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54 DESIGN007 MAGAZINE I DECEMBER 2025 per on the outer layer. Every component of the design layout, from pads and through-hole pads to trace routes and ground fill, goes onto the manu- facturing panel. Let's dive into how we make this happen on the shop floor. Dry Film vs. Liquid Photoimageable (LPI) Photoresist After prepping the surface, we apply the photore- sist to the board, which accepts the digital design and creates it physically on the panel. There are two application methods: dry film and imag- ing using a liquid polymer that is similar to solder mask. Dry film is a thin layer of polymer film, usually blue or purple. It comes in rolls like kitchen plas- tic wrap, and you apply it with heat and pressure to the copper surface on each side of the panel. As the roll unwinds, it physically laminates to the sur- face of the panel. This re-sults in a uniform (usually a 2.3 mil) photoresist for outer layers. Dry film is more forgiving and more uniform in thickness, but that thickness can limit some of the fine line capabilities. Dry film is gen-erally prefera- ble, depending on how small your design features are. It's an effective choice in the hands of a crafty, experienced manufacturer. However, once you reach less than 2.5 mils of thickness, liquid resist is better. If we use dry film below 2.5 mils, yield and quality can degrade a little. Classic dry film below 2.5 mil can create more of a yield problem when the design calls for narrower traces and spaces. Materials do not generally affect the imaging process, even with advanced applications like RF, though t. hey may present challenges during etch- ing. It depends on the copper's adhesion to the laminate, but all copper is equal, and the imaging process does not differentiate 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 sen- sitive to light—not just high-intensity light, but any light in the room—the imaging process takes place under yellow lights. The concept 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, which functions like a laser printer. The 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. This creates channels where we want copper to remain, so we can add more later for a more de-sirable thickness. Once we identify what and what not to keep, we remove from the outer layer the photoresist from the features we want to keep by developing it the way we used to develop photographs. We spray a mildly caustic solution onto the back and front of the panel, and the unwanted material dissolves. This creates channels where the copper traces go and clears the way for electroplating copper in the channels. We have to be extra careful during this process. Many microscopic effects can snow- ball and leave us with a board different from the intended design. Every process has some variation. In the imag- ing process, we often see variation in placement, size, or thickness. We avoid these issues by con- sidering 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. This helps us meet our annular ring requirements down the road. It also shows why it's important to collab- orate with your fabrica-tors. Take their advice when they want you to consider manufacturing re-ality as conservatively as possible or reasonable when crafting your design. Doing so improves the manu- facturing process, improves yield, and drives down cost. Outer Layer Design Best Practices Focusing on manufacturing reality during board design is an excellent strategy, but there are spe- cific design considerations for the outer layer pro- cess that not only lower cost and improve yield but also opti-mize the functionality of your board. Trace Routing and Width Stability is the name of the game with trace routing and width. Consider the following when creating your outer layer design: C O N N ECT T H E D OTS