Integrated flow-thermomechanical and reliability analysis of a densely packed C4/CBGA assembly

This paper presents an integrated flow-thermomechanical and reliability analysis of a C4/ceramic ball grid array (CBGA) under a mini power-cycled load of 0 to 5 W at a frequency of 3 cycles per hour. A computational fluid dynamics model was used for conjugate conduction-convection heat transfer analysis to determine the local heat transfer coefficients of the package under various low air flow conditions of 0.5 to 2.0 m/s. The determined local heat transfer coefficients were specified as the thermal boundary conditions in a thermomechanical model for transient heat transfer and nonlinear thermal stress analyses to predict the local temperature profiles and associated viscoplastic deformation of CBGA solder joints. By using a deformation base lifetime analysis method, the predicted mean fatigue life of the power-cycled solder joints is compared to that of the temperature-cycled solder joints under a 0/100°C load. Predictions of temperature-cycled solder mean life show good agreement with experimental data. Analysis shows that the leading edge and trailing edge CBGA solder joints in the package have different fatigue behaviour. This air flow induced orientation effect grows significantly as the air flow velocity increases. Both temperature-cycled and power-cycled solder fatigue results were used to estimate the equivalent solder fatigue lives of the C4/CBGA packages with application to laptop and desktop computer systems at an average on-off temperature rise ΔT of 20°C and 30°C