Laser drilling of microvias in epoxy-glass printed circuit boards

The results of an investigation exploring the feasibility of laser-drilled microvias are reported. The process relies on the use of a 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. Stop-and-go and on-the-fly drilling methods were examined. 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. Based upon these results, a production-type laser system was constructed capable of drilling sub-0.004-in-diameter holes over 18 in×24 in panels at rates of up to several hundred holes per second. High resolution and a high absolute accuracy were achieved. A description of the system is provided