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Overcoming Package Stress in Flip Chips with Low Outgassed Silicone Materials to Reduce Contamination

The EU mandates for lead free solders are often soldered at higher temperatures than lead and have created a need for electronic packaging materials that can handle these thermal cycling extremes. The low modulus of silicones as compared to epoxies overcomes the stress due to large CTE differences in the package with the heat sink, heat spreader, die and substrate. The lap shear, bulk conductivity (W/mK) and ionic content of three TIM materials were tested and compared to each other. The TIM materials used were a low outgassing silicone, standard silicone, and an epoxy. Both the standard silicone and epoxy materials tested had higher weight loss at 275ºC for 1 hour than the low outgassing silicone. Thermal Interface Materials (TIMs) and Underfills based on low outgassed silicone chemistry can aid the designer in overcoming thermal cycling stress while maintaining minimal contamination due to low levels of contaminates from mobile polymer chains that have been known to cause fogging.