IWLPC (Wafer-Level Packaging) Conference Proceedings

Marked Reliability Increase of Plastic-Cored Solder Ball for Large Size Wafer-Level CSP

Authors: Hiroya Ishida and Kiyoto Matsushita
Company: Sekisui Chemical Co., LTD, Japan
Date Published: 11/5/2012   Conference: IWLPC (Wafer-Level Packaging)

Abstract: A unique plastic-cored solder ball (PCSB) developed by Sekisui Chemical Co., Ltd consists of plastic core, copper layer and solder layer. It enables high reliability as a solder bonding material in Wafer-level Chip Size Package (WLCSP) by dispersing the occurred stress via its flexible plastic core. Currently, PCSB has been used commercially in analog devices within mobile phones. However, the demand for higher reliability is increasing as manufacturing shifts to smart phones that utilize packages of larger body size, a finer ball pitch, and thinner chip. Particularly, the reliability performance of board-level temperature cycling tests (TCT) is a great concern to the use in a wide flexibility of package designs.

In this study, we focused on the composition of PCSB in order to maximize its reliability properties and obtain greater reliability in larger-sized WLCSP that use fine ball pitch. We evaluated various compositions of PCSB and compared the reliability performance with conventional solder ball technology. The advanced type of PCSB achieved over 1000 temperature cycles in WLCSP with a package size of 10.2×10.2 mm square, and a ball pitch of 0.3-mm in a temperature range of -40 deg. C. to 125 deg. C. According to the observation of cross sections after TCT, the major crack of the advanced PCSB occurred along the copper electrode, while one occurring using conventional PCSB occurred along its copper layer. SEM analysis revealed that the intermetallic compound (IMC) was formed in the interfaces of the copper layer and the electrode. The IMC of the advanced PCSB was (CuxNiy)6Sn5 and fine in form, whereas the IMC of conventional PCSB was Cu6Sn5 and rough in form. This results in the advanced PCSB demonstrating better TC life.

This technique contributes to excellent reliability and underfill-resin-less in WLCSP, and proves that advanced PCSB is capable of large-size packaging design applications.

Key Words: 

WLP, CSP, Reliability, Solder Ball

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