Laser drilling of blind via holes in aramid and glass/epoxy composites for multi-layer printed wiring boards

The authors discuss the comparison of aramid (Technora by Teijin)/epoxy and glass/epoxy composites for multi-layer PWB (Printed Wiring Boards) in relation to blind via laser drilling. A CO2 laser with a maximum attainable power of 100 W is used. The spot diameter is 0.3 mm. The layer build up has a thickness of 0.3 mm with the outer copper etched. The blind vias are formed by laser irradiation from the outer layer side, since the base copper foil (inner copper thickness 18 mum) tends to reflect light strongly below the ablation threshold of copper. First, SEM observations of rim quality were carried out. The surface appears uneven due to melted glass fibers when using glass/ epoxy materials, while it exhibits little residual fibers when aramid/epoxy materials are used. Second, conditions which are promising for the formation of blind via holes were researched. It was found that a few pulses with irradiation time of 0.5-2.0 ms per pulse are needed for completing a blind via hole. However, it was shown that only half the number of pulses is required with an aramid/epoxy composite. The reduction in fabrication time is significant, since a typical multi-layer panel has thousands of via holes. Finally, it is clear that the laser-drilled via hole has a uniform taper to the sidewall. There is better metallization with this type of hole than one with no taper of the sidewall. Additionally, the surface roughness on the sidewall is smaller with an aramid/epoxy composite than a glass/epoxy one. Therefore, the laser via formation method for multi-layer PWBs of aramid/epoxy is also effective from the viewpoint of plating reliability in circuit registration. (C) 2001 Elsevier Science Ltd. All rights reserved.