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Bottom die crack issue is found in one type of thin-die and 4-stacked multi-chip-package (MCP) at a failure rate of almost 100 % after full decapsulation by nitric acid. Experiments and FEA simulation of the acid soak process are conducted to find the root cause of the failure. The residual stress redistribution after molding compound has been etched during acid soak is studied using advanced FEA element birth/death technology. If redistributed stress exceeds the strength of silicon, die crack will occur. Stress on the bottom die redistributed during acid soak is dominated by the package itself. The main influencing factors are bottom die thickness and the substrate. Using new destructive analysis method, no die crack found anymore.