Exclusive Series, Part 2 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 as to the compatibility between the lead-free solder joint and the Sn/Pb coating that is placed on the surface of the component leads and pads. The ternary phase diagram of Sb-Pb-Bi indicates the presence of a ternary eutectic composition of 16Sn/32Pb/52Bi, with a melting temperature of 96°C. In addition, the two low-temperature binary eutectic reactions of 43Sn/57Bi at 139°C and 43.5Pb/ 56.5Bi at 125°C extend into the ternary domains, causing concerns about the effects of Sn/Pb coating on Bi-containing lead-free solder joints. When a lead-free solder alloy is soldered on the Pb-containing surface, Pb will contaminate the lead-free solder alloy through metallurgical reaction. This reaction is fundamentally a secondary alloying process and is almost instantaneous under the common soldering conditions. This study investigated the effects of Pb contamination in lead-free Sn/Ag/Bi 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 Protocol Solder compositions were prepared 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 93.5Sn/6.5Ag, 96.5Sn/3.5Ag, 42Sn/58Bi and 63Sn/37Pb (in a bar form) were used. Alloying procedures were as follows: 1. The furnace was heated and controlled to a temperature of 500°C. 2. All 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*, one by one, and the mixtures were thoroughly stirred. 4. The resulting alloyed mixtures were homogenized at temperature for 20 minutes before casting. An example of the procedure for alloying the composition of 91.5Sn/3.3Ag/4.7Bi/ 0.5Pb (alloy AB+), is shown in Figure 1. Casting A cast mold was made of stainless steel and configured for making mechanical specimens for mechanical testing. This mold was preheated to 150°C on a hot plate before casting.
During casting, the mold was maintained at 150°C. Each time the cast mold generated two standard round tension test specimens with a gauge diameter of 6 mm. 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-Rupture, and Stress-Rupture Tests of Metallic Materials) and ASTM Standard E606-92 (Standard Practice for Strain-Controlled Fatigue Testing). Standard tensile and low-cycle fatigue tests were run in uniaxial tension at room temperature (300°K) using an electromechanical material testing system. The load was measured using a static load cell of 5kN capacity. Strain was measured using an extensometer of 1-inch gauge length, and test data were acquired by the built-in software. Tensile Tests Specifically, tensile tests were performed at a cross-head speed of 1-mm/minute, which produced a strain rate of 6.56x10-4 /second. Basic mechanical properties were determined in accordance with ASTM Standard E8-96. These tests 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.
Fatigue Tests 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 the ASTM Standard E606-92. The melting temperatures of lead-free alloys were measured through cooling curves by a thermocouple technique. Results Pb at 0.1%, 0.2% and 0.5% was added to the optimum lead-free solder composition (alloy AB: 92Sn/3.3Ag/4.7Bi) as a representative of an Sn/Ag/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. The table also includes the reference alloy of 63Sn/37Pb. Melting Temperature As shown in the table, the Pb addition in small amounts produced no notable influence on the alloy melting temperatures. Strength and Plasticity Figure 2 compares the tensile stress (σ) to the strain (ε) curves of alloy AB2: 91.9Sn/3.3Ag/4.7Bi/0.1Pb, alloy AB1: 91.8Sn/3.3Ag/4.7Bi/0.2Pb, and alloy AB+: 91.5Sn/3.3Ag/4.7Bi/0.5Pb with alloy AB: 92Sn/3.3Ag/4.7Bi and 63Sn/37Pb. As observed in Figure 2, the Pb-free solder, alloy AB, had a superior strength to 63Sn/37Pb. When Pb is added to alloy AB, there was no apparent effect on alloy strength, but the alloy plasticity was significantly reduced. These phenomena can be better observed in Figure 3 by plotting Pb dosage vs. the yield strengths (σy), tensile strengths (σTS) and plastic strains (εp) at fracture. A significant reduction in plasticity took place at the Pb content of 0.1%. The presence of lead in Sn/Ag/Bi system largely reduced the low-cycle fatigue life (Nf). Figure 4 displays the low-cycle fatigue life (Nf) vs. Pb dosage. We found that Pb at 0.1% caused an abrupt drop in fatigue life, but that fatigue life somewhat recovered when the Pb dosage increased towards 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. We may expect, therefore, that the Pb atoms in an Sn/Ag/Bi system will 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 effect on alloy strength. With the presence of the softer Pb particles in the Sn matrix, plastic deformation tends to concentrate on these particles. This, in turn, 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 second phase Pb in the microstructure should also play a role in the fatigue mechanism. We further postulate that a small amount of Pb precipitates tend to reside preferentially at the grain boundaries, causing early grain boundary fracture.
Conclusion The contamination of Pb up to 0.5% in 92Sn/3.3Ag/4.7Bi produced no apparent effect on the alloy melting temperature. Its impact on the strength of Sn/Ag/Bi composition was also negligible. However, the Pb addition reduced alloy plasticity and fatigue life. * J. S. Hwang, "Solder Materials," SMT, March 2000, p.81.
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