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°C to 125°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 μm and 500 μm) 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.