The definition of Blow Hole
Below hole is a type void in the solder fillet.
And the below hole caused by outgassing from the barrel of a plated through hole.
Then the question is what is outgassing.
What is Outgassing?
If you have learned PCB design theory, you may find the definition of outgassing is similar with Below Hole.
The gaseous emission from a laminate rigid PCB or component.
Maybe someone will confuse the feature of outgassing.
Set the ougassing on the rigid PCB is aimed to protect the board.
And the board or PCBA is exposed to heat or reduced air pressure or both.
Some common senses of outgassing
Rigid PCB outgassing is still a common problem.
This kind of outgassing associated with wave and hand soldering.
You know when a rigid PCB board is soldered, there was some moisture in the board close to the hole.
And the solder process must in high temperature, that the reason why those moisture will heated and turned to vapor.
This is safety in most normal rigid PCB board assembly process.
Two ignore of the outgassing should be careful about
The first condition is that there was a thin plating or a void in the plating.
The outgassing can come through the plated hole wall.
Usually you do worry about this condition will be good.
Then solder is present in the hole.
This condition should be careful about, due to the solder will be produce voids in the solder as it solidifies.
If solder is present in the hole, this will produce voids in the solder as it solidifies.
In other words, the voids may appear as small holes in the surface of the joint.