Effect of simulation methodology on solder joint crack growth correlation and fatigue life prediction

A generalized solder joint fatigue life model for surface mount packages was previously published in [1,2]. The model is based on correlation to measured crack growth data on BGA joints during thermal cycling. It was subsequently discovered by Anderson et al. that the ANSYS(TM)(1) 5.2 finite element code used in the model had an error in its method for calculating plastic work (3). It was shown that significant error in life prediction could result by using a recent version of the code where the bug has been fixed. The error comes about since the original crack growth constants were derived based on plastic work calculations that had the bug. In this paper, crack initiation and growth constants are recalculated using ANSYS(TM) 5.6. In addition, several other model related issues are explored with respect to the crack growth correlations. For example, 3D slice models were compared to quarter symmetry models. Anand's constitutive model was compared with Darveaux's constitutive model. It was shown that the crack growth rate dependence on strain energy density always had an exponent of 1.10+/-0.15. This is in the range of the original correlation. The model was applied to several data sets of tape-based and laminate-based BGAs. The accuracy of absolute predictions is mostly within +/-2X. The accuracy of relative predictions is within +/-25%. However, the accuracy of absolute predictions could be off by a factor of 7 in the worst case, if the analyst uses a modeling procedure that is not consistent with that used for the crack growth correlation. The key to good accuracy is to maintain consistency in the modeling procedure.