Surface enabling technology for direct copper drilling in thin PCB manufacture

Abstract

Laser technologies are being investigated and adopted to form very small blind holes (blind micro-vias) by PCB manufacturers for high-density interconnect (HDI) printed circuit boards (PCBs). Conventionally, CO2 laser systems are used by the industry due to their higher productivity if only epoxy resin is to be ablated. Therefore, to form micro-vias by CO2 lasers, chemical etching of the top copper layer of the PCB is necessitated as copper's absorption of the CO2 laser is very low. The etched copper "window" serves as a conformal mask for subsequent CO2 laser drilling. As there are limitations and problems in (i) chemical etchants flowing into micro-vias of smaller size, and (ii) registration in the conformal masked laser drilling method, "direct" copper drilling by CO2 laser irradiation without using a conformal mask can be a promising alternative for HDI PCB manufacture. This research work aims to investigate and develop direct laser drilling technology by studying different control factors. Firstly, experiments were performed to investigate and evaluate different dielectric materials by CO2 laser ablation in terms of micro-via qualities including blind hole shape, resin residue inside the blind hole, and the roughness of the blind hole walls. The results of the experiment indicate that resin-coated copper is more suitable and appropriate for the development of direct copper drilling technology (CDD). Secondly, the effects of surface pre-treatments in enhancing the laser absorption of the copper surface were studied. Different black oxide and oxide replacement treatments were investigated for CDD by CO2 laser ablation. The laser absorption of the treated copper surfaces and the surface topography were evaluated as well. The critical processing parameters affecting the quality and reliability of the oxide coating were also studied in this research. Irradiating at the same laser energy, the oxide replacement treatment (Multibond) showed a higher surface roughness (Ra=0.5606) and a coarser grain structure, enabling higher laser energy absorption and thus resulting in laser-drilled micro-vias with better hole shape and quality. Thirdly, experiments were carried out to study different technologies for etching down the copper thickness (copper reduction), as copper thickness is believed to play an important role in enabling CDD. The uniformity of copper thickness across the PCB panel resulting from different etching processes was investigated. The characteristics of the different copper reduction processes were compared and studied, including the critical process factors, chemical characteristics, and machine design and configuration. It was found that the requirement of the uniformity of copper thickness could be achieved by some of the copper reduction technologies in the study. There was no sharp increase of surface roughness and no significant change in surface topography by increasing the number of cycles of the copper reduction process. The result of the research revealed that Multibond was the most appropriate copper reduction process for CDD. This is because Multibond was able to reduce the copper thickness very evenly (+-0.5um) and at the same time provide an oxide coating on the copper surface for enhancing laser absorption. The last part of the research focused on the optimization of the laser drilling conditions (such as energy level per pulse, pulse width, number of shots) for different copper thicknesses. It was found that the amount of total laser energy level required for CDD was proportional to the thickness of the copper layer to be "burnt-off". Also, longer cycle times were needed to "burn-off' micro-vias with a thicket copper layer. As a threshold, the copper layer thickness has to be reduced by an appropriate reduction process to around 5 to 7um to enable CDD. At this range of copper thickness, a minimum of two pulses of laser irradiation with a short pulse width are recommended for direct copper drilling. For example, in drilling 100um micro-vias on resin-coated copper of 5um thickness and 65um thickness epoxy, it was found that the best set of laser drilling parameters are: one shot of l0milli-joules laser energy followed by two shots of 2milli-joules laser energy, all with 9us of pulse width. Finally, the reliability of laser-ablated micro-vias was checked and verified by the Interconnect Stress Test (IST) test and the thermal stress test. The critical processing parameters (energy per pulse, pulse width and number of pulses) were optimized to establish conclusive recommendations in enabling direct copper laser drilling technology for PCB micro-via formation.