Most of the credit to temperature withstanding is the composition of FR-4 and the soldermask with its primary heavy constituents of titanium dioxide and barium sulfate.
FR-4 retains 50+% of it tensile strength at 350C (660F) under short term exposure.  TiO2 and BaSO4 do not degrade until exposed to 600C (1100F).  The remaining fillers are subject to burn off at temperatures well short of 105C (220F) leaving the remaining matrix to support the PCB.  The Dagobah (just kidding DGEBA or BADGE if you prefer) will not reach boil off temperature until 450C (850F), so the matrix may droop if too heavy of a load is applied, but if done properly, it can be processed at temperatures as high as 350C pretty safely.  Part of the reason I use Durastone is that the continuous operating temperature is 300C (575F) and can take short term exposures over 350C (660F)  This is useful if you ever have to create a brazed connection to a PCB.  *TIP* always remove the feed wire before removing the heat or you will rip the pad or pads clean off.  <<< My memory is somewhat foggy regarding how I know this >>>