Effects of Adhesion and Delamination on Stress Singularities in Plastic-Packaged Integrated Circuits

Abstract

The reliability of plastic-packaged integrated circuits is severely affected by the occurrence of delamination. Several types of electrical and mechanical failures are introduced as a result of the delamination, such as metal smear, pattern shift, open channels, passivation cracking, and fracture of the plastic encapsulation. An analysis on the basis of fracture mechanics is applied to wedges of two dissimilar elastic media as a model for the interfaces between silicon chip and plastic mould compound. The stress state inside the encapsulation and in the vicinity of the edges of the silicon chip is characterized by a stress-singularity parameter. It is shown that loss of adhesion aggravates the singular behavior of the strees components and that delamination induces higher stresses inside the packaging material. This will lead to crack initiation in the plastic encapsulation and also to more rapid crack growth in comparison with undamaged devices. This stress-singularity approach provides an explanation for the observed relation between the occurrence of delamination and the increase of the failure frequency in plastic-encapsulated integrated circuits. The thermal effects that arise during the temperature cycling tests are also investigated. For uniformly distributed temperatures it is shown that the stress singularity is determined by the elasticity constants and by geometric effects and that it does not depend on thermal influences.