Modeling and process development of Die Edge Protection to alleviate thermo-mechanical stresses on silicon dies in PBGA packages

Mechanical stresses induced by the mismatch of coefficient of thermal expansion (CTE) among different materials are the major driving force of fracture failures in silicon dies of microelectronic packages. The stresses induced are concentrated at locations where different materials interfaced and geometrical singularities such as die corners and die edges. Stress analyses using finite element mechanical modeling demonstrated that the stresses close to the die corners are 30% ~ 80% higher compared to the inner regions of the die. In addition, cracks inside the bulk mold compound and interfacial delamination on top of the die have been found due to these high stresses. DEP (Die Edge Protection) epoxy, a low-stress high CTE polymeric material, has been used to cover the corners and edges of the die to reduce stress levels with the objective of eradicating failures associated with these stress singularities. It acts as a buffering medium to mechanically shield the die corners and edges from being in direct contact to the high modulus molding compound materials. The DEP coating is achieved by dispensing the liquid DEP epoxy outside the wire bonded die area and allowing the epoxy to creep up along the die corners and edges. The DEP dispense process takes place after wire bonding. In the subsequent molding process step, areas coated with DEP will be shielded from the mold compound. It was observed from experimental evaluations that the DEP coating managed to prevent die corners cracking and delamination. This paper examines the simulation of the DEP coating, and discusses the DEP process development required to implement such a scheme into high volume production.