Can you identify this substance on PCB pads?
It is likely a combination of various salts, mostly Copper Bromide, Lead Chloride, Lead Carbonate, and Lead Bromide. Most likely Lead Carbonate is the vast majority of the build up due to the high \$CO_2\$ levels experienced from the exhaust. It is a reaction byproduct between the Lead and the \$CO_2\$.
It should be noted Lead Salts, like most salts, are conductive.
For more information see this detailed analysis on white residue commonly found on PCB boards: https://www.kester.com/Portals/0/Documents/FAQs/White_Residue.pdf