Optimizing maximum equivalent strain on solder balls in flip-chip packages

摘要

Flip-chip packaging provides a high-performance low-cost approach for development of electronic packages. A three-dimensional (3D) viscoelastic-plastic finite element analysis using the commercial software ANSYS has been performed to study the thermo-mechanical behavior in flip-chips assemblies, i.e., the four components: chip, solder ball, underfill, and substrate. The viscoelastic behavior of underfill is modeled by a Maxwell constitutive equation, while the viscoplastic behavior of solder balls is modeled by an Anand model. Both chip and substrate are assumed to elastic materials modeled by Hooke's law. As in standard industry practice, temperature cycling from 125 to -40 degrees C is used. Thermo-mechanical behavior of solder balls is presented, and the effects of underfill material properties are investigated. Further, Taguchi methods are used to optimize flip-chip package performance. The design goal is to minimize the maximum equivalent strain on the solder balls. The eight flip-chip assembly factors chip-thickness/substrate-thickness ratio, underfill modulus (G(i)), underfill relaxation time (lambda(i)), solder height-to-diameter ratio, chip coefficient of thermal expansion (CTE), underfill CTE, solder CTE, and substrate CTE are chosen for optimization.