Characterization of Thermal Cycling Ramp Rate and Dwell Time Effects on AF (Acceleration Factor) Estimation

Acceleration factor (AF) empirical equation hasbeen developed for many years to predict product lifetimeunder field-use condition based on accelerated thermal cycling (ATC) test result and loading condition, such as well-known Norris-Landzberg (N-L) equation with operating temperature and frequency terms. However, some studies in literature have indicated opposite test results to that using current N-L AF empirical equation in high ramp rate case. That means with higher ramp rate, i.e. higher frequency, smaller AF number is obtained based on current AF empirical equation, which indicates longer test lifetime. However, the test data shows opposite result, shorter test lifetime, which is due to solder strain rate effect induced by high ramp rate loading. That is also because current AF empirical equation combines both ramp rate and dwell time factors in one frequency term, and strain rate effect cannot be well reflected. Such risk becomesmore critical especially for widely-used lead-free soldermaterial which owns higher strain rate property than SnPbeutectic solder. The study to modify current AF empiricalequation is necessary to better address such phenomenon. In this paper, a modified AF empirical equation has beencharacterized with SnAg based lead-free solder on wafer level chip scale package under various thermal cycling profiles. This study use finite element method (FEM) with a temperaturedependent Young's modulus and Garofalo-Arrhenius creep equation to describe the strain rate and creep of solder properties during thermal cycling loading. Several effects are observed regarding solder strain rate effect, and thermal cycling ramp rate to solder fatigue lifetime. A novel AF empirical equation has been successfully developed with two new terms instead of original frequency term, i.e. cycle time, to distinguish thermal cycling ramp rate and dwell time effects. The derived AF empirical equation shows good correlationwith test results, including the tests with various dwell timeand ramp rate profiles. In addition, new AF equation has been further verified by others package types reported in the literature. This new AF equation can serve as useful guidance for microelectronics reliability assessment.