Pull testing of lead-free QFP solder joints

Eutectic 63Sn/37Pb solder has been the dominant soldering material in the joints of electronic components for decades. The harmful impact of lead on environment and human health has, however, led to an international trend to ban Pb from all electronic products. Efforts have been made to find a viable lead-free solder for replacing tin-lead. The SnPb replacing lead-free solder should have good enough mechanical properties both in the as-soldered condition and in the service. Many studies have shown that SnAgCu solder is one of the most promising soldering materials. This paper concentrates on determining the best composition of SnAgCu solder paste to guarantee reliable joints between substrate and components. QFPs (quad flat packages) with Ni/Pd/Au plating were reflow soldered on NiAu plated circuit boards. The used lead-free solder compositions were 96.9Sn/2.7Ag/0.4Cu, 95.5Sn/3.8Ag/0.7Cu and 95.5Sn/4.0Ag/0.5Cu. Eutectic SnPb 63Sn/37Pb solder paste was used as a reference. The pull strength, microstructure and fracture surfaces of the joints after pull tests were examined in the as-reflowed condition and after 2500 thermal cycles.