, the Pb addition, in small amounts, had no notable influence on the alloy melting temperatures.
Strength and Plasticity Figure 1 compares the tensile stress (σ) vs. strain (ε) curves of Alloy 367c: 93.2Sn/3.1Ag/3.1Bi/0.5Cu/0.1Pb, Alloy 367b: 93.1Sn/3.1Ag/3.1Bi/0.5Cu/0.2Pb, and Alloy 367a: 92.8Sn/3.1Ag/3.1Bi/ 0.5Cu/0.5Pb with Alloy 367: 93.3Sn/3.1Ag/ 3.1Bi/0.5Cu 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 is caused by an inhomogeneous plastic deformation somewhere in 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 fractures for high temperature plastic deformation. As shown in Figure 1, the Pb-free solder Alloy 367: (93.3Sn/3.1Ag/3.1Bi/0.5 Cu) had a superior strength to 63Sn/37Pb. When Pb is added to Alloy 367, there was no apparent effect on alloy strength, but the alloy plasticity was reduced. Low Cycle Fatigue Life The presence of lead in the Sn/Ag/Cu/Bi system was found to reduce the low cycle fatigue life (Nf) significantly. Figure 2 shows the low cycle fatigue life (Nf) vs. Pb dosage over 0.1-0.5%. Discussion The maximum solid solubility of Pb in Sn is 2.5.% and the solid solubility of Pb in Sn at room temperature (300°K) approaches zero. It is thus expected that the Pb atoms in Sn/Ag/Cu/Bi 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 had little effects on alloy strength, as reflected by the test results in the table.
With the presence of the softer Pb particles in the 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 concentrate on the soft Pb inclusions, accounting for the deterioration of the fatigue life. Additionally, the distribution of Pb second phase in the microstructure should also play a role in the fatigue mechanism. It is postulated that a small amount of Pb precipitates tend to reside preferably at the grain boundaries, contributing to an early grain boundary fracture. Conclusion The contamination of Pb up to 0.5% in 93.3Sn/3.1Ag/3.1Bi/0.5Cu did not effect the alloy melting temperature. The impact of the contamination on the strength of Sn/Ag/Bi/Cu composition was also negligible. However, the Pb addition reduced the alloy plasticity and fatigue life. * J. S. Hwang, Chapter 27, Environment-Friendly Electronics: Lead-Free Technology, Electrochemical Publications Ltd., Great Britain, 2001.
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