Corrosion is caused by an electrochemical reaction between two dissimilar metals, one acting as an anode – one as a cathode. Halide ions, such as fluoride, bromide, and chloride, are the usual culprits for electrolytic mobility. Although they are commonly found in fluxes, I was surprised to find they are also used in etching, plating, sawing, and encapsulation. When used in flux, the chemistry is designed to either entrap the halide so it is not harmful, or ease its cleaning so it can be fully removed (such as in a water soluble flux). On low standoff flip chips, water soluble flux cleanability is of utmost importance during formulation.
All hope is not lost, and you can still assure high reliability by using a no-clean flux appropriately, or using a water-soluble flux and verifying your water washing process. Large chips with low standoffs (50 to 100 microns) require a precision water jet setup to help the water reach the center of the chip-to-substrate interface.