Allan Cairncross

Tacky Dots/sup TM/ 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 /spl mu/m or larger, much too large for the typical flip chip applications of 150 /spl mu/m or less. To address this need, Texas Instruments and DuPontB have jointly developed a process for transferring 127 /spl mu/m diameter solder spheres to wafers. The process, called Tacky Dots/sup 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/sup TM/. The compliant polyimide sheet used in Tacky Dots/sup 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/sup TM/ at transferring spheres to electronic packages and substrates other than wafers.

Tacky Dots/sup TM/ technology for flip chip and BGA solder bumping

As the electronics market moves toward higher performance Integrated Circuits (ICs), each IC requires larger numbers of Inputs and Outputs (I/Os). This has resulted in a strong need in the marketplace for a low cost, high resolution method for placing controlled volumes of solder (or other metal alloys) on bond pads of ICs and area array semiconductor packages, such as Ball Grid Arrays (BGAs), and Chip Scale Packages (CSPs). To satisfy this need, DuPont has developed the concept of Tacky Dots/sup TM/, which utilizes proprietary technology in photoimageable adhesives to form a pattern of tacky areas, which are subsequently populated with conductive particles and then transferred to ICs or packages. DuPonts expertise and effort have been focused on developing a systems approach to the front end population process, while working closely with Texas Instruments who has developed technology to enable the effective transfer of the conductive particles. This paper contains details of the imaging and population technology as well as a discussion of the overall progress of this new wafer bumping process.

Silver masters for high-quality electrostatic printing

Du Pont has developed new silver electrostatic master technology suitable for making multiple copies of high quality prints. A digital laser image is captured by a multilayered silver halide film and diffusion transfer chemistry is used to form a unique conducting silver image in a polymeric insulating layer. For prints the silver master is corona charged; where there is a silver image, charge is bled off to an underlying ground plane and where there is no silver, charge is retained by the insulator. The charged master is then developed with high resolution liquid toners and the toner image transferred to paper. The charging, toning, and transfer steps are repeated for multiple copies. Quality is maintained at printing rates exceeding 8 inches per second. The silver master looks like a normal photographic film and is processed like one but has dual functionality; it can be used as an electrostatic printing master and as a normal photographic film, i.e., as a graphic arts intermediate. The master will be described in more detail and examples shown.