Driving mechanisms of delamination related reliability problems in exposed pad packages

Exposed pad packages were introduced in the late 80-90´s because of their excellent thermal and electrical performance. Despite these advantages, the exposed pad packages experience a lot of thermo-hygro-mechanical related reliability problems during qualification and testing. Examples are dielift, predominantly after MSL conditions and die/attach-leadframe delamination leading to downbond stitch breaks during temperature cycling. In this study, we have combined novel interfacial adhesion test techniques with non-linear FE models using fracture mechanics based J-value calculations to assess the reliability problems of the exposed pad package family. Using the parametric FE models any geometrical and material effects can be explored to their impact on the occurrence delamination, dielift, etc. For instance the impact of diepad size is found to be of much less importance as the impact of die thickness is. Using the fracture mechanics approach, the starting location for the delamination from thermo-hygro-mechanical point of view is deducted. The results indicate that when diepad delamination is present, cracks are likely to grow beneath the die and dielift will occur. The interaction between dielift and other failure modes, such as lifted ball bonds, are not found to be very significant. In our future work we will combine the FE modeling with simulation based optimization methods to deduct design guidelines for optimal reliability of the exposed pad family.