The influence of fillet height of a low modulus die attach on the wirebondability of a plastic package

There is a general shift in plastic package size towards larger and thinner dice. This can be seen in the dramatic increase in the number of QFP type packages produced in the semiconductor industry. This shift has pushed molding compound and die attach materials requirements towards a lower modulus to comply with reliability requirements. This has brought with it a number of processability concerns. One of the foremost concerns is that of wirebondability of packages assembled with a thin die using low modulus die attach and overcoming the phenomenon which is commonly known as "bouncing". In this paper, we report on one such low modulus die attach material (JM2500 from Johnson Matthey) which met reliability requirements but had a wirebondability issue when used with small, thin dice. This study used a dynamic mechanical analyzer (DMA) to study the bouncing phenomena. This was done by first bonding the die to a leadframe with low modulus die attach. After cure, the DMA was used to observe die depression during a simulated wirebonding process. The effects of high and low fillet heights on the bouncing effect was then studied. It was found that increased fillet height tends to act as a damper for the bouncing effect. This observation was then tested on an actual bonding process in a production line where packages with fillet height <30% were compared with packages with fillet height >50%. The results showed a significant improvement in package wirebondability when fillet height increased. The merits and limitations of the technique for understanding wirebondability are also discussed.