Board level reliability of mixed solder interconnects

While the majority of electronics has converted to lead-free materials, a portion of the industry that provides electronics for some applications, with limited availability of some tin-lead (SnPb) parts, has resulted in the potential use of lead-free (Pb-free) parts in a SnPb assembly process as mentioned as the backward assembly process. While the material supply base converts from leaded to lead-free components, there will be a combination where leaded solder paste and lead-free solders are being used in the same assembly. Maintaining solder joint reliability is one of the key challenges as the industry complies with new lead-free requirements. The primary objective of the study was to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly process and completely Pb-free assembly process. The effect of the pure Sn plating thickness and SMT reflow profile were also investigated using a design of experiments (DOE) approach. The test vehicles were TSOP 48 and TSOP 56 with SnPb and pure tin plating finish respectively on organic solderability preservative (OSP) printed circuit board surface finish. The SnAgPb and SAC solder paste was used. Reflow profiles with peak temperature at 213?C and 225?C were employed on backward assembly condition. The lead pull test was performed in the ?as-soldered? condition. The performance of the solder joints were quantified in term of pull strength. The assemblies were subjected to thermal shock test as per Interconnecting and Packaging Electronic Circuits/Joint Electronic Device Engineering Council (IPC/JEDEC) standard at condition of -40/100?C, 15 minutes dwell time, 1 cycle per hour and 2,000 cycle. During the test the resistance of daisy-chained solder interconnects were monitored. The performance of the TSOP solder joint was quantified in terms of the number of cycles to failure, which was analyzed by Weibull statistics. The microstructural analysis of the solder joint indicated the distribution of Lead (Pb) in the solder joint matrix and the intermetallic formation at various reflow profile condition as comparison. The results show that the mean cycle to failure of Matte Sn solder finish leadframe packages with SnAgPb (backward assembly process) and SnAgCu paste (Pb-free assembly process) were not significantly different and they were lower than those with SnPb solder finish and SnAgPb paste (conventional SnPb assembly process). For backward process, it was found that the mean cycle to failure significantly decreased when the Sn plating thickness increased.