The quality of the die attach is crucial for almost all power devices, as in most cases thermal and electrical transport is vertical through the die and its backside. For glue inspection, C-SAM through the lead frame is widely used. Ordinarily, one of the three following states of delamination is found: (A) no delamination, (B) delamination at the die, or (C) delamination at the lead frame. A general rule for the assignment of the brightness in C-SAM amplitude images to these states of delamination cannot be given. This is evidenced by contrast inversions observed with frequency variation of the applied ultrasound or variation of the glue thickness. Contrast inversions at images through the lead frame occur between areas of states (A) and (B). The calculation of ultrasonic echoes for a three-layer model (copper, glue, silicon) shows that the contrast inversions are connected to the first resonance of the glue in state (B). Here complicated shapes of the echoes are found in experiments and calculations, which helps to correctly assign brightness levels to delamination states. Additionally a flow for reliable glue investigation with the use of through-transmission SAM inspection is proposed.