Challenges of Ultra-Thin 5 Sides Molded WLCSP

Abstract

As mobile electronics are continuously driven for compact, slim and lightweight, miniaturization of IC packaging has been a must. There are increasing Wafer Level Chip Scale and fan-in Package (WLCSP) employed in electronics to achieve the miniaturization. Since WLCSP just is the Die with solder balls attached, WLCSP has the smallest footprint and the lightest weight compare to substrate base and lead-frame base package. However, WLCSP side wall with pure silicon is brittle, it is easily damaged and cracked during transportation and SMT processes. How to protect the WLCSP sidewall and to prevent the cracks is a rising issue for WLCSP applications. Therefore, as a molded WLCSP has aroused lots of attention in IC semiconductor industry. It not only provides side wall protection but also enhances package board level reliability by additional molding fixture around solder balls. By using a wafer form molding technology, it solves side wall crack issue and has no concern for high cost. Without rigid support and with unsymmetrical structure, the major challenges of this molded WLCSP come from the wafer warpage post backside grinding process. In order to diminish the warpage during backend processes, lots of experiments were conducted extensively to reduce the wafer warpage, including molding compound selection (which focus on its CTE and Tg adjustments), post-mold cure optimization, die thickness decision, and so on. In this paper, a test vehicle with ultra-low profile was evaluated. Stress simulation was conducted to determine the package construction and work out the bill of material. Screen and corner DOEs which includes molding compound selection and post-mold cure parameters were performed to come out the optimal material and process window. Functional test and Board reliability test have been passed as well. Additional collision test also shows good side wall protections. Hence five sides molded WLCSP has been proven to be a feasible and reliable way for the miniaturization in assembly industry.