If the PCB's are HASL finished, the problem may be due to improper rinsing of the HASL fluids/fluxes used. They absorb moisture like mad and can cause long term reliability issue's.

If its moisture you are having a problem with run the board through a reflow cycle or bake them over 100 degC to remove the moisture.

Most likely this would be what is called blow holes. They will have the appearance of a volcano top shape and edge, They will be predominately appear on the bottomside solder joints after a wave soldering process.

As you indicated, these defects are caused by moisture in the board. The moisture accelerates into vapor from the heat or the wave process and escapes after exiting the wave. At times there is enough moisture that the barrel can actually be cracked by the escaping vapor.

Depending on the level of moisture entrapped you maybe be able to evaporate the moisture by using a pre-bake in a heat chamber.

It sounds like you have found out the problem, humid PCB storage. To salvage the remaining boards you should pre-bake them just like you would a PBGA component.

First make sure the temperature that the board's solder joints are being subjected to are within reasonable temperature limits by running a thermal profile of the board with thermocouples located on the solder joints specifically where the "explosions" are happening.

This will assure that the temperatures are not too hot and are of a reasonable time duration. Not to hot is: no more then the component's maximum temperature rating as specified by the manufacture, and yet well above the solder's liquidous temperature, and the correct duration is typically 2 to 4 seconds.

Besides moisture, one possible cause of these explosions (AKA: blow holes) is air escaping from between the layers of the PCB through pin holes in the barrel of the plated through hole. This will happen no matter how much you pre-bake the board because it's air and not moisture being forced out as the board heats.

If pre-baking the bare boards to drive off moisture does not solve the problem, then the only solution is to look to your board vendor to make sure the through-hole plating is 100% with no pin holes. undefined undefined

It is difficult to diagnose accurately without seeing an example of the failure and doing a cross section on it.

However, this problem could be caused by either circumferential or just large voids in the plated through holes of the bare board. This would allow the fibers of the glass in the laminate to absorb moisture, not just water but possibly acids or other process chemicals.

This moisture boils when the board is soldered causing the "explosion" like occurrence you describe. Some fabricators do a final bake to avoid this problem and in fact that is something that you can do to reduce the incidence of these failures.

Be aware however that baking might improve the apparent solder joint quality (the symptom) it will not fix the underlying issue of poor quality bare boards with voids or possible very thin or under spec hole wall plating, if, in fact that is the cause.

The fact that it is batch related makes me very suspicious of the underlying quality of the batch in question and I would certainly inspect the holes for voids and I would cross section a sampling of any suspicious areas for voids as well as for minimum hole wall plating thickness.