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The aim of this paper is to investigate temperature and thermal stress responses at various joint arrangements during the cooling stage of the reflow process using an effective numerical approach. In this approach, numerical techniques for computational fluid modeling of the internal flow in the reflow oven were coupled with the structural cooling modeling at the board and package levels using a multi-physics code coupling interface. A thermal profiling experiment was conducted using the forced convection reflow oven to validate the simulation model. The numerical results were found to be in agreement with the experimental results. Results showed that the full-grid ball grid array (BGA) package has greatest temperature deviations, indicating different time responses between the start of the solidification process at different locations of the soldering process. Moreover, the solder joints experienced phase change from liquid to solid during the cooling stage of the reflow process. The large time interval for mushy zone of the full-grid BGA package indicated that the latent heat in a solder joint was hardly released to the environment. Consequently, this breaks the balance of the wetting force and increases the chances of the full-grid BGA package to skew. Generation of thermal stress at the interfaces of different materials occurred due to the mismatch of a variant coefficient of thermal expansion. Analysis and visualization of simulation results also showed that the maximum von-Mises stress of critically affected joints is influenced by solder joint arrangement patterns and not by the number of solder joints. The recommendation was also made to place dummy joints at the center of a package if routing and solder cracking problems were critical for peripheral array BGA package. The maximum von-Mises stress was reduced by 25.78% through improvement of solder arrangements. On the whole, the newly developed approach greatly helps reduce soldering defects and enhances s- lutions to lead-free reliability issues.
Components, Packaging and Manufacturing Technology, IEEE Transactions on (Volume:2 , Issue: 12 )
Date of Publication: Dec. 2012