Tacky DotsTM transfer of solder spheres for flip chip and electronic package applications

The use of preformed solder spheres for bumping flip chip wafers has not gained wide acceptance within the semiconductor industry. Due in part to equipment shortcomings, solder sphere transfer until now was commonly limited to spheres 300 μm or larger, much too large for the typical flip chip applications of 150 μm or less. To address this need, Texas Instruments and DuPontB have jointly developed a process for transferring 127 μm diameter solder spheres to wafers. The process, called Tacky Dots^TM, forms are array of sticky or tacky dots in a photoimageable adhesive coating. Solder spheres sprinkled on the adhesive coating are then captured and retained by the tacky dots until the spheres are aligned and reflowed to the wafer. This paper describes the equipment and processes developed for bumping wafers using Tacky Dots^TM. The compliant polyimide sheet used in Tacky Dots^TM required a new and unique equipment design that aligns the solder spheres to the wafer and then reflows the solder without moving the wafer. Post reflow analysis of the bumped dies before and after environmental testing is reviewed. Tests conducted with a leadless chip carrier package design are also reviewed to demonstrate the capability of Tacky Dots^TM at transferring spheres to electronic packages and substrates other than wafers