Stencil printing on bumped wafer for pre-applied underfill application

For a successful implementation of wafer pre-applied underfill application, an appropriate underfill coating process has to be developed. A successful wafer level underfill coating process must produce uniform films of appropriate thickness across a single wafer, as well as films of consistent thickness from wafer to wafer. It requires a development of material printing properties and optimization of process parameters. Stencil printing is employed to coat underfill on bumped wafers. In addition to the large number of process variables and complex nature of the material flow, the presence of solder bumps complicates the stencil printing process. Uniformity of the coating thickness on single wafer, as well as consistency of the coating thickness from wafer-to-wafer, are studied in this paper. The printing void entrapment and the solder bump contamination by printing are also discussed.