The reliability of plastic packaged integrated circuits was assessed from the point of view of interfacial mechanical integrity. It is shown that the effect of structural weaknesses caused by poor bonding, voids, microcracks or delamination may not be evident in the electrical performance characteristics, but may cause premature failure. Acoustic microscopy (C-SAM) was selected for nondestructive failure analysis of the plastic integrated circuit (IC) packages. Integrated circuits in plastic dual in line packages were initially subjected to temperature (25 °C to 85 °C) and humidity cycling (50 to 85 %) where each cycle was of one hour duration and for over 100 cycles and then analyzed. Delamination at the interfaces between the different materials within the package, which is a major cause of moisture ingress and subsequent premature package failure, was measured. The principal areas of delamination were found along the leads extending from the chip to the edge of the molded body and along the die surface itself. Images of the 3-D internal structure were produced that were used to determine the mechanism for a package failure. The evidence of corrosion and stress corrosion cracks in the regions of delamination was identified.
A model is constructed to consider the stresses (analytically and with Finite Element Analysis (MiA)) which result from the thermal mismatch between the die and the substrate. FHA is used to simulate thermal stresses induced from temperature cycling with voids and without voids in the die-attach at the die-substrate interface. Local stress concentration caused by voids is found to be dependent on the location of the voids. The presence of an edge void at the die-attach interface changes the local stress and creates a longitudinal stress field. It is also observed that for die-attachment without voids or some center voids there will be no cracking whereas specimens with voids near the edge of the die are likely to have vertical die cracks. Using the void growth, stress relaxation equations, the void growth is simulated yielding an exponential relationship to void grow th and a saturation of void volume w ith time. Stress relaxation and void growth during cool down are simulated, once the material parameters and cooling rates are known. It yields a time dependence of the relative void volume (exponential decay).
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