Fatigue life evaluation of wire bonds in LED packages using numerical analysis

Reliability of LED packages is evaluated using several tests. When a thermal shock test, which is one of the reliability tests, is conducted, the most common failure mode is wire neck breakage. In order to evaluate the wire bonding reliability of LED packages, performing the thermal shock test is time-consuming. In this paper the wire bonding reliability for LED packages is evaluated by using numerical analysis. A wire bonding lifetime model for the thermal shock test was developed, which is based on Coffin-Manson fatigue law. The model was calibrated from fatigue data of thermal shock tests and volume averaging accumulated plastic strains. The accumulated plastic strains were calculated by using finite element analysis corresponding to the test conditions. The test conditions were changed by silicones, package sizes, wire bonding diameters, heights, and lengths. The calibrated model was used to estimate the number cycle to failure so that the wire bonding reliability for the thermal shock test was evaluated by performing the numerical analysis. Furthermore, we used a response surface methodology to study the relationship between the wire loop and the accumulated plastic strain to determine the optimal wire loop. The plastic strain was a function of diameter, height and length. At the optimal point, the number of cycle to failure for the thermal shock test was suggested using the wire bonding lifetime model.