by Bob Chandler
1. December 2009 20:08
Non-Plated Through holes (NPT).
NPTs are deceptively simple in most CAD systems. So much so, they are frequently over-looked, with disastrous consequences. An NPT is used for mounting holes on a board, for alignment pegs on connectors, and for tooling holes for assembly. The entire idea of the NPT is that it doesn’t have a large copper pad around it. In most CAD systems, you don’t actually “see” the holes. You see the pads. You route around pads and keep the soldermask back from pads. But what happens when you can’t see the hole. More than once, we have had boards go to fabrication with traces and soldermask too close to the NPT. If the soldermask gets too close, it can “flake off” during assembly, or during the life of the product, causing all kinds of reliability issues. If the trace gets too close to the NPT, it can be cut by the drill when the hole is made – again a real problem. To avoid these problems – our policy is to add a “Route Keep-Out” on all layers around the NPT, making it “visible” while the designer routes.
If you are an engineer, you might want to add NPTs to your standard check list of items to review. It’s on ours now.
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