Extending the storage life of stencil printed wafers for small die size on wafer mounting tape

One of the most recent developments in die attach adhesive technology comes in the form of Wafer Backside Coating (WBC) materials. These new materials overcome the perennial manufacturability issues with traditional dispense paste such as inconsistent fillet height and width, die tilt, insufficient coverage, etc. But unlike traditional dispense pastes, these WBC materials need to be stencil printed at the back of a wafer, b-staged cured and then mounted onto a wafer mounting tape. However, mounted stencil printed wafers have a limited storage life because the adhesion between the stencil printed material and the mounting tape tends to increase with time. The problem is more evident with wafers that have been sawn into very small die size since there would always be some excess dice left on the wafer tape after every production run. These remnant dice need to be stored in dry cabinets until the next production run is scheduled. The storage time could take months, depending on the device forecast. As a consequence, stencil printed wafers that exceed the storage life are scrapped. This paper presents the work done on material characterisation and process characterisation in order to define the maximum allowable storage life of stencil printed wafers with the objective of minimizing wastage. Material tests such as DSC, TGA, peel test were performed in parallel with process characterisation to relate die attach process responses with the degree of cure, adhesion strength between stencil print material and wafer mounting tape, as well as thermal stability of the Wafer Backside Coating material (or adhesive). MSL1 was performed to assess the package reliability with the extended storage life of stencil printed wafer on wafer mounting tape.