The present paper studies several failure mechanisms at both UBM and Cu substrate side for flip-chip die open contact failures in multi-chip-module plastic BGA-LGA packages. A unique failure analysis process flow, starting from non-disturbance inspection of x-ray, substrate and die level C-SAM, bump x-section followed by a bump interface integrity test including under-fill etching and bump pull test and/or substrate etch has been developed. Four different types of failure mechanism in multiple chip module that are associated with open/intermittent contact, ranging from device layout design, UBM forming process defect, to assembly related bump-substrate interface delamination have been identified. The established FA process has been proved to be efficient and accurate with repeatable result. It has facilitated and accelarated new product qualification processes for a line of high power MCM modules.