New insights in critical solder joint location

Current numerical modeling approaches to assess BGA solder joint thermal mechanical fatigue, typically identify the max DNP (distance from neutral point) solder joint as the first to fail. An experimental and numerical analysis has shown that critical solder joints can be located interior to the max DNP location. In this work, FCBGA packages were subjected to accelerated thermal cycling stress. At regular intervals, subsets of the units being tested were extracted and analyzed with conventional dye and pull failure analysis techniques to better understand the progression of crack growth. Failure analysis revealed that cracks initiated on the diagonal axis and on the periphery, interior to the max DNP location. Numerical models using an accumulated damage metric to predict the critical solder ball, indicated that the max DNP solder ball would likely be the first to initiate a crack. However, examining the peel stress (D33) component on the solder balls revealed that the diagonal max DNP solder joints are under a compressive stress. Assuming that a crack will not preferentially propagate under a compressive stress, a qualitative approach using accumulated damage and peel stress from numerical modeling was defined to identify the critical solder joint. In this paper the experimental program, numerical model validation and results, and qualitative approach to couple accumulated damage and peel stress are discussed.