Changing states of delamination between molding compound and chip surface: a challenge for scanning acoustic microscopy

The states of the interface chip/molding compound under temperature cycling stress are described by a simple model. This interface is usually evaluated nondestructively by means of scanning acoustic microscopy (SAM). Applying the well known theory of elastic waves to rough interfaces we are able to predict some restrictions in sensitivity of SAM in this case. The reason is that acoustical bridges within gaps play a crucial role in observing delaminations between chip and molding compound. As a consequence, one is led to the conclusion that there are types of delaminations which cannot be detected by SAM. Experimental results are presented which show this