Effect of randomness of Cu-Sn intermetallic compound layer thickness on reliability of surface mount solder joints

A statistical reliability analysis on thermal fatigue lifetime of surface m ount solder joints, considering randomness of Cu-Sn intermetallic compound (IMC) layer thickness, is presented. Based on published thermal fatigue lif e test data, the two-parameter Weibull distribution of the thermal fatigue lifetime for a fixed IMC layer thickness Is found, and a K-S goodness-of-fi t test is conducted to examine the goodness of fit of the assumed Weibull d istribution. Then, the Weibull parameters as functions of IMC layer thickne ss are obtained. With considering the randomness of IMC layer thickness, th e MTTF and reliability of surface mount solder joints on thermal cycles are analyzed. For surface mount solder joints formed under the same conditions and loaded during the same thermal cycling as stated in the publication, n umerical results of the MTTF and reliability are presented. The results sho w that when the mean value of IMC layer thickness is low (e.g., smaller tha n 1.5 mu m), the effect of randomness of IMC layer thickness is significant ; i.e., the MTTF has strong dependence on IMC layer thickness distribution, and the reliability is significantly different at high thermal cycles. Whe n the mean value of IMC layer thickness is high (e.g., greater than 2.0 mu m), the effect of randomness of IMC layer thickness is negligible. Therefor e, the presented results are important to the reliability study of surface mount solder joints. Even though the validity of the presented results base d on the test data remains to be verified from other sources of data, the p roposed statistical method is generally applicable for thermal fatigue reli ability analysis of surface mount solder joints. By combining the proposed method with the forming mechanism of IMC layer under varying manufacturing and loading conditions, a comprehensive reliability analysis on thermal fat igue lifetime of surface mount solder joints can be expected.