Challenge to zero CTE and lower shrinkage organic substrate material for thin CSP package

Abstract

Along with the advancement in miniaturizing of mobile devices, typified by smart phones and tablet PCs, the semiconductor PKG substrate installed in these devices is demanded to be thinner and higher in density. As one of the most innovative solutions, the PoP (package on package) technology, which has the three-dimensional construction, has been expanding rapidly in recent years. However, thinner PKG such as PoP tends to warp at the assembly process and cause the decrease in the connection reliability. Therefore ultra low CTE (coefficient of thermal expansion) core materials have been needed as a key solution for the reduction of the warpage for PoP. We have already developed and mass-produced ultra low CTE core material named E-770G (CTE value: 1.8 ppm/°C) for this application. However, the demand for lower warpage is still stronger especially for thinner PKG applications. To meet this requirement, we confirmed that not only low CTE but also lower resin shrinkage control gives big impact to warpage. In this paper, we would like to introduce new ultra low CTE and low shrinkage core material, named E-777G for next generation thin CSP. At the same time, we would like to explain the relationship between the warpage and material properties (CTE and resin shrinkage). E-777G has ultra low CTE and low shrinkage. These properties were achieved by applying our original resin system designed by hard segments and soft segments. Hard segments have a stack structure of aromatic ring and the strong intermolecular force between them, which lead to the ultra low CTE and low shrinkage. Soft segments of low elastic modulus can follow approximately the thermal behavior of glass fabric when heated and cooled. Consequently, the material can show the ultra low CTE which is similar to that of glass fabric itself. As a result, E-777G has achieved the ultra low CTE of 0.7 ppm/°C by TMA method which leads to significant reduction of the warpage. In the cooling process after the reflow, the bottom PKG of PoP shows a convex warpage. It is caused by resin shrinkage of substrate in the cooling process. As the method of decreasing resin shrinkage in the cooling process, we focused on decreasing free volume of the resin. By using molecular dynamics simulation, we designed the most suitable resin composition. Confirming the warpage property, we evaluated the warpage behavior of the bottom PKG before/after assembly process. The bottom PKG using E-777G material showed lower warpage than E-770G. We confirmed that not only the ultra low CTE but also low shrinkage of material effectively contributes to the decrease of the warpage.