NOVEL LEAD-FREE WAFER-LEVEL UNDERFILL MATERIALS FOR CHIPSCALE PACKAGINGAuthor: Gyan Dutt et al.
Company: National Starch & Chemical Co.
Date Published: 10/10/2004 Conference: IWLPC (Wafer-Level Packaging)
We have developed novel wafer level underfill materials for chip scale packaging that are compatible with lead-free assembly. These materials, when coated on the wafer, form clear, transparent coating after B-stage that can be diced into coated dies without any delaminating and cracking. In this paper we discuss the effect of baking in the B-stage step on wafer warpage, underfill cracking, solder paste smearing and underfill flow during the reflow. The material attributes required for controlling solder paste smearing and ensuring solder interconnection are contradictory to each other. The underfill material design and the B-stage step must be controlled tightly to achieve the right working window. The underfill properties, critical for the success of the waferlevel underfill process, are also discussed.
Key Words: wafer-level, underfill, surface mount technology, flip chip, chip scale packaging, lead-free, Bstage.
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