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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?

Edit:

enter image description here

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).

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    \$\begingroup\$ What is the actual board temperature you measured? \$\endgroup\$ – W5VO Jun 27 '17 at 0:53
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    \$\begingroup\$ heat source ? and method of flow? forced or convection? makes a huge difference to heat absorption and profile and temp of each component's surface as well as material type of FR4 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '17 at 1:25
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    \$\begingroup\$ As Tony Stewart notes, the material used makes a difference: FR-4 simply means the material is flame retardent. Are you buying the material to one of the IPC4101/??? standards? \$\endgroup\$ – Peter Smith Jun 27 '17 at 6:48
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    \$\begingroup\$ "It turns from bright green to almost black." - None of the FR4 PCBs I've seen are bright green, they are invariably a dull semi-translucent beige colour. What's usually green is the soldermask. Can you post pictures of your PCB (preferably before and after the blackening)? \$\endgroup\$ – marcelm Jun 27 '17 at 9:51
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    \$\begingroup\$ Please post your complete temperature profile. \$\endgroup\$ – Glenn W9IQ Jun 27 '17 at 11:32

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