Mechanism of PCB milling

1. Milling Bit is lowered down with spinning by rotation of the spindle motor.
2. When the milling bit is lowered, it bits the copper layer and starts cutting.
At the tip of the milling bit, copper chips are shaved off and ejected outward
through the rake face.
3. Lower the milling bit further down and the tool tip bits into and start cutting dielectric layer.
When the material is cut away, the energy is consumed and heat is generated. It is important to
remove the heated portion immediately, and let the thermal energy escape with shaved chips.Frictional heat is generated where the milling bit contacts the material.
Heat is discharge with shaved chips.

* Milling bits cut the material by “High Speed Milling” method and
“Flat Face Cut” method simultaneously.


It is very important to set the cutting parameters appropriately
for the material to be cut.

If board cutting parameters are adequate, shaved chips are ejected without any difficulties, resulting in clean cutting surface. On the other hand, if the cutting parameters are inadequate, or a worn out milling bit is used, the bit cannot make sharp cutting, causing difficulties in ejecting shaved chips smoothly. If this happens, clean cutting becomes impossible due to shaved chips being stuck in the milled trench, or cutting edges get burred. In addition, shaved chips stuck in the milled trench melt by functional heat and get stuck to the milling bit. It makes it even more difficult for the milling bit to cut the material cleanly.
4. In order to discharge the heat quickly, it is necessary to run the spindle motor at fast speed.
However, running the spindle motor at fast speed tend to cause “run-off” and other problems.
Tool run-out due to the spindle motor
As discussed in “The relationship among the spindle rotating speed, processing speed, and the tools” section, tool run-out tends to be higher as its rotation speed increaces.
If the tool run-out is high, the cutting edges become rough. Furthermore, it tends to cause tool breakage. It is very important to choose appropriate rotation speed for the tool you use.

Mits opinion:

Faster tool rotation speed does not necessary mean high quality cutting. It is very important to choose appropriate rotation speed and feed speed for the material to cut.( In our experience, 100,000 RPM is not very practical with small spindle motors except for very particular applications. )

Difference in cutting cleanliness due to spindle motor speed

Faster spindle motor speed does not mean sharp cut!

Milling of hard, fragile SrTiO3
Printed board is soft, which includes surface metal layer and plastic board

Milling parameters optimization

Milling parameters

HF Milling Bit 200 µm
Rotation speed from 10 to 40 Krpm
Milling depth up to 5 µm
Lateral milling speed 0.5 mm/s