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Blow Hole
Pin holes or blow holes are the same thing and caused by the printed board outgassing during soldering. Moisture in the board escapes through either thin copper plating or voids in the plating. The term pin or blow hole are normally used to indicate the size of the hole, pin being small. The size is solely dependent on the volume of water vapour escaping and the point the solder solidifies. The only way to eliminate the problem is improve the board quality with a minimum of 25um of copper plating in the through hole. Baking is often used to eliminate the gassing problems by drying out the board, Baking the board takes the water out of the board but it does not solve the root cause of the problem. Have your boards tested for outgassing by EPS.