Effect of solder ball pitch and substrate material of printed wiring board on reliability under thermal cycling - a finite element analysis

A detailed Finite Element Model was created to study the effect of different parameters, structural and material, on the stresses/strains experienced by the different features in a composite containing chip package/solder ball/PWB, during thermal fatigue (-40/spl deg/C to 125/spl deg/C). The most prevalent failure mechanisms are the solder-ball failure due to cumulative creep-energy-density of the critical-solder-balls and the rupture of the surface dielectric (HDI Layer) of the PWB due to in-plane tensile strains. This analysis studied the effect of the solder ball pitch (800 /spl mu/m and 500 /spl mu/m) and the material properties of the HDI substrates, on the stresses at the solder balls. In general, a decrease in the BGA pitch increases the cumulative creep strain-energy density at the critical section of the solder ball, by about 50%. That, in turn, will increase the propensity of creep failure of the solder balls under thermal fatigue. Reduction of pitch-size also increased other stresses/strains of the solder balls (e.g. Von Misses stress). These stresses/strains at the solder balls were lower, when the HDI substrate in the PWB had low coefficient of thermal expansion (CTE). The adverse effects of decreasing pitch-size can be compensated by use of a reinforced surface dielectric with low CTE, and thus increase the reliability of the structure under thermal fatigue.