Effects of Pb Contamination on the Material Properties of Sn/Ag/Cu Solder
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.

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