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|Latest Develoments in Bumping Technologies for Flip Chip and WLCSP Packaging|
|Keywords: flip chip bumping, balling, stencil printing|
|This paper intends to present all advancements in stencil printing technology pertinent to innovative stencils, ultra fine solder powders which along with alternative “Paste-in-Resist” and balling technologies have enriched IZM’s technology Portfolio for cost-effective WLCSP bumping and Flip chip bumping even down to 60µm pitch. Major advancements in stencil manufacturing and ultra fine powder production have enabled bumping at pitches smaller than 100µm. In specific, this study brings into light new developments in laser-cut stainless steel stencils with the introduction of nano-coatings to the bottom stencil surface and on the wall surface of the stencil apertures. The nano-coating treatment follows after the laser-cutting process. Very fine apertures of 40µmx90µm have been designed and state-of-the-art 20µm thin stencils have been used. Paste printing at 80µm UFP has used a newly developed type 8 (2µm-8µm) lead-free solder paste. Our studies extend also to electroplated stencils which offer economical benefits to industry for bumping large wafers with more than 50000 pads compared to the cost of a laser-cut stencil. Advanced very thin electroformed stencils (20µm in thickness) at 60µm pitch have been manufactured by IZM Industrial Partner with extreme care on foil expandability, roughness and stencil frame handling. Fraunhofer IZM has performed printing of UFP type 8 pastes and reflow soldering at 60µm pitch. The Feasibility studies on 6 inches wafers have yielded bump height of 28µm ± 3µm. UFP studies have shown extreme smoothness of the 35mx80µm aperture walls with outstanding paste release characteristics. However, the quality of electroplated stencils still varies from different manufacturers and thickness uniformity can become a major parameter of concern. Alternatively to stencil printing, technology researchers in Fraunhofer IZM are developing processes to print pastes in dry resists (“Paste-in-Resist” Technology). Very advanced laminated resists up to 50µm in thickness can be lithographically opened to provide apertures for paste printing. By this technology route, paste release issues are not longer the determining factor for bump uniformity and yield. Subsequent reflow soldering yields high quality bumps of any desirable tin-lead or lead-free composition on electroless Ni/Au UBM pads. Results with resist designs down to 50µm pitch will be shown and printing issues with very fine type 8 pastes will be discussed.|
|Dionysios Manessis, Principal Technology Scientist
Technical University Berlin/Fraunhofer IZM Berlin