Effect of NCF design for the assembly of Flip Chip and reliability

Abstract

Recently, design of Flip Chip (FC) Package is changing with the higher density of the Package. Conventional process with Capillary Underfill (CUF) is not applicable to PKGs such as 3D and chip stacked types. To solve these problems, other processes are being developed in which an encapsulant is applied on a substrate before bonding IC. One of those is the process with Non Conductive Film (NCF). In this process, after NCF is applied, IC is bonded. Interconnection and NCF cure are done at the same time. Therefore, the design of NCF has great influences to FC assembly in terms of void, interconnectability and reliability. Thus, in this paper, we are mainly discussing the optimization of NCF design. At first, we looked at the aspect of voids. One of the causes of voids is captured air which generates when an IC connects to NCF. This relates to the flow of resin. Regarding this flow, we looked into what encapsulant's behavior is effective in controlling voids by measuring temperature and viscosity with a rheometer. As a result, we could decrease the voids by optimizing the minimum melting viscosity. As another type, the void from volatilization gas may occur from an organic substrate. We looked at the quantity of substrate using TG-DTA, and found that it decreased by 0.4% till the temperature reached 260°C. Then we found that the higher minimum melting viscosity is, the more effectively this type of voids can be controlled. Moreover we tried to optimize minimum melting viscosity, curability and flux-ability for good interconnection. Regarding the minimum melting viscosity, when it is too high, the connection will be poor. Regarding curability, when cure speed is too high, solder melting will be blocked. We also attempted to optimize flux activity, and found that gelling time, minimum melting viscosity and oxidation-reduction power need to be controlled. Based on these approaches, it became possible to design the NCF which is voidless, has good connection, and can pass the reliability test (JEDECL3, TC1000cyc).