3240 can be anything. And stuff sold as FR4 can be outright crap.
So, there's the enterprising Chinese entrepreneur who sells very old FR4 on Amazon... It may once have qualified as FR4, but not today. These plates were old, ugly, damaged, full of scratches, and definitely contain formaldehyde, bromide, and probably tons of other halogens.
Moral of the story: made sure you know what and where you buy!
















































