Development of Micro-ball placement technology for WLCSP

Abstract

The trend of semiconductor advance packaging development is toward fine pitch and high I/O density. Wafer level package is good way to resolve fine pitch and high I/O density IC productions, especially package tape of Wafer Level Chip Scale Package (WLCSP). In traditional bumping process, such as printing [1], electric-plating [2] and ball mount [3–5], it is done by producing bumps on wafers. The paste printing technologies are very versatile with respect to the alloy composition that can be use, but is limited to pitches around 200um for 100um tall bumps. The electric-plating technique is somewhat limited for use in smaller facilities due to the high capital and operation costs. In addition, ternary alloys, like SnAgCu are difficult to plate with consistent results. There is also a practical upper limit to the size of the bump that can be produced, and most applications rare for fine pitch bumping. Ball mount technology uses performed solder spheres dropping through a metal template onto wafer at once. This technology is directly producing bumps on wafer which with high throughput and consistent bump results. Ball mounts process without using electric-plating electrolyte decrease cost and chemical pollution. This technique is applicable for many applications but there are several issues are associated with this technology that limits its widespread use in high volume and high yield applications. These limitations include of there is a practical lower limit to the size of sphere that can be dropped, the stencil between the performed solder spheres and the wafer can fail, causing a release of all the spheres into the tool (often referred to as bursts or escapes), and the yields are statistically low. To get high I/O density IC request, the trend of WLCSP I/O pad distributed design is toward to reduce I/O pitch and increase I/O density, and therefore impact solder ball size application of ball mount process, WLCSP Micro-ball mount technology is requested. There are several issues are associated with this technology that certain ball dropping position and escape issue. The most important factors associated with performance of Micro-ball mount technology are accurate dropping parameter, stencil quality and reflow condition. In this paper, we successfully produced WLCSP Micro-ball which diameter lower than 100um with bump pitch 130um onto 300mm wafers. Yield more than 99.99% without missing bump and bridge bump were realized for placing 70um spheres onto wafers with ∼2KK I/Os.