Injection molded solder technology for Pb-free wafer bumping

A new wafer bumping technology is described that is especially suited for Pb-free applications. Although capable of using standard PbSn eutectic solder, IMS (injection molded solder) has been found to be especially suited for accommodating a wide range of Pb-free alloys with equal ease. The development of IMS technology was driven by the need to reduce wafer bumping costs while simultaneously addressing the conflicting demands of increasing wafer dimensions to 300 mm and decreasing bump and pitch dimensions below 75 μm on 150 μm centers. The IMS wafer bumping process uses a new head assembly that melts bulk solder alloys with precisely controlled compositions and dispenses the molten solder into multiple cavities of a wafer-sized mold plate. The mold plate is CTE matched to silicon and is reusable many times, thus reducing the per wafer bumping cost. In the process, a mold plate is scanned and filled with molten solder and inspected after solidification. Thereafter, the mold plate and device wafer are aligned and adjoined in a mirror image fashion for processing through a solder reflow furnace to transfer solder to the wafer. In this paper, early manufacturing challenges and solutions are described which allow IMS to be considered as an attractive technology for 300 mm Pb-free wafer bumping. Early process feasibility data for 200 mm wafer bumping are reviewed. Economical and environmental advantages are also discussed in relation to key process characteristics, such as solder waste reduction, use of low-cost bulk alloys, and others.