PCB SolderPad Yellowish

I hv a PCB (mtrl is RF35). After going thru' reflow, we found yellowish stain on the solder pad (solder mask). It looks like burn-marks. The reflow temp use is 245oC. We tried running bare pcb under 220oC, there's minimum stain found (acceptable),however yet to test out component solderability. We are trying to get the mtrl of the solder pad finishing and it's Td temp from the pcb maker but the reply is very slow. That's why we need to do something while waiting for their reply. Does anyone came across this problem? Could it be most probably due to low Td of the finishing mtrl?

Thks...

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