Microdrills Dry Lubricant

Microdrills are used to make holes on printed circuit boards (PCB). Due to the small diameter (e.g. 0.3 mm) and the high aspect ratio of the hole, debris may be trapped inside that can grip the drill and break it. In order to lubricate the flutes of the drills, DLC was coated on the surface. The coating on the cutting tips may be rubbed off quickly during the drilling of the PCB, but the cutting debris can be extruded readily along the slipper surface of DLC. As a result, the microdrills can continue to penetrate the board without been overly twisted. The breakage of microdrills due to twisting has been the limiting factor for the holes drilled from the same tool. Figure 10.5 shows the sample of DLC coated microdrills.

Printed circuit boards were drilled by DLC coated microdrills. Table 10.1 summarizes the testing conditions.

Figure 10.5. The appearance micro drills with hydrogenated DLC coating (left) and Fluorinated DLC coating (right).
Table 10.1. The drilling parameters of DLC coated microdrills

Size

0.3 mm

Speed

140K RPM

In Feed

49 mm/s

Temperature

34°C

Pressure

1400 mm Aq

PCB

FR-4

Circuitry

2 Layers

Thickness

1.6 mm

Board

2 Plates

Figure 10.6. Examples of the good holes with surface roughness less than 25 /m (a); and the bad ones, more than 25 /m (b).

Figure 10.6. Examples of the good holes with surface roughness less than 25 /m (a); and the bad ones, more than 25 /m (b).

The quality of the drilled holes is in PCB is typically judged by the roughness of the hole wall. In particular, the copper laminate must not smear to cause excessive relief on the wall. The following photos show the examples of the hole profiles in PCB. Figure 10.6 illustrates the hole quality based on the smoothness of the wall as examined by polishing the cross section.

Normally, the microdrills can sustain a drilling life of about 2000 holes through multiple boards (e.g. 2), but DLC coated ones demonstrated the capability of passing 50,000 holes as shown in Fig. 10.7 (Sung et al, 2006).

Figure 10.7. Hydrogenated DLC coated microdrills showed that the wall smoothness could be maintained until reaching 50,000 holes that are about 25 times the life (about 2000) of uncoated microdrills.

Number of Holes Drilled (><1000)

Figure 10.7. Hydrogenated DLC coated microdrills showed that the wall smoothness could be maintained until reaching 50,000 holes that are about 25 times the life (about 2000) of uncoated microdrills.

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