Studies on Bending at Small Hole Drilling of Printed Wiring Boards.

In recent years, needs for drilling small diameter holes (φ0.2-0.4 mm) are increasing, accompanying the development of higher wiring density of printed wiring boards. However, the drilling of small diameter holes includes many technically difficult problems which must be solved. In contrast to drilling of normal diameter (φ0.5-1.2 mm) where epoxy smear, surface roughness of the hole wall, tool life, etc. are important, small diameter drilling has a different set of performance criteria : drill breakage, initial positional accuracy and hole bending. This investigation was conducted to examine hole positional accuracy and hole bending in relation with tool wear using φ0.4 mm drill to several kinds of glass epoxy circuit board. Furthermore, to limit initial slippage and then bending, vibrant drilling and drilling with a cross-thinned drill were carried out. A numerical analysis was executed to obtain accurate flexural rigidity of the drill which has an effect on the slippage before the drill point has stayed at a certain position and on hole bending even more. Conclusions are summarized as follows : (1) Amount of hole bending is generally affected by that of tool wear, but initial positional accuracy is less affected. (2) Hole bending develops with drilled depth without hole wondering (Burnhum, 1980). (3) Vibrant drilling is effective for minimizing hole bending but less for limiting initial slippage. (4) The effect of cross-thinning on hole bending is as evident as of vibrant drilling, but not on initial slippage.