When a blow hole defect occurs during the assembly process as a result of the PCB card, the primary culprit tends to be entrapped moisture or air. With moisture, any non-plated and non-masked areas on a bare circuit board that expose internal laminate can be suspect to absorbing moisture. Absorption can occur either during the board fabrication process or from improper storage. Examples of highly suspect areas include non-plated drilled holes and routed features.
Blow Hole on Printed Circuit Board Magnified
Baking the circuit boards before assembly can help resolve these issues, although proper board storage is just as important. Bare PCBs should be stored in vacuum sealed bags with desiccant and humidity indicators. The packages should then be stored in areas devoid of high temperatures and high humidity.
In rare instances, boards can entrap air pockets behind plating. This is a process indicator for the PCB manufacturer. Rough drilling of plated through holes can lead to slight gaps between plating and the laminate within those holes. When the board is exposed to high temperatures during assembly, the entrapped air can expand rapidly and "blow" through thinner copper plating. This can be verified via micro-section of circuit board blow holes.