SQUEEGEE INFLUENCE ON BUMP METRICS FOR STENCIL PRINTED WAFERS
Authors: Jeff Schake and Guy Burgess Company: DEK USA Inc. and Flip Chip International, LLC Date Published: 11/1/2006
IWLPC (Wafer-Level Packaging)
Abstract: Stencil printing fine powder solder paste on wafers and reflowing leverages the flexibility and productivity of standard SMT assembly equipment. As with any process, a vast number of variables need to be considered before launching successfully. Design of a proper stencil template is critical to print consistently formed individual solder paste deposits of correct dimensions. Stencil foil thickness and aperture size selection not only determines the capacity available for formation of the solder paste deposits, but also affects the ability of solder paste within the aperture to be conveyed to the wafer. Compliance to area ratio rules will result in high paste transfer efficiency and predictable bump size. Maintaining a sensible spacing between adjacent apertures will minimize defects. With the demand for finer pitch escalating, conventional design rules are being stretched to their limit. This study investigated the influence of five different squeegee designs on bump size and distribution using several aperture designs intended to challenge fineness of spacing. Metal trailing edge squeegees deposited the largest average paste volume. The experiment also produced excellent yield at tighter than recommended aperture spacing attributed to technology advancements in tools and materials.