Reflow profile optimization for lead-free (SAC) alloys in BGA applications

Even though lead is supposed to disappear from most of the electronic components in the near future, there is still an industry wide intense effort to find new lead-free alloys, or variations within the existing general class of widely accepted lead-free SnAgCu alloys. For each of these alloys it is important to find optimum, applications based/specific and general process conditions. For these alloys there is a general question concerning the processing condition, what is the widest process window one can have without compromising performance? At Cookson Electronics the authors have an aggressive on going program of alloy and process development for BGA applications. For BGAs, especially used in the portable electronics, there is a trend to go to low silver alloys for improved drop shock reliability. Lowering silver moves the SAC alloys away from the eutectic around 217-221degC. An extensive study of a range of low Ag alloys along with SAC 405 and SAC 305 has been conducted to find the optimum process window. Measured performance parameters are: Solder joint strength, ball shear/pull (strength and failure mode), intermetallic layer thickness (as soldered and post high temperature aging). IMC growth at high temperature was used to verify and understand the effect of initial IMC thickness and morphology on its growth as reported in a few recent publications. Results show that for each alloy there exists an optimum reflow process window within which one can expect the best solder joint reliability performance.