Development of Encapsulant Material for Molded Underfil for Fine Pitch Flip Chip Packages

Abstract

Encapsulant material for molded underfill (MUF) is widely applied to flip chip type semiconductor packages for mobile applications like as smart phone. Along with high functionality of the semiconductor device, the number of copper pillar for electric contact between die and organic substrate interposer is increasing and the gap between pillars / die and substrate is reducing. As a result of this trend, unfill of encapsurant material between die and substrate becomes more difficult issue. Unfill causes electrical failure during mounting process. For the reason, higher filling ability is required to the encapsulant material. On the other hand, reducing package warpage is important function required for the material. Because large warpage during mounting process causes failure of solder connection between package and mounting board. Behavior of warpage depends on the package structure. Recent trend of thin and coreless substrate adoption, warpage control has become more difficult challenge. In this seminor, we will report the development of advanced encapsulant material for MUF to achieve both filling narrow gap and reducing package warpage. Encapsulant material is composed by epoxy resin, hardener, catalyst, fused silica as filler and other some minor components including coupling agent, releasing agent, pigment and additives. In molding process, it is heated up in the mold die which is kept around 175 degree C and melted, after that filled into narrow gap between die and substrate. While filling, large size filler particles prevent flowing in narrow gap. However, reducing size of filler usually causes reducing flow length because specific surface area of filler increased. These cause worse filling ability. On the other hand, to reduce warpage it is needed to optimize physical properties. To reduce fluctuation of warpage during reflow mounting process, to reduce fluctuation of CTE at high temperature is effective. To reduce fluctuation of CTE, to increase filler content is one of the major method, but it causes reducing flow length and worse filing ability too. For these reasons, we studied the design of encapsulat material to achieve high flow length with small filler size and low CTE fluctuation. We examined the material the fine-pitch flip-chip packages which gap between die and substrate is below 30 microns to the target. Maximum size and size distribution of silica affect to filling ability to narrow gap and flow parameters like as spiral flow and viscosity. We studied effect of silica size to flow parameters and filling ability. After that we optimized the silica design using a maximum particle size of 10 microns. Furthermore we studied composition of resin, latent catalyst and additives to keep low viscosity during filling by controlling cure reaction and achieve good narrow gap filling ability. We developed encapsulant material for MUF applicable to very high pin count, fine pitch and narrow gap flip chip package with low warpage fluctuation by the techniques based on these studies.