The possible cause for yellow/brown discoloration are the flux viscosity and the reflow profile.
You need to check the flux property like activation temp, viscosity, helide contains in the flux and temperature profile of soldering.Also check the amount of flux apply to the solder joints.
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Sandip Thakor has 9 years of experience in electronics industry specializing in soldering technology. Thakor has experience in lead free installation, process optimization and developing quality standards.
Yellow / Brown color on silver is silver tarnish which is a combination of silver sulfide and silver oxide which occurs over time as silver is exposed to air. Heat accelerates silver tarnishing, so it is easily visible after the first reflow cycle.
Unfortunately silver tarnish can cause solderability issues and subsequent reflow or wave soldering may be difficult.
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.