No. 94 · APRIL 2022

SUMITOMO ELECTRIC TECHNICAL REVIEWNew Products and Techniques

AF905 - New Cemented Carbide Material for
High-Reliability PCB Drills
1. Outline

2. Features

In recent years, there has been an increase in dataprocessing capacity due to the widespread use of high-performance PCs and automatic driving systems and thecommunication of high-quality video data by smartphones
and other devices. Therefore, printed circuit boards (PCBs)
used in these products are required to be more heat resistant and durable than ever before. To improve heat resistance and durability, there has been progress in improvement and increase in the content of heat-resistant andheat-dissipating fillers, super-multilayering of boards, andincrease in the thickness of boards, all of which make it
more difficult to drill holes in boards. Specifically, it leads
to increased drill wear and consequent degradation of holeposition accuracy, resulting in decreased board reliability
due to degraded hole accuracy and hole inner wall quality.
Therefore, it has been desired to develop a new drillmaterial that achieves high accuracy and high quality for
drilling holes in PCBs, which is becoming increasinglydifficult.
In response to market demand, we have developed
and launched a new material, AF905. AF905 has achieved
an unprecedentedly dense structure by controlling the grain
size of tungsten carbide (WC), which is the hard phase in
cemented carbide, and controlling the thickness of thecobalt (Co) phase, which is the binder phase that exists
among the WC grains. Furthermore, by improving thebonding strength of the interface between the WC grains
and between the WC grains and the Co phase, wesucceeded in significantly reducing the dropout of the WC
particles, which causes wear when drilling in PCBs,
thereby achieving excellent wear resistance and high holeposition accuracy.

2-1 Improvement of wear resistance
When drilling holes in heat-resistant PCBs that
include many hard glass fibers and ceramic fillers, theiringredients are extremely difficult to cut, which causes thedropout of WC particles as shown in Fig. 1 and results indrill wear. Therefore, in order to achieve high wear resistance, it is important to suppress the dropout of WC particles. In AF905, we have improved the raw materials, additive elements and alloy manufacturing process to control
the size of WC grains and the thickness of the Co phasebetween WC grains, thereby succeeding in achieving densestructures.
By distributing closely packed WC particles with
controlled particle size, the contact points between WC
particles are increased, and thus even if one WC particle isabout to drop out, it is held in place by other WC particles,
thereby suppressing the acceleration of wear due to thedropout (Fig. 2). Furthermore, we have improved thebonding strength of the interface between WC grains andthe interface between WC grains and the Co phase byimproving and controlling the additive elements. As aresult, high wear resistance, 1.2 times higher than that of
the conventional product, has been achieved in drilling ofdifficult-to-cut boards.

Tip of the cutting edge
(drill diameter: 0.30 mm)
Cutting edge

Enlarged

AF905

ConventionalMuchdropout

Gray: WC
Black: Co

Fig. 1. Photo of the cutting edge after drilling

AF905

Cutting edge

Conventional

Photo 1. PCB drills

WC particles make contactwith each other.
=> Reducing dropoutDropout
WCCoWCCo

Fig. 2. Concept of the new material
AF905 - New Cemented Carbide Material for High-Reliability PCB Drills

109（1）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 94 · APRIL 2022

2-2 Improvement of hole position accuracy
Hole position accuracy is important for the operational reliability of PCBs. The hole position accuracy in theinitial stage of drilling is correlated with the Young’s
modulus of cemented carbide that the drill is made of.
However, in the case of heat-resistant PCBs, wear
progresses significantly due to the dropout of WC particles
in cemented carbide, and cutting resistance suddenly
becomes high, leading to degradation of hole position
accuracy. The AF905 has achieved excellent hole position
accuracy (1.2 times higher than that of the conventional
product) by improving the Young’s modulus compared tothat of the conventional product and suppressing wear due
to the dropout of WC particles and thereby reducing cuttingresistance.

Cutting edge wear [µm]

2015
1050

Conventional
AF806H

Fig. 3. Wear resistance

Conventional
AF806H

3. Example of Drilling-X

AF905+X+Y-X+X-Y-Y

80Hole position accuracy(averageThe following is an example of actual hole drilling of
an automotive PCB with high heat resistance under theconditions shown in Table 1.
According to Fig. 3, which shows the amount of wearon the cutting edge (decrease in drill tip diameter) after
4,000 drilling cycles, the AF905 suppressed the wear
progression by suppressing the dropout of WC particles,
and the wear was reduced by approximately 20% comparedto that of the conventional product.
In addition, Fig. 4, which is a diagram indicating thebehavior of hole position accuracy in the same drilling
process, demonstrates that even after 4,000 drilling cycles,
the variation of the hole position from the center of thecircle in the AF905 is smaller than that in the conventional
product. As an effect of improved Young’s modulus andreduced cutting resistance due to the suppression of thedropout of WC particles, the “average +3σ,” an index ofvariation in hole position accuracy, was reduced by approximately 20% compared to that of the conventional product.
The above results support the expectation that theAF905 has excellent wear resistance in the drilling of difficult-to-cut PCBs, whose use is anticipated to expand in thefuture, resulting in excellent drilling hole position accuracy
and hole quality, and demonstrates high reliability in thedrilling process of heat-resistant PCBs.

AF905

6040200

Conventional
AF806H

AF905

Fig. 4. Hole position accuracy

Table 1. Drilling conditions
Drill
Work material(PCB)

Drilling conditions

Drill diameter

0.30 mmShank diameter

3.175 mmBoard typeFor automobiles

Board thickness
x number of stacks

0.8 mmx4

Number of rotations

100 krpm

Chip load

8.0 μm/rev

Feed rate

0.8 m/minNumber of drilling

4,000 hit

AF905 - New Cemented Carbide Material for High-Reliability PCB Drills

110（2）