Effects of Solder Paste Volume and Reflow Profiles on the Thermal Cycling Performance of Mixed SnAgCu/SnPb Solder Joints

Manufacturers of most high-reliability microelectronics products are still exempt from the requirement to eliminate Pb from their soldering process, and neither their components nor their printed circuit boards can always survive a lead-free reflow profile. When faced with the need to incorporate a lead-free ball grid array or chip-scale package into one of their products, they are thus faced with three alternatives: attaching it separately using a rework station to establish a localized lead free profile; reballing the component with eutectic SnPb solder; or soldering it to the board with a eutectic SnPb solder paste to produce high-Sn joints with a few percent of Pb. Any of these approaches may affect the final reliability of the assembly in its own way, and comparisons between them by accelerated testing are confounded by very different acceleration factors. Optimization of solder paste volumes and reflow profiles for mixed soldering is of course less affected by this, but common experiences have been that optimized mixing requires a typical lead-free reflow profile. This clearly defeats the purpose of mixing in many cases. Two different sets of experiments were conducted to assess the consequences of soldering SnAgCu (SAC) balls with a eutectic SnPb paste for the thermal fatigue resistance of the resulting solder joints. The first experiment addressed effects of solder paste volume and common reflow profile alternatives. Based on the results of this, the second set of experiments involved the development of an improved low-temperature profile and the comparison of this with common SnPb and lead-free profiles in terms of thermal cycling performance. Five different high and moderate strain area array components were tested with SnPb, SAC, and mixed SAC/SnPb solder joints, and the results are discussed on the basis of known test parameter dependencies and trends. The improved profile was shown to optimize the performance of the mixed joints, but indications are that these may still compare poorly to pure eutectic ones for some components. All five components may have shorter lives with lead-free than with SnPb joints in service.