Issue link: https://iconnect007.uberflip.com/i/1526407
38 DESIGN007 MAGAZINE I SEPTEMBER 2024 of the fine line capabilities. Dry film is prefera- ble to a certain point, depending on how small your design features are. Dry film can be a very effective choice in the hands of a cray, experi- enced manufacturer. However, once you reach a certain point, LPI offers a better solution. If we use dry film below 2.5 mils, yield and qual- ity can degrade a little. e classic dry film cre- ates even more of a yield problem when traces and spaces get down to 2.25 mils. ough it may seem counterintuitive, mate- rials do not generally have an impact on the imaging process, even when we are talking about advanced applications like RF. Materials might present some challenges during etching depending on how the copper is adhered to the laminate, but all copper is essentially cre- ated equal, and the imaging process is agnostic when it comes to materials. Ready For Imaging So, we have a board with a blue or purple layer on both sides. Now what? Since the photoresist on the panels can be sensitive to light—not just high-intensity light but any white light in the room—the imaging process takes place under yellow lights. e con- cept is the same as using red light in a darkroom to avoid triggering the film exposure process. We use laser direct imaging (LDI) to apply the design pattern to the board. It functions like a laser printer. e laser scans an entire side of the panel, turning off for traces and turning on when there should be an absence of copper on the outer layer. is creates channels where we want copper to remain, so we can add more copper later for a more desirable thickness. Now that we have identified what to keep and what to get rid of, we remove from the outer layer the photoresist from the features we want to keep. We remove it by developing it the way we used to develop photographs. We spray a mildly caustic solution on the back and front of the panel, which dissolves the stuff we don't want. is creates channels where the copper traces go and clears the way for fun stuff like putting copper in the channels and electroplat- ing. Having fun doesn't mean we don't need to be careful. ere are a lot of microscopic effects that can snowball, leaving you with a board that differs from what you expected and your intended design. Every process has some type of variation. In this process, we oen see some level of varia- tion in placement, size, or thickness. We avoid these issues by considering the placement of the drill hole on the pad, the placement of the pad relative to the drill hole during imaging, and the etching of that pad. is helps us meet our annular ring requirements down the road. It is also an example of why it's important to collaborate with your fabricators. When they say to be as conservative as possible or reason- able in your design, I recommend taking their advice. Doing so improves the manufactur- ing process, improves yield, and drives down costs. To learn more about outer layer imaging, check out this episode of I-Connect007's On the Line with… podcast. DESIGN007 Matt Stevenson is vice president and general manager of ASC Sunstone Circuits. To read past columns, click here. Download Matt's book, The Printed Circuit Designer's Guide to… Designing for Reality and listen to the podcast here. You can view other titles in the I-007eBooks library. " Every process has some type of variation. "