Drilling in Printed Circuit Board

A circuit board drilling is just like a common drill, except that it is optimized for drilling holes in the printed circuit board as opposed to drilling holes in other materials such as wood. Holes are normally drilled in circuit boards which made from tungsten because of its mechanical properties. Another difference of circuit board drilling from wood drilling is that, the size of this type of drill can vary greatly because it typically is very small. Sometimes, this drill is made by people, but it more often is combined with a drill press for automated drilling by the machines.

There are two main drilling holes in the circuit board. One is NPTH hole, another is PTH hole. All the holes in the single sided printed circuit board are NPTH hole. That’s because no need for electronic conduction between layers in single sided PCB. It has just one copper foil layer. PTH hole will appear in double sided or multilayer PCB.
These holes exist for the electronic conduction among layers. So it may have Copper in these holes.

While as explained, circuit board drilling holes must be very small. We usually use unit mil to indicate it. What’s more tungsten is a very hard metal. So circuit board drilling holes are made by drill bits. The PCB drill bits are made of micro grain carbide. They are used primarily in CNC machines for drilling printed circuit boards for electronics, but their toughness and accuracy lend them to uses in other industries. When a hole has to be drilled through a circuit board, the bit must go through copper traces, which are sturdy and hard. That’s a subtle work. A piece of printed circuit board may have several thousand of drilling holes including NPTH and PTH holes, so many PCB boards are finished drilling by machines!

One thing we should pay attention to is the smear when drilling holes in a PCB. For multilayer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. We should remove the smear before plating through these holes. So we need to have a chemical de-smear process, or plasma-etch. The de-smear process remove smear to ensure that a good connection or electronic conduction among layers when the holes are plated through. If smear remains in the PTH holes, it may cause short cut in the printed circuit board. Some bad PCB products are rejected just because the smear of the holes!