Effect of Underfill Property Evolution on Solder Joint Reliability in Automotive Applications

Underfills are used for reliability augmentation of flip-chip solder bumps in flip-chip ball-grid array (FCBGA) architectures. Incorporation of the advanced driver-assistance systems has resulted in incorporation of advanced packaging architectures including the FCBGAs in automotive platforms. Automotive electronics often resides underhood where it may be exposed to temperatures in the neighborhood of 150-200°C. Models and material evolution data are needed for assurance of 10-year, 100,000 mile reliability. Evolution of viscoelastic properties and effect of evolution of non-linear constitutive behavior on the plastic work in the solder joints has not been studied. In this paper, the evolution of properties of underfills over 1-year of sustained high temperature operation have been measured for four-underfills. The effect of the non-linear property evolution on the solder joint reliability has been assessed. The study relies on measurements using dynamic mechanical analyzer for the measurement of the viscoelastic properties including storage modulus, loss modulus, tangent delta, and the glass transition temperature. Study of viscoelastic behavior of underfills is achieved by performing TTS (time-temperature superposition) experiments at 7 discrete frequencies 0.1, 0.21, 0.46, 1, 2.15, 4.64, and 10 Hz using DMA in three-point bend mode. The measurements have been used to fit the data to Prony-Series for all the sustained high temperature exposure durations. The viscoplastic underfill models have been coupled with non-linear solder models to evaluate the effect of material constitutive behavior evolution on the inelastic strain energy density and thermo-mechanical fatigue life. Model results have been used to assess the effect of sustained thermal exposure on plastic work in the solder interconnects.