The reliability of mixed solder interconnects under thermal cycling conditions is related to the degree of Pb mixing and the formation of microstructures within the solders. Many studies of the mixed soldering technique have focused on: (1) optimizing reflow profile parameters for acceptable mixed solder joints - (2) evaluating Pb diffusion and its uniformity throughout the solder joint microstructure, - (3) evaluating the reliability of mixed solder joints under different thermal cycling conditions, , (4) measuring the properties and failure modes of mixed solder joints under vibration and shock loadings and (5) evaluating the effect of the rework process on the mixed solder joints according to the peak reflow temperature and rework counts. Mixed alloy assembly serves as an alternative to immediate or complete conversion to Pb-free manufacturing, but its implementation has been limited due to concerns about the long-term reliability of the mixed solder joints. The impact of package dynamic warpage during the reflow soldering process, on the solder joint shape, collapse and percentage of Pb mixed within a solder joint will also be described As part of the study, recommendations will be made for an optimum reflow window that can be used to achieve acceptable degree of mixing in solder joint formation. The ball collapse and the percentage of Pb mixing achieved in the solder joints of these BGAs during the various iterations of the reflow profile development will be presented. This paper will illustrate the challenges encountered in reflow profiling of a thick (>90mils), printed circuit board test vehicle, which contains four each of two large (>37mm, >1200 balls), high density BGAs. Backward Compatible reflow profile development has always been a challenge, but recently, this challenge has been further exacerbated by the increase in size and complexity of high density BGAs. One key challenge to maximizing both the solder joint yield and reliability has been to achieve adequate collapse of the SAC solder balls and sufficient Pb mixing when the reflow soldering process is performed at temperatures below the Pb free solder liquidus temperature. Acceptable mixing of the two alloys is critical for solder joint yields during high volume manufacturing, as well as for long-term solder joint reliability in the field. The two main concerns with Backward Compatibility are the quality and reliability of the solder joints formed when mixing SAC solder balls of the BGA with eutectic SnPb solder paste. Some customers for these BGA packages, whose products have exemptions from the use of Pb free solders, are still employing SnPb solder paste for reflow soldering their products. Most BGA package suppliers have converted their BGAs ball alloys to Pb free, using SAC solder. Backward Compatibility of Pb free SnAgCu (SAC) solders with conventional SnPb soldering has been a subject of considerable interest since the introduction of Pb free solders earlier in this decade.
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