Mechanics modeling of eMUF FC BGA molding process for bump deformation control

eMUF (exposed die molded underfill) transfer-mold process of FCBGA (flip chip ball grid array) is one of the most popular integrated circuit package assembly solution, which has the benefits of low cost, high throughput, high thermal dissipation, low profile, good protection of the bump and the top side solder ball of PoP (package on package) bottom package, available laser abrasion process for PoP bottom package. One of the most important problem of this process is there have external forces (a part of mold clamping force and the fluid transfer pressure of the molding compound) applied on the package before bump protection. For molding mechanics analysis, the structure of the eMUF molding process is a substrate with flipped chip placed on the bottom mold and a releasing film covered lid type top mold pressed on the substrate meanwhile contact the backside of the chip. The clamping force is partially applied on the substrate and partially on the chip. After mold clamping, the molding compound is pumped into the surface of the substrate include the bump area and then compressed into high pressure to make sure the mold cavity is full of molding compound and avoid the existence of the void. Generally, the clamping force is dynamic controlled simultaneous with the increase of the transfer pressure of molding compound. The problem could be separate into two parts, force and displacement balances before and after the molding compound transfer pressure build up. The force unbalance may damage the weakest part of the package; generally, the damage will appear at the UBM, IMC or C4 bump. Additionally, die back side mold flash is another defect may appear if the displacement balance not deal well. A static force and displacement balances mechanics model proposed in this article provides a good comprehension on the mechanism of the eMUF molding process. It indicates that the solder bump is easy to have plastic deformation, and gives a reasonable bump deformation prediction. This model is a powerful tool to help the process engineer to select process and mold parameters, include the molding compound transfer pressure, adaptive and full clamping force, and the mold cavity depth, for different product and process incoming situation. Meanwhile, for next generation assembly, it indicates that the molding process window of eMUF should have a stricter control, due to the smaller of the bump size. Furthermore, it told that a process refinement action for reducing bump bearing external force is pressing.