Failure rate prediction and prevention of die cracking in over molded plastic packages

Package designs in RF applications usually require large exposed ground pad in order to obtain sufficient RF grounding. But this design also leads to risks of die cracking during the molding process. In this paper we discuss the observed die cracking failures on land grid array (LGA) packages due to transfer molding process and also propose a method to predict the failure rates and to create design rules to control the failure rate. In practice, the stress on the die and the die strength determines the failure rates. Die strength depends on the processes used for backside grinding and dicing. Three point bending test was used to obtain the die strength distribution. Several package designs with different solder mask openings and die sizes were tested and the failure rates due to die cracking were recorded. Finite element modeling (FEM) was used to evaluate the die stresses on these package designs. The stress levels from FEM were used along with the die strength distribution to predict the failure rates. These results were then used to create design rules to control the die cracking failure rate well below the allowable limits