Jun 15.2019, 09:09:48
Water absorption is a REAL problem, but it will NOT cause burning. it can cause blistering of the substrate or bubbling of large copper areas. A bake out at 50 C for one hour before stuffing and reflow will generally solve that problem.
But, you black board really looks like the temperature control failed, and the board got SEVERELY overheated. I have a setup that requires a thermocouple to be poked into a plated-through hole in the board. One time I forgot to place the thermocouple, and the boards caught fire, and looked a bit worse than your black one.
It appears both boards had an uneven temperature across the boards. The tops (as in the picture) got much hotter than the bottoms.
I use a toaster oven with thermocouple controller, and the only way to get proper temperature control is to have the thermocouple touching the board. With the thermocouple in the air, it can read 100 C or more BELOW the actual board temperature.