Reliability of high I/O FCBGA corner stake materials

Commercial-off-the-shelf area array package (COTS AAP) technologies in high-reliability versions are now being considered for use in a number of electronic systems. Although to improve mechanical resistance of fragile flip-chip die within package, these advanced electronic packages commonly use underfill for the die attachment; full or partial corner underfilling may also be required at the printed circuit board (PCB) level to improve assembly reliability, particularly under mechanical and fatigue loading. This paper first presents a comprehensive summary of literature surveyed on the application of underfill materials, thermal cycle reliability of underfill for flip-chip die with conventional balls, and compare to the recent advanced versions with fine copper-pillar interconnects. Then, it presents reliability due to underfilling at the board level, discussing key parameters that influence thermal cycle and mechanical reliability of AAPs with underfill, edge-bond, and corner stake. Finally, it presents thermal cycle test data with optical and microsectional photomicrographs of a high I/O flip-chip ball grid array (FCBGA) assembled onto PCB and then added corner staking or additional center staking to improve mechanical resistance to vibration and drop testing. Drop test results and failure mechanisms for a 1704 I/O FCBGA package assembly with and without corner/center staking also presented.