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Drilling Via Holes: Slow and Steady Wins the Race
Drill bit types.

Today's electronics carry everything to extremes — everything must be faster, use incredibly tiny electronics, and certainly must be light in weight — and all this helps to create difficulties for the PC board fabricator. Conductor line widths and the spacing between them both approach an incredibly tiny 50µm, and the corresponding hole diameters are only 0.10mm.

Drilling those holes can become a problem because of a need for low abrasion to the drill bits, keeping down the roughness of the hole walls, all while maintaining a high degree of hole positioning accuracy. The currently and commonly used technology for this operation has been to use high-speed CNC drills running at 300,000RPM or faster. But the drilling was resulting in rough via hole walls and the drill bits themselves were getting roughed up — resulting in even rougher holes and short useful life for the drill bits.

Testing Drilling Methods
We decided to test various methods of drilling to see exactly where we could make measurable improvements in the resulting vias. We focused our study on three principal elements during the volume drilling production: the abrasion to drill bits, the hole position accuracy, and hole wall roughness.

After evaluating the various factors involved in the drilling process — number of RPM, drilling force, and drill instability, it was decided to use only lower rotational speeds in the 120 to 160kRPM range.

In order to reduce abrasion to drill bits, we found that there were several contributing factors: screw angle, core thickness, core thickness grads and shape.

Hole Positioning Accuracy
We found that it was possible to increase hole positioning accuracy by changing the cover board texture, and adjusting the rotating speed and feed-in speed, we are able to control the hole position tolerance within ±30µm.

To reducing roughness of the hole wall, we evaluated the cutting ability under different conditions, such as drill bit diameter, board thickness, and stacking layers, and were able to control the hole wall roughness less than 20µm.

We then implemented test by using two different kinds of PC board materials — ordinary FR-4 and High Tg laminates.

Via hole drilling today is mainly performed with high speed CNC drilling machines, typically at 250kRPM and more. But, in the high speed CNC drilling, when the drill diameter is 0.20mm or more, the best feed-in speed of 3500mm/min and higher, the hole precision will suffer and there be other effects from mechanical vibration. The result is that some holes will not meet the quality requirements for printed substrate.
Hole condition distribution curves and tool wear state comparison chart.


To correct this problem, our company has instituted special micro via drilling technology in the difficult processing areas. When drilling 0.10mm via holes, we do not use high speed drilling machines, but a slower rotational speed of 160kRPM.

Experimental Verification
Because of the geometry of the drill bit itself affects the quality of the hole (the hole position accuracy, hole wall roughness), we needed to verify two variables: the shape of the drill bit front, and the tip angle of the drill.

The tests varied the drill bit shapes, the rotational speed, feed-in speed and the number of holes drilled, from 2,000 to 6,000. The results showed clearly that using Type C and Type D cutting tools, when cutting at a drill speed of 150kRPM and a feed-in speed of 900mm/min, the cutting conditions were relatively better, and the hole precision averaged stabilized within 20µm.

In another experiment, when evaluating the hole position accuracy and bit consumption with the different types of drill (Type C, Type D), drilling 6000 holes, for testing the bit life speed of rotation was 150 kprm, with feed-in speed of 900mm/min.

The results showed that the best drilling condition is Type D, bit cutting condition: drill speed of 150kRPM and feed-in speed 900mm/min, bit life 2000 Hits.

Evaluating the hole wall roughness of different materials, we used processing conditions that involved drilling material stack 0.1mm holes x 4 pieces, 0.2mm x 3 pieces, 0.3mm x 3 pieces, 0.4mm x 2 pieces, 0.5mm x 2 pieces, using normal FR-4 and high Tg material. The hole wall roughness (the micro-section after copper plating) was inspected, and there was a definite correlation with the material being drilled and its thickness. The best results were obtained with the thinnest section of High Tg material, followed closely by FR-4. Analyzing the outcomes of these experiments, it can be concluded that the 0.10mm via can be processed by the common drilling machine running at 160kRPM, with good results for both normal FR-4 and high Tg materials.

Contact: Dalian Pacific Electronics Co., Ltd., 127 Liao-he-xi Road, Jinzhou New District, Dalian, China 116600 (86-411) 8718 9366 fax: (86-411) 8792 1512 E-mail: victor.gao@dpmc.com.cn or marketing@dpec.cn Web:
http://www.dpec.cn

 
 
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