SMTA International Conference Proceedings

Epoxy Fluxes: Dip Assembly Process Issues and Reliability

Authors: Pericles A. Kondos, Ph.D., Michael Meilunas, and Martin Anselm, Ph.D.
Company: A.R.E.A. Consortium - Universal Instruments Corporation
Date Published: 10/14/2012   Conference: SMTA International

Abstract: Epoxy fluxes act like conventional fluxes in the initial stages of reflow, but then harden and form a “shell” around individual solder joints. This shell is expected to protect the joints from mechanical stresses, a protection not offered by conventional fluxes. For this reason, epoxy fluxes have been of increased interest lately in applications when use of capillary underfill after reflow is difficult or impossible to implement. Process development for the use of epoxy fluxes is not trivial and advantages in throughput and cost are most dramatic in dipping the material as compared to dispensing. However, reaching manufacturer’s recommended amounts of flux under the device can be challenging in dipping applications.

The assembly process was investigated during the initial stage of this project, and dipping and placement parameters that allowed consistent fluxing were determined. For this early research project only a single material was tested. Two different epoxy flux processing thicknesses were developed for a CSP and a WLCSP of slightly different sizes but the same pitch and ball size. The CSPs were placed on drop testing boards and were subjected to drop or bend tests. Assemblies with the WLCSPs were subjected to accelerated thermal cycling. The reliability performance of all these parts was compared to conventionally-fluxed, non-underfilled assemblies. Failure modes were studied and compared as well. It was seen that epoxy flux could indeed improve the performance as compared to non-underfilled parts if properly applied, but if not it might even reduce reliability.

Key Words: 

Epoxy flux, mechanical reliability, drop test, CSP, WLCSP, flux dipping, accelerated thermal cycling

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