Modeling Effect of Underfill Property Evolution on the FCBGA Reliability at Sustained Automotive Underhood Temperatures

The operating conditions inside the automotive underhood involve the sustained exposure of electronics to temperatures in the range of 150 to 200 ℃ for prolonged periods of time. The CTE (coefficient of thermal expansion) mismatch between the chip and the substrate causes assembly-warpage when operating at very high temperatures in flip chip packages. The underfills are used to support the solder balls, which reduces the solder joint strains and improves the fatigue life of the joints. In this paper, the effect of evolution of non-linear viscoelastic properties of two underfills were studied on plastic-work in the solder balls and other lidded FCBGA (Flip Chip Ball Grid Array) package components. To obtain the viscoelastic behavior of underfills, TTS (time-temperature superposition) experiments are performed at 7 discrete frequencies 0.1, 0.21, 0.46, 1, 2.15, 4.64, and 10 Hz in frequency sweep three-point bend mode in DMA. The shift factors are calculated from WLF (Williams-Landel-Ferry) equation as a function of temperature. Using the shift factors, TTS results, and at selected reference temperatures, the Master curves are obtained for storage moduli, loss moduli and tan-delta as a function of frequency. The relaxation modulus, thus bulk and shear modulus, are calculated in time domain using Schwarzl and Struik equation from storage and loss modulus master curves. The bulk and shear modulus curves are fitted with Prony series in ANSYS and Prony series coefficients are implemented in underfill properties. By taking the advantage of symmetry, quarter lidded FCBGA model was built, and two complete thermal cycles were applied from -40℃ to 125℃ to get a steady hysteresis loop. Aging samples include the pristine samples and the sampled aged for 30, 60, and 120 days.