Reliability of SnPb and Pb-free flip-chips under different test conditions

This paper presents the results of an experimental study of the lifetime of flip-chip solder joints on low temperature cofired ceramics (LTCC) substrates. Otherwise identical test vehicles were built with SnPb37 and SnAg4Cu0.5 solder. The mean lifetimes of the SnPb37 joints were - depending on test condition and die size - between 637 and 1465 temperature cycles. The failures occurred due to fatigue cracks in the solder. The components soldered with SnAg4Cu0.5 solder showed a significantly longer time until electrical breakdown. A partial stress relief caused by cracks in the metallization layers was identified as one reason for the long time to failure of the lead free solder joints. In the second part of the paper, a model for lifetime prediction of solder joints based on FEM simulation is derived from these results. The absence of underfill simplified the FEM simulation, because the solder is the only material with nonlinear material behavior in the test vehicle. By combining the experimental results with the computer simulation, it is possible to generate equations that permit the prediction of the lifetime of solder joints based on FEM simulations. They could be used to calculate the lifetime for similar flip-chip interconnects. The two common approaches (strain-based or energy-based) for generating such equations are compared. Both approaches were capable of describing the experimental observations.