- De-wetting is often due to a lack of flux activity. This behavior rarely occurs with water-washable type pastes since these pastes are highly activated. Lower activity solder pastes in the category of ROLO, halide free no-cleans pastes tend to create this on more difficult finishes such bare copper OSP or on Ni-Au where the nickel base metal, may have experienced oxidation or plating contamination.

- Below are test coupons on which SAC no-clean paste was applied to two surfaces.

- The test coupons were then reflowed in air using the manufacturer’s recommended thermal profile. The one on the right shows de-wetting while the one on the right exhibits good wetting. The pooling of the solder was due to the base metal being difficult to solder to. The molten solder initial spread across the surface but not a good enough intermetallic bonds was formed, resulting in surface tension pulling the solder away.

• Select a paste with excellent activity up to the melting point of the alloy, 217° C for SAC alloys

• Use a more active flux system

• Insure metals to be joined are oxide-free as possible

• Insure base metals are solderable with the selected flux type

• Reduce the preheat time or temperatures as to preserve flux activity

• Increase time above liquidus (217 ° C), if flux activity is good

SAC Good wetting