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70 The PCB Magazine • November 2015 by Dirk Müller, Ph.D. coherent Inc. While lasers have long been employed for via drilling in PCB fabrication, mechanical drill- ing 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 va- riety of laser technologies are now poised to step in to extend production via drilling down to the micron level. This article reviews the various la- ser sources that are available to support the lat- est packaging technologies as they become more widely adopted, and describes the characteristics and capabilities of each. Co 2 Lasers Carbon dioxide (CO 2 ) 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 pen- etration is simple. Even though CO 2 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 small- er vias, mechanical drill replacement costs start to increase exponentially, and the use of CO 2 lasers will expand significantly to cater to the growing demand for ever smaller micro-vias. As Vias Shrink, Opportunities for Laser Drilling Expand CO 2 lasers drill vias through a thermal inter- action. That is, the material absorbs the infrared light output of the CO 2 laser, which heats it un- til it vaporizes. Many dielectrics absorb well in the far infrared, while nearly all metals are high- ly reflective at these wavelengths. As a result, copper layers act as a natural stop when drilling with a CO 2 laser. In order to drill through cop- per (such as a top clad layer), it must first be oxidized to create a dark patina which absorbs the laser light. While the CO 2 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 infra- red) wavelengths, cannot be focused as finely as visible or ultraviolet wavelengths. Also, the thermal nature of the light/material interac- tion produces a small heat affected zone (HAZ) ArTiCle

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