Achieving crack free package through elimination of type II failure

Popcorn package cracking is a major package reliability issue for surface mount IC packages. Although many works have been done in the quest for a solution to this problem, the success is only limited. This paper reports on the Type II cracking mechanism in a TQFP-208 package and discusses a new leadframe design which can pass the stringent JEDEC J-std-020 Level 1 moisture sensitivity test. During soldering reflow exposure, delamination was found to initiate at die pad shoulder area and propagated into the die attach/die pad interface. In the worst case, the delamination extended to mold compound cracking at the tip of the die pad. Finite Element Analysis (FEA) suggests that the interfacial stresses along die attach/die pad interface is highest at the die attach fillet and the mold compound next to it. A die attach with a higher modulus provides stronger coupling between the silicon die and copper die pad, reduces relative displacement and hence preventing initiation of mold compound/die pad interfacial delamination at die pad shoulder area. However, this is only true if the die attach is also a strong adhesive to the die pad. To overcome this constraint, a new leadframe design is made with die pad which eliminates the die pad shoulder or die attach fillet. One of such design is a "small window pad" which minimises die pad area yet provides good mechanical stability for mold encapsulation and handling during assembly process prior to molding. The new die pad design is found to be applicable to both TQFP as well as QFP packages and passes the JEDEC J-std-020 Level 1 moisture sensitivity test.