Die cracking is an annoying problem in the packaging industry. In this paper, we identified the weak regions, in terms of mechanical strength, in chips in a semiconductor wafer using the three-point bending test. The weak regions were observed in two sectors approximately 45 wide, axisymmetric to the wafer center. The strength of the chips within these weak regions was about 30%-35% lower than the average chip strength of the whole wafer. The existence of these weak regions was related to spiral grinding marks, which, in turn, were formed by backside mechanical grinding. The probability distributions of the chip strength and the chip fragmentary pattern confirmed this relationship. When wafers were mechanically ground until they were 50-m thick, chip warpage was found to be oriented to the direction of the grinding marks. Meanwhile, by slowing the mechanical grinding speed by 50%, we were able to increase the average chip strength by 56%. Either plasma etching or polishing after mechanical grinding eliminated the weak regions, and the optimal amount of mechanical grinding and the polishing depths were observed, beyond which the chip strength would not increase. On the other hand, a preprocess for blunting a new saw blade for chip dicing was found to be essential as the chip strength increased five-fold, whereas increasing the dicing speed or using dual saw instead of a single saw had only small effects on the chip strength degradation.

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Die cracking is an annoying problem in the packaging industry.In our previous study, we have identified the weak region of the chip strength distribution in a wafer using the threepoint bending test. It was found that the strength of the chips within the weak region was 30% to 50% lower than the averaged chip strength of the whole wafer, and the cause of the weak region was due to backside mechanical grinding. In this paper, additional thousands of chips on different wafers were tested to find the solution to enhance the chip strength in these weak regions, which included the effects of grinding speed, fine grinding depth, post processing using plasma etching or polishing, and search of optimal polishing depth.It was found from the experimental results that (a) slow down the grinding speed can increase chip strength in both the weak region and the whole wafer by approximately 50%, (h) although use of polishing after mechanical grinding increased the chip strength in the weak region, only 1 to 2 umthick polishing is considered adequate. On the other hand, use of coarse mechanical giinding only reduced the chip strength drasticany to about 20% of the original value.

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