By Dr. Jennie S. Hwang and Dr. Zhenfeng Guo, H-Technologies Group Inc., Cleveland, Ohio Sn/Pb solder coating is a common surface coating for component leads and PWB pads. As some segments of the industry begin to convert to lead-free solders, there is uncertainty as to the compatibility between the lead-free solder joints and the PWB, which both employ this same coating. 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 these common soldering conditions(1, 2). To simulate this problem, we investigated the effects of Pb contamination in lead-free Sn/Ag/Cu solder by intentionally doping the lead-free solder with a small amount of Pb. In considering the thickness of the common surface coating, we examined a dosage up to 0.5% Experimental Solder compositions were prepared by melting, in sequence, commercial solder alloys and pure metals in a glass beaker in an electric resistance chamber furnace. The master solder alloys of 99.3Sn/0.7Cu, 95Sn/5Cu, 93.5Sn/6.5Ag and 63Sn/37Pb (in a bar form) were employed. Alloying procedures were as follows: 1. The furnace was heated and controlled to a temperature of 500°C in air. 2. Master solder alloys with the highest melting points were initially melted for 10 minutes. 3. Pure metals and solder alloys with lower melting points were then sequentially dissolved in the molten2 alloys, one by one. The mixtures were stirred thoroughly. 4. The resulting alloyed mixtures were homogenized at temperature for 20 minutes before casting. As an example, the procedures for alloying the composition of 94.9Sn/1.5Cu/ 3.1Ag/0.5Pb (Alloy 363+) are shown in Figure 1.
A cast mold was made of a stainless steel and configured for making mechanical specimens for mechanical testing. This cast mold was pre-heated to 150°C on an electric hot plate before casting. During casting, the mold was maintained at 150°C. All test specimens were produced in accordance with ASTM Standard E8-96 (Standard Test Methods for Tension Testing of Metallic Materials), ASTM Standard E139-95 (Standard Practice for Conducting Creep, Creep-Rapture, and Stress-Rupture Tests of Metallic Materials) and ASTM Standard E606-92 (Standard Practice for Strain-Controlled Fatigue Testing).
Tests Standard tensile and low-cycle fatigue tests were run in uniaxial tension at room temperature (300K), using an electro-mechanical material testing system. The load was measured with a static load cell of 5kN capacity, and strain was measured using a 25 mm gauge length extensometer. Tensile tests were performed at a cross-head speed of 1 mm/minute, with mechanical property testing equipment and software, which yielded a strain rate of 6.56x10-4/second. Basic mechanical properties were determined in accordance with ASTM Standard E8-96. These included yield strength (σy) at 0.2% offset plastic strain, ultimate tensile strength (σTS) at maximum load, and plastic strain (εp) at fracture, after a drop in load to 10% of the maximum values. Low-cycle fatigue tests were carried out under strain control at a frequency of 0.1Hz and an R-ratio of 0.8. The fatigue life (Nf) was defined as a number of cycles at which the tensile load on the hysteresis loop of load vs. elongation was dropped to a value of 50% of the maximum load according to ASTM Standard E606-92. The melting temperatures of lead-free alloys were measured through cooling curves employing a thermocouple technique.
Results Pb at 0.1 %, 0.2 % and 0.5 % was added to one of the lead-free solder compositions (Alloy 363: 95.4Sn/3.1Ag/1.5Cu) as a representative of Sn/Ag/Cu 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 of 63Sn/37Pb is also included. As seen in the table, the Pb addition in small amounts had no noticeable influence on the alloy melting temperatures. Strength and Plasticity The figure compares the tensile stress (σ) vs. strain (ε) curves of Alloy 3632: 95.3Sn/ 3.1Ag/1.5Cu/0.1Pb, Alloy 3631: 95.2Sn/ 1.5Cu/3.1Ag/0.2Pb and Alloy 363+: 94.9Sn/ 3.1Ag/1.5Cu/0.5Pb with Alloy 363: 95.4Sn/ 3.1Ag/1.5Cu 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 was caused by an inhomogeneous plastic deformation somewhere in the gauge length, and 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. Alloy Strength When Pb is added to Alloy 363: 95.4Sn/ 3.1Ag/1.5Cu, there was no significant effect on alloy strength, but the plasticity was slightly reduced, particularly at the low dosage, 0.1-0.2 %. These phenomena can be observed better in Figure 2 by plotting Pb dosage vs. the yield strengths (σy), tensile strengths (σTS) and plastic strains (εp) at fracture. A minimum in plasticity appeared at 0.1% Pb. Overall, the strength and plasticity of the Pb-contaminated solders in the Sn/Ag/Cu system still surpassed those of 63Sn/37Pb (see Figure 1).
Low-Cycle Fatigue Life The presence of lead in the Sn/Ag/Cu system largely reduced the low-cycle fatigue life (Nf). Figure 3 shows the low- cycle fatigue life (Nf) vs. Pb dosage. It was found that Pb at 0.1% caused an abrupt 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 system was still higher than that of 63Sn/37Pb (as in the table). Discussion The maximum solid solubility of Pb in Sn, Ag and Cu is 2.5 %, 5.2% and 0.29%, respectively(3, 4). However, the solid solubility of Pb in Sn, Ag and Cu at room temperature (300K) approaches zero. We thus expect that the Pb atoms in the Sn/Ag/Cu 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 had little effect on alloy strength, as reflected by the test results in the table. With the presence of the softer Pb particles in 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 the Pb second phase in the micro-structure should also play a role in the fatigue mechanism. We can postulate that a small amount of Pb precipitates tend to reside preferably at the grain boundaries, causing early grain boundary fracture(5).
Conclusion The contamination of Pb up to 0.5% in 95.4Sn/3.1Ag/1.5Cu has no apparent effect on the alloy melting temperature. Its impact on the strength of Sn/Ag/Cu composition is also negligible; however, the Pb addition has reduced the alloy plasticity and fatigue life. References 1. J. S. Hwang and Z. Guo, "Lead-Free Solders for Elec-tronic Packaging and Assembly," Proc. SMI 1993, p.732 2. J. S. Hwang, "Solder Materials," SMT, March 2000, p.81. 3. M. Hansen and K. Anderko, The Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 4. J. S. Hwang, Modern Solder Technology for Competitive Electronics Manufacturing, McGraw-Hill, New York, 1996. 5. J. S. Hwang, Environmental-Friendly Electronics-Lead Free Assemblies, Electrochemical Publications Ltd., in press.
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