Mechanical properties of solder joints in surface mounting

The feasibility of improving the solder-joint strength in surface mounting was investigated. The evaluated leads were gull-wing types, and quad flat packages (QFPs) were used. Fe-Ni alloy was examined as the lead material. A QFP was assembled on a printed wiring board (PWB) by vapor-phase soldering. After soldering, the strength of the solder joint was estimated by peel testing. The interfacial layer between the solder and the lead was investigated by scanning electron microscopy, electron probe microanalysis, and Auger electron spectroscopy. The cracking of the solder joint starts near the tip of the heel fillet and propagates along the interface between the solder and the lead (microscopically in the solder phase near the interface). The strength of the solder joint is mainly affected by the shape of the solder fillet and the mechanical properties of the interface between the solder and the lead. The lead shape, the lead surface, and the amount of solder influence the former. Soldering conditions influence the latter.<
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