Many people are perplexed by countersink holes (often called sink hole or countersunk hole) on PCB boards. Indeed these terms deserve a good explanation.

I  want to thank Mr.Li from our fabrication department.  He took me to school (maybe a little old school even) on countersinks and counterbores.

Countersink

A countersink hole is simply a normal right cylindrical through-hole with an additional right conical section removed on the top. The cone angle is typically 82 or 100 degrees. Why do you need Countersink?  Makes it possible for a flat head screw to sit flush with the surface to make for a very clean look and installation.

Counterbore

A counterbore is a cylindrical flat-bottom hole that is cut that also allows for a socket cap screw (has a flat head) to be used. The socket cap screw usually has an allen wrench drive (hex) hole.Why do you need a Counterbore? It is typically needed for much more secure and stable mountings.

Application
Countersinks and counterbores are not anything unique to printed circuit boards. They are used in a variety of manufacturing of both metal, wood and other materials.A common usage of countersink or counterbore holes is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material.

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