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22 PCB007 MAGAZINE I MAY 2024 aspect ratio for a 50-µm space on 35-µm foil becomes 0.8, well under the maximum aspect ratio of 1 we have determined is the limit for subtractive etching. For 25-µm lines in 17-µm foil, the aspect ratio is 0.88, making it theoreti- cally possible to etch 1-mil lines and spaces in half-ounce foil. I say theoretically because, not having access to a liquid resist coater, I've not been able to try this. Extending the theoretical a little further, it may be possible to go down to 15-µm lines and spaces in 9-µm foil. So, there is some hope for extending the limits of subtractive etching by using a liquid resist. is will still involve capital expense for coaters (roll coaters, spin coaters, or dip coat- ers) and ovens for baking and maybe multiple lines depending on production levels. You'll still need a very good exposure system as well, which may require an upgrade. is will give you the chance to remain competi- tive in the higher density circuit market and will be a lot less expensive than going to a fully additive process or opting out of this market altogether. PCB007 Don Ball is a process engineer at Chemcut. To read past columns or contact Ball, click here. Meet Indium Corporation's Evan Griffith, our featured young professional. He has a bachelor's degree in materials engineering and a master's degree in engineering management from Dartmouth College, and works in Utica, New York. In this audio interview, he shares his thoughts on this industry and advice to other young engineers. "What I enjoy most is getting to work with people from a wide range of disciplines and also a wide range of places," Evan says. Young Professionals Series: Meet Evan Griffith, Indium