| Popis: |
Solder interconnects are used for the majority of microelectronics products. Extensive studies led to (i) constitutive relation to describe the material, (ii) descriptions of the microstructure, and (iii) assessments of the fatigue life. Despite the tremendous research effort, mechanical simulations to accurately predict the solder fatigue life remain ambiguous. Experimental calibration of the results is often mandatory. In general, calibration is required per solder alloy, which makes a comparison between solder alloys for a specific product by simulation alone troublesome. The need for more accurate simulation of the solder fatigue life is fueled by the ever-increasing size of Wafer Level Chip Scale Packages (WLCSPs), and the reduction in WLCSP bump pitch. To cope with these trends new solder alloys have been introduced that are more stiff than traditional solder alloys. This has indeed increased the fatigue life of the solder bumps itself, but also introduced fails in e.g. the Re-Distribution Layers (RDL), or the Under-Bump Metallization (UBM), which did not occur for traditional SAC solder alloys. Mitigation of these fails requires a balanced product, which can be efficiently achieved by simulation models. The first step in such simulation model is an accurate description of the inelastic behavior of the solder material. In this work we present a viscoplastic description based on the Maxwell model with separate parameters for the creep and plastic behavior of the material. These elements are intuitive, and can be used to generate a consistent set of data for different solder alloys. We describe a routine to fit the parameters onto experimental data. Finally, we will indicate how the material description can be used to obtain a balance between the solder fatigue life, and the stress exerted on the RDL and UBM. This study thus gives handles to select an optimal bill of materials. |
