SMTA International Conference Proceedings


Authors: Nathan Blattau and Craig Hillman
Company: DfR Solutions
Date Published: 9/24/2006   Conference: SMTA International

Abstract: As the transition to Pb-free progresses from consumer and computer electronics to applications with longer life requirements and more extreme use environments, there is a rising concern regarding the long-term reliability of Pb-free interconnections. In response, several publications have recently presented Pb-free solder joint reliability models. All of these models, while useful, have severe limitations. Strain rate equations and damage models allow for the use of finite element modeling (FEM) to predict solder joint reliability, but this is a specialized tool that requires the use of scarce resources. A modified Norris-Landzberg equation provides a correlation between test results and field performance, but there is a strong impetus to ensure robustness during the design stage, rather than identify potential issues after thousands of hours of testing. Extending work done by Engelmaier [3] and available experimental results from literature, DfR proposes a Pb-free first-order solder joint reliability model based upon cyclic strain energy density. The maximum strain range of the solder joint is determined using formulas developed by Engelmaier. The stresses on the solder joint are determined by using a simplified structural model that accounts for the various stiffnesses of the structure. These strain and stress results are then used to determine the strain energy dissipated by the solder joint. The strain energy was then used to make life predictions using equations developed by Syed [6] and Dasgutpa [7]. These time-to-failure predictions are then compared to existing accelerated test data, which allowed for the calibration of model constants.

Keywords: Pb-free, failure, model, analytical, fatigue, thermal cycling, resistor, SnAgCu, solder

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