Challenges of Large Body FCBGA on Board Level Assembly and Reliability

Abstract

For years the IC industry has been driven by Moore's Law and the functionality that fits on a single die will double every 18 to 24 months. As minimum dimensions have shrunk from 14 to 10nm and now to 7nm, supposedly die size should shrink accordingly. But for the application of networking, things don't go that way. Nowadays we are in an era that the demand to improve networking performance exceeds Moore's Law, thus die size and package size of FCBGA for networking are increasing. With package size increasing in network application, there is a need to use heat sink with more rigidity to control the warpage of large FCBGA, therefore the package itself is more rigid and more heavier. Large FCBGA with package size 75x75mm is PLR (package level reliability) qualified with rigid and heavier heat sink [1], but when the package is mounted on to PCB (printed circuit board), the BGA made of solder balls melt during reflow which are not able to uphold heavier package and result in low standoff height and solder bridge happened because the balls connect to adjacent ones. The other challenge is even when the solder bridge issue can be resolved, there is risk of board level reliability (BLR) due to significant increase of package rigidity. There is CTE (coefficient of thermal expansion) mismatch between the package and the PCB, and during temperature cycling test (TCT) the package can hardly bend with PCB which can result in BGA even ELK crack. Finally, the heat sink is heavier as package size increases, and the large FCBGA will be vulnerable when it is mounted on PCB and in transit with vibration. In this paper, we will review the challenges of large FCBGA in terms of board level, meanwhile we will explore possible solutions to address each challenge, and then we conduct DOE (design of expirement) to verify the solutions for large FCBGA at board level.