Low-cost and fine-pitch micro-ball mounting technology for WLCSP

Ball mount technology uses performed solder spheres dropping through a metal template onto wafer at once. This technology is directly producing bumps on wafer with high throughput and consistent bump results. Ball mounting process without using electroplating decreases cost and chemical pollution. This technique is applicable for many applications but several issues associated with this technology that limits its widespread use in high volume and high yield applications. These limitations include: (1) a practical lower limit to the size of sphere that can be dropped, and (2) 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 meet high I/O density IC request, the trend of wafer level chip scale package (WLCSP) I/O pad distributed design is toward to reduce the I/O pitch and increase the I/O density, and therefore impact solder ball size application of ball mount process, WLCSP micro-ball mount technology is requested. In this study, a low-cost and fine-pitch micro-ball mounting technology is developed. Emphasis is placed on determining the most important factors such as accurate dropping parameters, stencil quality and reflow conditions for microball mounting design, materials, and process. Three different ball sizes are considered: 70μm, 100μm, and 250μm. Their corresponding pitches are 130μm, 180μm, and 400μm. WLCSP micro balls with diameter = 70μm and pitch = 130μm on 300mm wafers (with ~2KK I/Os) have been successful produced. The yield is more than 99.99% without any missing micro ball and bridging.