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High Reliability Flip Chip Assembly using Fluxless Reflow Techniques
Keywords: flip chip, fluxless, miniaturization
The demand for miniaturization and weight reduction in electronics continues to drive assembly solutions for high reliability flip chip technology in flight and space electronics. Solder interconnect technology provides a proven high reliability flip chip interconnect system. However, the flux residues typically associated with solder flip chip technology are sometimes found to compromise long term reliability. A comprehensive study is performed investigating fluxless reflow technology for solder flip chip assembly. Specifically, fluxless reflow is performed using a forming gas (a nitrogen and hydrogen gas mix) technique on high lead and eutectic lead tin solder flip chip bumps mounted on thick film ceramic substrates. This project looks at the different variables of fluxless reflow soldering including the substrate temperature, bonding collet temperature, bond force, forming gas flow rate (or pressure), and forming gas temperature. Parametric and design experiments are used to study the effects of the process variables on interconnect quality and interconnect yield. Both underfilled and non-underfilled assemblies are evaluated as baseline assemblies. The fluxless assemblies are compared to baseline solder flip chip assemblies built with both no clean and water soluble flux. The water soluble components are cleaned by using a hot water soak and agitation technique. The baseline assemblies process with flux were dip fluxed, placed using an automated flip chip bonder, and reflowed in a convection reflow furnace. A detailed reliability screening is performed on the assemblies to assess the reliability robustness of the fluxless solder flip chip assemblies compared with the baseline flux assemblies. Reliability testing conditions for air to air thermal cycling are from -55 C to 125 C. Statistical analysis of the reliability results is presented.
Daniel F. Baldwin, Ph.D., President and Chief Techical Officer
Engent, Inc.
Norcross, GA

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