The main purpose of PCB baking is dehumidification: removing moisture contained in the PCB or absorbed from outside, because some materials used in PCBs readily take up water molecules.
PCB PCB popcorn delamination PCB 100℃ hot-air
The faster the PCB is heated, the faster water vapour expands; the higher the temperature, the larger the vapour volume; if vapour cannot escape from the PCB in time, it can blister the board.
The Z-axis of a PCB is especially weak: vias between layers may be pulled apart, layers may delaminate, and in severe cases blistering, swelling or board explosion is visible on the surface.
Even when the PCB looks fine outside, internal damage may already exist and later cause unstable electronics, CAF or product failure.
