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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Be sure to optimize the preheat to improve wetting.

This month we we illustrate high-temperature soldering of through-hole connectors. In this case both the PCB nickel/gold surface and the tin surface of the pin were perfectly solderable. The time to preheat the board and connector before soldering was not optimized for robotic soldering, however. To obtain the correct process setting, balance all settings, but to increase throughput in an inline process using a robot, many applications require preheating. Robots are flexible, but an operator can see and judge the time required.