Development of a stacked WCSP package platform using TSV (Through Silicon Via) technology

Abstract

To enable the miniaturization, electrical performance and heterogeneous functionality needs for emerging Analog applications, a stacked Wafer-level Chip Scale Package (WCSP) package platform has been developed using Through-Silicon Via (TSV) technology. This allows stacking of ICs, MEMS, passives and other components in the vertical direction onto active or passive TSV wafers, to create innovative System-in-Package (SiP) product solutions. Since Analog devices are small in size and cost is a key care about, a careful selection of the integration flow is required to achieve a low cost packaging solution. In this work, an integration flow for the stacked WCSP package is presented, along with development details for the Chip-on-Wafer (CoW) bonding and wafer overmolding unit processes. The test vehicle was 3mm × 3mm in size and used 25u diameter Cu TSVs in a 200mm diameter wafer. Interconnect reliability evaluations were done with different micro-bump Under Bump Metallurgy (UBM) and TSV tip surface finish metallization combinations. Wafer ovemolding development included warpage, saw and adhesion evaluations with multiple mold materials. A back-end assembly flow was established with a mass reflow bonding process and an overmold material with low CTE and intermediate Tg and modulus. Samples were prepared with mold-on-die and exposed die package structures. Excellent time-zero yields were obtained, with an average TSV micro-bump interconnect resistance of 25 mohms. Results and failures modes from preliminary reliability testing are included.