A novel method to predict die shift during compression molding in embedded wafer level package
The increased functionality of cellular phones and handheld devices requires system level integration. Thus there is a strong demand in cell phone maker to move to embedded micro wafer level packaging (EMWLP). But the major problem encountered is die shift during compression molding. This paper presents a novel method to predict the die shift during wafer level molding process. A series of parametric studies are performed by changing the die thickness, die pitch distance and top mold chaste compression velocity. The effect of thinning down the chip thickness affects the pressure difference and local shear rate on the chip surfaces. The rate of change of epoxy mold compound fluid pressure across the die top surfaces is not constant. The local shear rate is increasing linearly from the centre of the wafer to the outermost die. From the parametric studies, the die shift is inversely proportional to the die thickness for wafer level molding. Such a phenomenon will reduce the lithography alignment error in the next process. This paper also shows that by reducing die pitch distance of a 5 × 5 mm2, 500 µm thick chip, the die shift decreases by a factor of 12%. In addition, the top mold chaste compression velocity contributes to the die shift by as much as 28% when the velocity is reduced by 50% from 100 µm/sec to 50 µm/sec Finally it is observed from experiment result that the die shift is not constant in all directions.