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While lasers have long been employed for via drilling in PCB fabrication, mechanical drilling still remains the predominant production technology. However, as via diameters shrink to support various advanced packaging techniques, mechanical drilling becomes more expensive, and ultimately technologically unfeasible. A variety of laser technologies are now poised to step in to extend production via drilling down to the micron level. This article reviews the various laser sources that are available to support the latest packaging technologies as they become more widely adopted, and describes the characteristics and capabilities of each.
Carbon dioxide (CO2) lasers have been used in PCB via drilling for more than two decades and currently service about 20% of the market. The reason for this relatively low market penetration is simple. Even though CO2 lasers are a non-contact method that eliminates the need for frequent tool replacement, their sweet spot is at hole diameters around 100 µm diameter and below. As the industry transitions to smaller vias, mechanical drill replacement costs start to increase exponentially, and the use of CO2 lasers will expand significantly to cater to the growing demand for ever smaller microvias.
CO2 lasers drill vias through a thermal interaction. That is, the material absorbs the infrared light output of the CO2 laser, which heats it until it vaporizes. Many dielectrics absorb well in the far infrared, while nearly all metals are highly reflective at these wavelengths. As a result, copper layers act as a natural stop when drilling with a CO2 laser. In order to drill through copper (such as a top clad layer), it must first be oxidized to create a dark patina which absorbs the laser light.
While the CO2 laser can readily produce a smaller via than a mechanical drill, there are limitations on the smallest via diameter it can reach. One limit is caused by light diffraction. Specifically, the smallest focused spot size to which a laser beam can be focused is directly related to its wavelength. Longer (e.g., far infrared) wavelengths, cannot be focused as finely as visible or ultraviolet wavelengths.
Editor's Note: This article originally appeared in the November 2015 issue of The PCB Magazine.