Interfacial fracture analysis of underfill delamination and flip chip reliability optimization

Abstract

A generalized plane strain condition was assumed for an edge interfacial crack between die passivation and underfill for an organic substrate flip chip package. C4 solder bumps are explicitly modeled. Temperature excursion is applied as loadmg condition. The design factors studied include underfill modulus, underfill coefficient of thermal expansioR CTE, fillet height and die overhang. Underfill modulus and CTE are functions of filler content. Varying underfill modulus and CTE produces a different stress field at underfilVdie passivation interface, different Stress Intensity Factor (SIF) and phase angle even under same loading condition. The baseline case uses underfill with elastic modulus of 6 GPa, CTE of 36 ppdT and fillet height equal to half die thickness. Four more cases involving underfill material properties are investigated by varying elastic modulus between 3 GPa and 9 GPa, and by varying CTE between 26 ppm/C and 46 ppd0C. Effect of fillet height is also studied by assuming no fillet and full fillet, i.e. fillet height equal to die thickness. Lastly, two cases concerning the influence of die overhang, defmed as the nominal distance between outmost solder joint and die edge, are investigated. The effects of these factors are evaluated on fracture parameters including energy release rate (G) and phase angle (v). Underfill material properties (elastic modulus and CTE), fillet configuration and die overhang can be optimized to reduce the risk of underfill delamination in flip chip or DCA applications.