The drive for increased circuit density in printed circuit board (PCB) technology has led to the introduction of ultra-small internal vias (commonly referred to as microvias) in new designs of epoxy-glass multilayers boards (MLB's). These microvias have dimensions comparable to linewidths and are used without lands. Their use simplifies the design of complex MLB's, facilitates conductor routing, and may even allow for a reduction in the number of MLB layers. Several techniques may be employed to produce the desired microvias. The most common technique, mechanical drilling, is limited by low drilling rates and excessive drill bit wear, particularly as hole dimensions shrink. In this paper we report the results of an investigation exploring the feasibility of laser drilled microvias. The process relies on the use of single pulses from a CO2 laser to drill small holes in panels of epoxy-glass. It is suitable for the generation of both buried and blind vias. Two different drilling methods were examined: stop-and-go and on-the-fly. In the former technique sub-0.004-in holes were achieved at a drilling rate of several holes per second. In the latter approach, slightly larger holes were obtained at a drilling rate of several hundred holes per second. To test the quality of laser drilled holes, daisy chain patterns including these holes were fabricated using conventional subtractive technology. These patterns were subjected to successive thermal shocks to detect connection failure via resistance changes. The specimens tested showed no degradation when certain metallization procedures were followed. Other samples were subjected to reliability testing and compared to mechanically drilled vias. No statistical difference between the two groups could be established. Panels containing up to 12 000 holes were drilled and processed into boards. Functional testing results indicated no significant difference between laser drilled boards and their mechanical counterparts. Based upon these results, a production-type laser system was constructed capable of drilling sub-0.004-in diameter holes over 18 in by 24 in panels at rates of up to several hundred holes per second. High resolution and a commensurate high absolute accuracy were achieved. A description of the system is provided. These accomplishments endow the laser process with unique capabilities currently unmatched by any alternative technique. The process, therefore, offers certain advantages which could advance the state of PCB manufacturing. © 1990 IEEE
