Moisture is only one of the potential culprits.
PCB delamination can be subtle or obvious. It is caused by expansion of moisture in the PCB laminate, but that may not be the root cause. Eliminating moisture often prevents the energy buildup that forces apart different layers, but this is not the complete story. Poor bonding during manufacturing of the multilayer board or some form of contamination may result in poor adhesion on innerlayers, permitting moisture to accumulate on these surfaces.
FIGURE 1 shows solder mask cracking around a through via. The PCB expanded during reflow, then contracted during cooling. This resulted in lifting and cracking of the solder mask, plus an intermittent electrical connection. FIGURE 2 shows the innerlayer surface of the board after separation. The through vias are separated and there is no visible adhesion on this layer.
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