Failure to apply the proper systematic analysis procedure can result in loss of valuable evidence required to understand the root cause of package failures. For example, in the case of marginal current leakage fail, decapsulation from package front-side may result in loss of the electrical failure signal so that root cause of the leakage failure cannot be understood. In such case, a systematic backside fault isolation method can improve the success rate of isolating the defect. These electrical failures are often due to zero solder bond line thickness (BLT), or filler particle compression on the die, which are key assembly defects encountered in clip style surface mount packages (SMX). In this paper, the first case study is to determine the failure mechanism of an electrical short. A silicon micro-crack propagating through the junction at the dimple clip center, which is due to the ultra-thin solder BLT close to zero micron is found to be the root cause of failure. The second case presents the failure mechanism for a low leakage fail. The pointed tip of a silica filler particle compressed on the die surface leads to excessive leakage.