Is it normal for exposed (no solder mask) FR-4 to blacken during reflow? We're using a fairly ordinary profile with a max at 233C, about 5 minutes total and only about 60 seconds above 200C. I realize FR-4 has a max temp of 130C+/- long term but I thought it should be able to take a reflow cycle.
It turns from bright green to almost black. It's darker in areas exposed on both sides, not quite so bad over large copper floods. Electrically everything is fine and the boards are functional, and the solder looks great down to 20 mil pitch QFNs. The gold connectors and traces aren't discolored so I don't think it's something else on the board or in the oven fuming off, and there was no smoke.
Are we doing something wrong, or is solder mask simply mandatory for reflow on FR-4?
The SMA connectors were hand soldered on after the fact and the white stuff is rosin from that (doesn't clean well off the blackened surface).