Drilled hole damage of small diameter drilling in printed wiring board

Of late, it has become necessary to improve the packaging density of printed wiring board (PWB), because the demand for size reduction of the electric devices is increasing. Therefore, the pitches between through holes in electric circuits have become fine for the high package density. On the other hand, various forms of damage occur around the holes after drilling. Much research has been done on internal damage in order to prevent the reduction of the insulation resistance between the holes after through hole plating, and the blow holes after soldering. This study is about the damage factors around the drilled hole wall in small diameter drilling of an PWB in order to prevent the ion migration between the holes. It is shown that there are two important factors of internal damage around the drilled hole: (1) the relative angles between cutting directions and fibre directions; (2) fibre bundle thickness of glass cloth at the drilled hole wall. In particular, it is demonstrated that the reduction of the fibre bundle thickness is effective to decrease the internal damage of drilled hole.