Blow Holes or Pin Holes
07 November 2006
These are considered by many people as defects in plated-through hole solder joints. In reality they have little effect on the reliability of the solder joint but they can be avoided or eliminated. The last time this author conducted reliability testing on boards exhibiting blow holes components failed before the joints. Holes in joints caused by out gassing are considered a process indicator in IPC 610 for class 2, 3 and acceptable for class 1. They are a defect in class 2, 3 if the void reduces the solder connection below the minimum acceptable.
This type of process problem is seen during the soldering process where gases can burst through the solder during solidification. This can be one escape of gas that just happens to coincide with the solidification of the solder or a continuous escape of gas. Blow hole or pin holes are basically the same thing, some engineers use the term to indicate the size of the problem which is usually seen at the bottom of the solder fillet. Blow hole is used for large voids and pin for small imperfections. The reason they are mainly seen on the base of the board is simply the solder in the bottom of the hole is the last to solidify during wave or selective soldering, so gas can only escape in this direction.
The main reason for outgassing on a printed circuit board is thin or poor plating in a plated through hole. The gassing that takes place is due to moisture in the board which turns to water vapour during soldering and looks for a path to escape. Many years ago in work conducted by the National Physical Laboratory, NPL they were able to capture the gas coming out of plated through holes during soldering. They were then able to analysis the gas and determine that it was simply moisture. Testing boards for out gassing can be fairly simple and a test kit with instructions, video of how to do the test and references is available from the SMART Group www.smartgroup.org
The minimum plating thickness of copper on a through hole is 20um, however most good suppliers have 25um as there limit to eliminate the possibility of outgassing during soldering. You can reduce or eliminate the problem by baking the board to eliminate the moisture but this creates other solderability problems. So make sure you have a good specification and use a supplier with a well-controlled process - not one that operates on the limits of international standards. You should not have to bake boards as standard practice for assembly even with lead-free. New surface finishes like copper, OSP and tin that are being adopted by engineers don't like extended baking. Any baking operation needs to be over 100°C, the time will depend on the size and number of the boards, the capacity of the oven and other factors relating to the board materials.
Pin or blow holes can also be seen on surface mount joints after reflow, however as these are surface pads and not plated through holes they are caused by a completely different phenomena.
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