Defining application spectrum for acrylic based backside tape on QFN leadframes

Abstract

Constant search of new Bill of Material (BOM) has enable back end manufacturing industry to provide lower die free package cost annually, thus stretching the product lifecycle. A thorough characterization and optimizing the new material behavior in mass manufacturing environment is as important as obtaining life test results of the end product meeting customer requirements. BOM contributing to high in-process yield loss, though with low cost commodity price will defeat the bottom line of reducing die free package cost, as more rework and scrap can be generated. Indirectly, the added screening process will increase operational cost and has higher probability of reject escapee causing customer complaints. In the effort to qualify an alternate source of backside tape with Acrylic based adhesive on QFN leadframes, material characteristic comparison between two adhesive layers on the tape was performed; Olefine Styrene-Ethylene-Butylene-Styrene (SEBS) and Acrylic (Poly-Butyl-acrylate-acrylic-acid). The differences were significant on Coefficient of thermal expansion (CTE) properties, moisture absorption, adhesion strength, and 3D displacement showing warpage level at elevated temperature. The characterization test results were used as a guideline to confirm the behavior of the leadframes while it undergoes series of heating and cooling process at die bond, snap curing, wire bonding before the tape could be removed from the leadframe after molding. In the DOE conducted, leadframe design, wire bond duration and leadframe thickness were varied to ensure the new tape could be used over various leadframe design and leadframe thickness. Besides leadframe, the DOE were used to confirm if it is critical to establish initial curing process step to remove moisture from tape before assembly process. In every process steps, warpage value of the leadframes were measured and inserted back to the magazine to continue to the following processes. The experiment revealed that warpage level was at its highest after wire bonding and remained permanent until the tape was removed. The leadframe design and thickness influences the warpage level, while there was minimal effect of introducing initial curing process and prolonging the wire bonding duration at 225°C. No glue residual were found on the leadframe after detaping that matches to adhesion test after heat treatment. The findings is used to identify robust application spectrum of acrylic based adhesive tape for leadframes in mass scale, while newer tape adhesives continue to be explored.