Abstract
Thermal fatigue of interconnects is a common root cause of failure for microelectronics. The estimation of cycles to failure for a solder layer or joint is one of the critical requirements in the development of reliable electronic packages. For an assembly, the thermal-mechanical strains are induced mainly due to the temperature non-uniformity and the mismatch of Coefficients of Thermal Expansion (CTE) at multi-material joints. Temperature cycling results in stress cycling that in turn, causes fatigue damage to accumulate in the solder joints. Many previous studies have reported that the solder deformation due to the combination of plastic and creep is much larger than the values expected from a simple calculation based on the thermal expansion mismatch. And the time-dependent creep of solder causes more damage than the time-independent plasticity. The structural and material complexities of a microelectronic package, however, often makes an evaluation of the stress condition in solder interconnect a highly challenging task. In this paper, Digital Speckle Correlation (DSc) technique is employed to directly measure the solder deformation. By recording digital images of the solder layer with high magnification, and processing the greyscale image files, full-field information of the six in-plane deformation parameters in areas with high stress concentration was obtained in a test designed to simulate the package's operating condition. The steady state sine hyperbolic creep law is applied to relate the measured strain to stress, and the strain energy density was further determined from the stress vs. strain graph. Finally, neglecting the portion of the energy due to plastic deformation, the failure criterion of Critical Accumulated Strain Energy (CASE) was applied to correlate the creep strain energy density to the solder fatigue life. Transactions on Engineering Sciences vol 40, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533
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