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JANUARY 2026 I I-CONNECT007 MAGAZINE 53 are at least five chemical components that make up a copper bath, and they must be in the correct ratio for the plating cell's intended output. Depending on the setup of the chemical ratio, the bath can per- form differently: higher throw—slower plating times, lower-throw—faster plating times, high current den- sity plating, low current density plating, etc. In copper plating, it is not just about the copper concentration and the sulfuric acid (electrolyte) concentration. We must consider other components in the bath, including proprietary levelers and bright- eners. Brighteners brighten the copper deposit. The copper from electroless and foil is matte and pink- ish. Once plated, the result is a shiny copper deposit we expect, with a grain structure that optimizes current flow. Levelers help fill in small imperfec- tions that may have existed at the start of the plating process. They do an excellent job of filling in gaps and creating a uniform, level copper surface. Components in the bath are consumed or broken down at varying rates during the plating process, jeopardizing our chemical ratios. We use chemi- cal analyses across all our baths to confirm the chemical concentrations of all components. Tech- nicians take samples and measure sulfuric acid, copper, chloride, brightener, and leveler content. They make additions or dilutions to keep the bath within the optimal operating window. The brighten- ers and levelers break down over time and create organic impurities. When they reach the tolerance threshold, we take the bath offline and clean it with a carbon filtering process to remove the impuri- ties. The filtering and chemical oxidation process reduces the bath to nearly just sulfuric acid and copper sulfate, so we must add back the brighten- ers and levelers to the solution. Preparing for the Next Phase of Manufacturing With plating complete, our boards are now freshly coated with copper and dripping sulfuric acid and copper sulfate. We rinse them off, and they go directly into an electrolytic tin plating process, the same process as with copper. The result is a layer of tin atop the copper that becomes our etch resist. After we add it to the board, we remove the photo- resist and expose all of the unwanted copper below it. The tin protects the copper we do want removed during etching. Now that we have protection over the copper traces, pads, and vias, the next phase of production is the strip-etch-strip (SES) process. Design Best Practices To ensure your design provides the foundation for a smooth pattern plating process, collaborate with your manufacturer early and often. Tolerances for UHDI boards are extremely tight, and working closely with your fabricator will ensure your design aligns with their specific process capabilities. To create a uni- form, precise design, keep the following top of mind: • Design for balanced copper distribution to minimize uneven copper density. You can achieve this in part by adding non-functional copper areas to promote uniform plating. Avoid leaving signal traces out "in the middle of nowhere." • Specify UHDI-friendly surface finishes like electroless nickel immersion gold (ENIG). • When planning your layer stack, use thin dielectrics to help control impedance and manage microvia aspect ratios. This will also help prevent warping and twisting during pro- duction that can affect copper distribution. Thin dielectrics can also improve signal integ- rity by keeping traces close to ground planes. • Optimize via structures to save space and enable the tight component placement UHDI requires. SES is the next manufacturing step, and design- ing for it will be the subject of my next article. If you want to learn more now, listen to episode 9 of "On the Line with…" where we discuss SES. DESIGN007 Matt Stevenson is vice presi- dent 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. C O N N ECT T H E D OT S