Evaluation and Improvement of Micro-Drilled-Hole Wall Quality in Printed Wiring Boards Made of Glass Fiber-Reinforced Plastics

With recent worldwide-use and the rapid spread of electric and electronic equipment, printed wiring boards (PWBs) are being miniaturized and made multifunctional. Miniaturizing the equipment and making it high performance are accompanied by an increase in the number of small diameter through holes for circuit connections available to the PWBs. Such tendency needs the large number of smaller diameter drilling with shorter pitches. Moreover, the improvement of drilled-hole wall quality for PWBs might be demanded by such trends. Here, PWBs are often made of fiber-reinforced plastics (FRP) for the insulating layers and copper foil for the circuit layers. A woven glass fiber cloth is generally used for PWBs. Therefore, delamination might occur during drilling and affect the surface roughness of the drilled-hole wall. Such wrong surface roughness of drilled-hole wall should be avoided because it might reduce the reliability of insulating between next holes by ion migration of copper plating progressing along the delamination. Revealing of its mechanism is expected to be more important with smaller diameter with smaller electric devices. However, it might be difficult because the micr-drilling of FRP shows is a complex phenomenon because it consists of different materials. Therefore, this paper investigates the proper method to improve drilled-hole wall quality by evaluating the drill temperature rising mechanism and the surface generation mechanism of micro-drilled-hole walls of PWBs made of GFRP. The following results were reached: (1) A drill temperature rising mechanism was conducted. (2) Drill temperature tends to increase with the frictional torque between the hole wall and the margin part of drill. (3) Frictional torque increases with the spring-back of the drilled-hole wall during drilling. (4) The surface roughness of the drilled-hole wall of the GFRP plate is affected by the breakdown of the glass fiber and is mainly caused by the cutting edge of the outer corner except the margin part of the drill. (5) A drilling method with higher fiber bonding strength, such as drilling with compressing PWBs in the thickness direction, effectively improves the drilled-hole wall quality.