On the role of adhesion in plastic packaged chips under thermal cycling stress

Scanning acoustic microscopy (SAM) was used to evaluate the degradation of adhesion on a blown up sample, i.e. a Si-beam coated only on the passivated side with the molding compound under consideration. Such a bending beam sample offers two advantages for evaluation by SAM over conventionally plastic mounted chips: (1) for the acoustic waves the sample is flat, which helps to eliminate or reduce the influence of chip topography, and (2) the bending stress tends to open any partial delamination, giving better defined conditions for SAM examination. From the SAM analysis of the bending beam sample a very low stress molding compound, which had caused an unexpectedly high failure rate, was found to show poor adhesion after thermal cycling in delamination near the corners of the sample. No delaminations could be observed by means of SAM at molded product test chips, however. Thus, it is concluded that poor adhesion of the molding compound which caused delaminations in the bending beam´s corner regions resulted in an increased chip failure rate