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72 The PCB Magazine • November 2015 around the drilled hole, limiting how close it can be placed to other features. As a result, the practical lower limit on via diameters for CO 2 lasers is about 70 µm. But what if the hole di- ameter needs to be smaller than 70 µm? Co Lasers This is where carbon monoxide (CO) lasers come in. This type of laser was first developed about 50 years ago, but lifetime and reliability issues prevented this technology from becom- ing commercially viable. However, in the past year, the advent of new technology is making CO lasers practical, yielding products with very high output powers, and which demonstrate lifetimes in the thousands of hours range. The reason that CO lasers are of interest is that they output over the 5–6 µm spectral range, or about half the CO 2 wavelength of 10.6 µm, allowing for a smaller focused spot. For via drilling, this shorter wavelength provides several important advantages. For example, it lowers the minimum via diameter that can be produced down to about 35 µm (due to diffraction). But even when producing larger diameter vias, the CO laser has an edge over CO 2 . Specifically, the focusing lens used to achieve a 70 µm diameter via with a CO laser has twice the focal length of the lens required to achieve the same via size with a CO 2 laser. This longer focal lens provides greater depth of focus, which increases the field of view. The longer focal length and increased depth of field facilitate an increase in scanning speed, and therefore faster via production, with the shorter wavelength CO laser. Because the CO laser can be focused to a smaller spot, it's easier to reach higher power densities with it than with a longer wavelength CO 2 laser of the same power. (Since the CO laser has roughly half the wavelength of the CO 2 , it forms a spot size that is half as big, and which therefore has one-quarter the area, or four times the power density.) Conversely, achieving a given power density requires only one-fourth the total output power with a CO laser as with a CO 2 laser. Depending upon the exact param- eters of a particular via drilling task, this makes it possible to use a much lower power CO laser for a specific job. This lowers the cost of the la- ser and the cost of the electricity and reduces the carbon footprint for the process. In addition to the optical benefits, there are also differences in light absorption character- istics at the shorter wavelengths. This can be especially true in the case of polymers, which have an infrared absorption spectrum that con- sists of numerous sharp peaks. As a result, some polymers exhibit high absorption at 10.6 µm, and much less in the 5 µm to 6 µm band, and vice versa. Another important material which exhib- its very different absorption characteristics at 10.6 µm and 5 µm is glass, a material now of great interest for interposers in 2.5D and 3D advanced packaging techniques. Specifically, although glass has much lower absorption at the shorter wavelength, the use of the shorter wavelength actually produces superior results. This is because the lower absorption enables the CO laser beam to penetrate farther into the ma- terial. Together with the superior focusing abil- ity of the 5 µm wavelength, this enables very small holes with high aspect ratios to be drilled in glass with precise depth control. The photo shows 35 µm diameter vias drilled in glass. Vias of this size and quality simply couldn't be pro- duced with a CO 2 laser in glass. AS VIAS SHrInK, oPPorTUnITIeS For LASer DrILLInG exPAnD ArTiCle Figure 1: a 50 µm-thick glass substrate drilled with successively more pulses from a co laser demonstrates the ability of this source to drill glass interposers. co 2 drilling of this material typically results in heat-related cracking.

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