Drilling is a slow process as each hole must be drilled individually. So depending on the drill size we drill a stack of one to three PCB panels together. We can drill holes down to 100 microns in diameter. To give you an idea of the size, the diameter of a human hair is about 150 microns. Drill change is fully automatic. The machine selects the drill to use from the drill rack, checks that it is the correct size, and then loads it into the drill head.
PCB drilling steps
X-ray drill of reference holes
Now we drill the holes for leaded components and the via holes that link the copper layers together. First we use an X-ray drill to locate targets in the copper of the inner layers. The machine drills registration holes to ensure that we will drill precisely through the centre of the inner layer pads.
Prepare the stacks for drillng
To set up the drill the operator first puts a panel of exit material on the drill bed. This stops the drill tearing the copper foil as it comes through the PCB. Then he loads one or more PCB panels, and a sheet of aluminium entry foil.
Drilling the holes
The drilling machine is computer-controlled. The operator selects the right drill program. This tells the machine which drill to use and the X Y co-ordinates of the holes. Our drills use air-driven spindles which can rotate up to 150,000 revolutions per minute. High speed drilling ensures clean hole walls to provide a secure base for good plating on the hole walls.
Once all the holes are drilled the operator unloads the panels from the drilling machine and discards the entry and exit material.
Different holes of PCB drilling
A raw circuit board can have many fabricated properties, including cut-outs, slots, and the finished board’s overall shape. The largest number of such properties will be the holes that are drilled into the board. The purpose of these holes can be broken down into three categories:
Small holes plated with metal are used to conduct electrical signals, power, and ground through the board layers. These holes are known as vias, and they come in different types depending on what is required:
Thru-hole: This is the standard via, and it goes through the entire board layer stackup from top to bottom. These vias can connect to traces or planes on as many internal layers as needed.
Buried: These vias start and stop on internal board layers without extending to an external layer. This structure takes much less room than a standard thru-hole via, making them very useful in high-density interconnect (HDI) circuit boards. However, they are also much more expensive to fabricate.
Blind: A blind via starts on an external layer but only goes partway through the board. As with buried vias, they are more expensive to fabricate, but they free up room for routing, and their shorter barrel can also help improve the signal integrity of high-speed transmission lines. Micro: This via has a smaller hole size than the others because it’s drilled with a laser. Microvias are usually only two layers deep due to the difficulties of plating smaller holes. These vias are necessary for HDI boards or high pin-count fine-pitch devices like BGAs that require their escape vias to be put into their solder pads.