Die cracking and reliable die design for flip-chip assemblies

Die cracking during underfill cure or thermal cycling is a cause for concer n in hip-chip assemblies. In this work, an integrated process-reliability m odeling methodology has been developed to determine the stresses at the bac kside of the die during underfill cure and subsequent thermal cycling. The predicted die stresses have been compared with experimental data, and excel lent agreement is seen between the theoretical predictions and the experime ntal data. The modeling methodology has been used to understand the effect of material and geometry parameters such as substrate thickness, die thickn ess, standoff height, interconnect pitch, underfill modulus and coefficient of thermal expansion (CTE), and solder mask CTE on die stresses and thus d ie cracking, Based on underfill-cure and thermal cycling models for specifi c eases, the critical flaw size to induce catastrophic die cracking has bee n calculated using linear-elastic fracture mechanics. Design recommendation s, including die thinning and polishing, have been made to reduce the tensi le stresses on the backside of the die and thus die cracking.