Low cost wafer bumping processes for flip chip applications (electroless nickel-gold/stencil printing)

This paper describes a wafer bumping process and some of the realities associated with implementing the technology into a high volume production facility. The manufacturing process is based on electroless nickel/gold under-bump-metallurgy (UBM), stencil based printing of solder/polymer paste, thermal reflow of solders, and automated inspection for yield (bump and die) and bump height/uniformity. Each of these processes is discussed relative to the technology employed, the manufacturing equipment used, and the process controls required to ensure reliable production. Examples of two different devices (9 mil and 17.5 mil pitch) are described in the context of wafer lots going through the manufacturing process within the facility (inquiry to invoice). Bump yields of 99.992% and 100%, with bump heights of 109.8 mu m (3.4 sigma) and 138.3 mu m (2.9 sigma), respectively, were realized for these two devices.