A Modified Acceleration Factor Empirical Equation for BGA Type Package

In order to estimate and to describe the actual fatigue lifetime of advanced packaging, utilizing the methodology of accelerated thermal cycling (ATC) experiment is an essential procedure to hasten the fatigue growth time through the manner of placing concerned vehicle in a severe condition as compared with the real working environment. Behind ATC tests, the experimental results and acceleration factor (AF) formula are integrated to assess the fatigue span of the actual use or other loading conditions. Therefore, the accuracy of the AF will affect the lifetime prediction significantly. The original AF model, more suitable for ball-grid array (BGA) type with lower ramp rate, was presented by Norris and Landzberg (N-L). It is basically composed of the range, cycling frequency, and the peak value of temperature. It is noted that the creep behavior of solder is required to be taken into account due to the applied temperature during ATC test constantly surpasses one third of solder melting point. Many evidences shown the original N-L AF is not adequate for predicting the reliability life when the test devices subjected to different ramp rates. To make original N-L AF formula have a better capability of predicting solder joint lifetime under various ramp rates, a modified AF model considered the creep behavior of solders and the ramp rate effect of ATC is proposed in this research. In the present AF model, the term of maximum temperature is modified by the mean temperature because of the accumulation of creep strain. Moreover, the thermal loading frequency separated into two portions, the ramp rate and the dwell time, is adopted for adjusting the ramp rate effect. To reveal above-mentioned mechanical behavior, a simulated model of the vehicle of wafer level chip scaling package (WLCSP) is built in this study. In addition, the creep phenomenon of the SnAg lead-free solders is emulated by using the Garofalo Hyperbolic Sine equation. To explore the peak value effect of applied temperature effect on the WLCSP fatigue lifetime under ATC test, two fatigue span estimated models, strained-based Coffin-Manson empirical formula and energy-based Darveaux model, are introduced. The modified AF model proposed in this research shown better prediction results than the original Norris and Landzberg AF empirical equation in many packaging cases.