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Connect the Dots: The Future of Designing for Reality—Outer Layer Imaging

If you read my column regularly, you know I’m passionate about helping designers get the most from their designs. As the demands on electronic devices and board complexity increase, it is essential to look to the future of designing for the realities of manufacturing.

The future for many electronic products is ultra high density interconnect (UHDI) PCBs. When designing UHDI boards, the choices you make are key to achieving more functionality in your PCBs and the electronic devices they power.

In my November column, I focused on designer best practices for the electroless copper component of the manufacturing process. The next step is outer layer imaging: the transition from digital to physical, and where the designer’s IP meets the board.

The conversion of digital designs to physical products was the topic of an episode of I-Connect007’s On the Line with…  podcast, explaining how the outer layer imaging process maps the design’s unique features onto the board.

Designers must have a solid working knowledge of how their manufacturing partners apply the photo image to the external layers—the method of physically transferring the digital image onto copper 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 manufacturing 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 photoresist to the board, which accepts the digital design and creates it physically on the panel. There are two application methods: dry film and imaging 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 plastic 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 surface of the panel. This results 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 generally preferable, 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 etching. 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 sensitive 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 desirable 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 snowball and leave us with a board different from the intended design.

Every process has some variation. In the imaging process, we often see variation 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. This helps us meet our annular ring requirements down the road. It also shows why it’s important to collaborate with your fabricators. Take their advice when they want you to consider manufacturing reality as conservativelyas possible or reasonable when crafting your design. Doing so improves the manufacturing 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 specific design considerations for the outer layer process that not only lower cost and improve yield but also optimize 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:

• Power traces: Where possible, use wider traces for high-current paths to reduce resistance and voltage drop. This is especially critical on the outer layers, where etching tolerances can be greater. If you need more robust power handling, expose copper and add solder for thicker traces.
• Signal traces: For high-frequency signals, keep your traces short. Use inner layers for tight impedance control, because outer layer tolerances can be less precise. Route high-speed digital and RF signals separately to prevent coupling and keep them away from high-power traces.
• Spacing: Appropriate spacing helps prevent crosstalk. Separate spaces by at least three times the height of the dielectric between the signal layer and the nearest ground or reference layer. 

Component Placement and Thermal Management
Managing heat and protecting against external interference can be challenging with smaller, more complex boards. Here are four methods to achieve both:

Heat dissipation: Use the outer layer as a heat sink or thermal conductor by increasing the copper area on the outer layer. Use thermal vias to lower resistance and call out any use of high-conductivity materials. Cluster power components together and place them near heat sinks or thermal vias to spread heat evenly. 

1. Grounding: To reduce noise, use solid ground planes and connect power circuitry grounds to the system ground at a single point. 
2. Shielding: It is important for designs to protect against external interference such as electromagnetic interference (EMI) or radio frequency interference (RFI). Place sensitive components within grounded areas to minimize EMI and pay close attention to material selection that aligns with the interference vulnerabilities of your device. 
3. Sensitive components: Heat can create issues with sensitive components, so protect them from thermal variations. Designers can create barriers against heat transfer by minimizing thermal bridging and using control layers and trapped air spaces to resist heat flow.

Manufacturing and Tolerance
Keep your manufacturer’s processes in mind as you design. These best practices can help facilitate a smooth manufacturing process:

• Impedance control: Designers can help the manufacturer precisely set impedance to prevent signal reflections and ensure clean transmission. Mate signal layers with plane layers across the piece of laminate to ensure impedance accuracy. The plane acts as a reference for the signal, creating a controlled capacitor that dictates the signal's impedance.
• Edge clearance: Maintain appropriate copper-to-edge clearance for manufacturing. A minimum of 0.010 inches is common for outer layers. 
• Imaging process: Outer layers may use positive film, and their imaging process includes applying photoresist, UV exposure, and chemical development to remove unexposed areas. Etching of the outer layer defines the final traces. 

By keeping these best practices top of mind and working closely with manufacturers, designers can spend less time worrying about yield or cost and focus on unlocking the innovative power of modern PCB technology like UHDI.

This column originally appeared in the December 2025 issue of Design007 Magazine.