Die cracking and reliable die design for flip-chip assemblies

Die cracking during underfill cure or thermal cycling is a cause for concern in flip-chip assemblies. In this work, an integrated process-reliability modeling methodology has been developed to determine the stresses at the backside of the die during underfill cure and subsequent thermal cycling. The predicted die stresses have been compared with experimental data, and excellent agreement is seen between the theoretical predictions and the experimental data. The modeling methodology has been used to understand the effect of material and geometry parameters such as substrate thickness, die thickness, standoff height, interconnect pitch, underfill modulus and coefficient of thermal expansion (CTE), and solder mask CTE on die stresses and thus die cracking. Based on underfill-cure and thermal cycling models for specific cases, the critical flaw size to induce catastrophic die cracking has been calculated using linear-elastic fracture mechanics. Design recommendations, including die thinning and polishing, have been made to reduce the tensile stresses on the backside of the die and thus die cracking.