An interim report on the comparisons of polymide to non-polyimide reliability for C4

Thermal cycling of non-underfilled C4 assemblies with polyimide shows that they are more reliable than C4 without polyimide. However, the reliability of underfilled assemblies of C4 with or without polyimide appears to be the same under thermal cycling conditions. The reason is that in non-underfilled assemblies without polyimide, the failure mode is adhesive failure of the UBM/passivation interface, with subsequent silicon cracking. In all other cases the failure mode is solder fatigue cracking. Finite Element Method analysis indicates that polyimide has no advantage over non-polyimide C4 in stress reduction at the interface. Thus, it is inferred that polyimide must enhance the adhesive strength of the system. However, such enhancement is unnecessary in underfilled systems, due to the significant decrease in the stresses from the presence of the underfill.