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This paper presents a reliability model for wafer level chip scale packages (WLCSP) assembled with Sn4%Ag 0.5%Cu (SAC 405) solder. The reliability model is based on a creep constitutive model that takes into consideration the dimensions of the solder joints and a thermo-mechanical fatigue crack growth model. The creep constitutive model was derived from over 250 constant load creep tests performed on micro scale specimens with specimen dimensions in the size range of 300 to 450 mum and specimen aspect ratios in the range from 1.2 - 5.1. In this work, data from experiments that range from 1E-3/s to 1E-9/s. while the applied stress levels ranged from 5 MPa to 40 MPa is presented for 3 different temperatures: 40degC, 70degC, and 100degC. Different SINH creep law based creep models were generated for the different specimen dimensions which are presented in this work. Fatigue crack growth during thermal cycling of soldered assemblies is measured on assemblies with 10 times 10 mm WLCSP's for temperature cycle conditions of 0-100degC, -40 to 125degC and 0-150degC. Strain and strain energy estimations were done with FE analysis and these were correlated with the crack growth rates measured for the different assemblies and different test conditions. The N63 (Number of cycles to 63% sample set) failure data based on 2 parameter Weibull fitting was correlated to the crack growth rate estimations and the strain energy estimations to develop a general failure life model.