In a typical electronic assembly, many of the solder joints undergo multiple reflow cycles during the course of a complete manufacturing process from wafer bumping to the board level assembly. Effect of these reflows on the solder joint formation and its evolution during these cycles was investigated. In the first part of the study, small size BGA's were assembled with 12mil spheres of a number of low-silver lead-free solder alloys. Substrate surface finish used was Cu-OSP. Components were subjected to up to six Pb-free reflow cycles. After reflows, joints were cross sectioned and the thickness of the IMC layers formed was measured. Also, SEM images of the cross sections were recorded to see any change in the interfacial IMC morphology as well any change in the bulk alloy microstructure. The mechanical integrity of the solder joints was evaluated using high-speed ball pull test. In the second part of study solder joints were formed using Pb-free solder pastes and reflowed two times under three different reflow profiles. The main measure of solder joints used was the voids formed in the joints. Effect of the reflows on the short term and long term solder reliability is discussed.
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