It is relatively common for questions about white deposits on the solder beads of copper foiled pieces to be raised. In reflecting on the cause of the white deposit on solder beads, I recalled that some people use baking soda to neutralise the flux. I put this together with some work on lead corrosion.
I have been doing a bit of research on lead came corrosion in another context. One of the forms of lead corrosion is white lead corrosion, or lead carbonate. It has the chemical compound PbCO3. It is a curious compound, as it is soluble in both acid and alkali. This much you will have seen from a previous posting about lead corrosion.
In that it is possible for excess whiting left on lead cames to give rise to this form of white corrosion. Baking soda has a chemical formula of NaHCO3. Solder contains a significant amount of lead – usually 37-40%. The chemical reaction of lead and baking soda gives lead carbonate - PbCO3 and NaH -sodium hydride. The sodium hydride is soluble in water, leaving the white deposit of lead carbonate as a corrosion product on the surface.
Putting these things together leads me to recommend that baking soda and other carbonates should not be used in cleaning solder beads. Some other non-carbonate neutralising or rinsing agent should be used instead.