Disturbed state constitutive modeling and testing of solders in electronic packaging

Abstract

Accuracy of fatigue life prediction of solder joint materials using the finite element method (FEM) depends on realistic and reliable constitutive models and the FEM procedure itself. The effectiveness and accuracy of a constitutive model to simulate and predict the behavior of materials strongly depends on consistent, reliable and precise experimental data. In this research, a thermomechanical digital image correlation test device has been developed to investigate solder joint material behavior under thermal mechanical and cyclic loading. The test device is composed of two high precision actuators driven by two brushless servo-motors; a vacuum chamber used to prevent the heating and cooling plate from heating and cooling the air in the vacuum chamber and other components to save the energy needed to heat and cool the specimen; a heating and cooling unit composed of four Peltier devices; a digital correlation deformation and displacement measurement system and various load and displacement sensors. The test device is controlled by Labview. Two closed loop system control programs in Labview were developed to control the device and perform tests under specified loads, displacements and temperature profiles. The temperature, loads, and displacements can be applied manually and automatically. A series of tests such as normal load tests, temperature and rate dependent tests, isothermal mechanical fatigue and thermomechanical fatigue tests have been carried out on 63Sn/37Pb and 60Sn/40Pb solder joint materials. The rate and temperature dependent test results are consistent, which shows that newly developed test device is reliable. Material parameters for the HISS and DSC models were computed from the test data, which can be used to investigate the failure mechanism of solder joints in electronic packaging to improve the reliability of electronic packaging. Disturbed State Concept (DSC) constitutive model is a powerful tool for the simulation and prediction of material damage, failure and fatigue. Strain localization and material softening cause negative-semi definite finite element stiffness-matrix, which create numerical difficulties for the calculation of DSC. A new DSC finite element scheme was proposed and implemented to overcome the numerical difficulties. The finite element thermoplastic formulation was modified and implemented based on Drucker's postulate. Thermoplastic deformation and viscoplastic deformation usually happen sequentially or combined. An alternative finite element computational method for viscoplastic problem was proposed and the FE program is modified. Computational examples show the new procedures produce accurate and consistent results and the program is robust. Material parameters for the HISS and DSC models were used to back predict the rate and temperature dependent test results using the modified finite element program. These back predictions showed that the HISS and DSC constitutive models and the FE program can be used to accurately simulate solder joint material behavior and predict solder joint damage and failure.