Thermomechanical reliability study of flip-chip solder bumps: Using laser ultrasound technique and finite element method

Current techniques for nondestructive quality evaluation of solder bumps in electronic packages are either incapable of detecting solder bump cracks, or unsuitable for in-line inspection due to high cost and throughput. As an alternative, a solder bump inspection system is being developed at Georgia Tech using laser ultrasound and interferometric techniques [1]. This system uses a pulsed Nd:YAG laser to induce ultrasound in electronic packages in the thermoelastic regime; it then measures the transient out-of-plane displacement responses on the surface of the chip or packages using laser interferometric technique. This paper presents a systematic study on thermo-mechanical reliability of flip chip solder bumps using laser ultrasound-interferometric inspection system and finite element (FE) simulation. The correlation between the failure parameter of solder bumps extracted from FE simulation and the quality degradation results of solder bumps from laser ultrasound testing has also been studied. Accelerated thermal cycling (ATC) tests were performed in two phases on flip chip package (FCP) test vehicles with 63Sn37Pb solder bumps. In phase I, four boards were thermal cycled up to 70 cycles. Every ten cycles, the boards were taken out and FCPs on each board were tested for presence of cracks using resistance and laser ultrasound testing methods. These measurements were repeated every 10 cycles and stopped after 70 cycles. Laser ultrasound testing results show that beginning from 10 cycles, testing values began to increase gradually until 50 cycles when there was a sharp jump (through crack verified at 50 cycles with cross-section). Cross-section was done only after 70 cycles, and the results show that solder bumps at the corners had through cracks, while solder bumps at the center had partial cracks. Phase II studies were refinements of phase I studies. In phase II studies, seven boards were thermal cycled starting from 27 cycles to 56 cycles at an increment of approximate five cycles. After every five cycles, only one board was pulled out and tested in order to track crack initialization and propagation in solder bumps. Smaller thermal cycle increments were used to help determine exactly when cracks initiated and propagated. It was shown that laser ultrasound technique could track solder bump crack propagation induced by thermal cycling. The elastic-plastic-creep and Anand's viscoplastic constitutive models have been used in FE simulation to describe the inelastic deformation behavior of solder bumps. A three-dimension FE model was implemented using ANSYS
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on the same FCP as used in the ATC testing. The stress-strain results were extracted from FE modeling and the inelastic strain energy density (SED) per cycle calculated at the critical solder bump position was used as a failure parameter. It was found that there was a good correlation between inelastic SED extracted from FE simulation and results from laser ultrasound testing.