Construction of FO-MCM with C4 Bumps Built First Using Chip Last Assembly Technology

Abstract

Chip last assembly technology is complex and higher cost for fan-out wafer level package (FOWLP). But, this technology is fit well for very high density interconnection packages. This article presents chip last assembly technology using C4 bump-first for fan out multi-chip module (FO-MCM) package. The objective is to reduce cycle-time. A chip module with 28 x 30 mm was fabricated using 2 daisy-chain Si dies that bonded onto 2/2 µm line/space redistribution layers (RDLs). This module was then assembled on high density substrate with size of 70 x 70 mm. This FOMCM package is constructed using C4 first process. C4 bumps were built on same side of the carrier after RDL was fabricated. The assemblies were protected and bonded on the carrier using temporary bond glue. The 1st carrier was then de-bonded. High I/O Si dies were attached onto the opposite side of the carrier followed by molding. The difference between C4 first and C4 last is the Si dies that were attached and molded with the carrier first then fabricated the C4 bumps. C4 first process has the challenge is micro-pads pattern shift between Si dies. By increasing the RDL density, one could reduce the irregular of micro-pads pattern shift. Additionally, by reducing the thermal budget and using higher Tg of the temporary bond glue, the pattern shift was improved to less than 5 mm. Additionally, the wafer warpage of C4 first was found consistently warped at the same side, thus the process was easier to control as compared to C4 last. The assembled FOMCM packages were then stressed for reliability tests. It passed 1000 hours of high temperature storage life test; MSL4 preconditioning with 1000 thermal cycles under B-conditions (-55~125 °C) and 192 hours unbiased high accelerated stress. Details of the results will be presented and discussed.