IPC International Community magazine an association member publication
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72 I-CONNECT007 MAGAZINE I MARCH 2026 legend layer on top with an inkjet printer, so there is good adhesion between the solder mask and the legend ink. Then we thermally cure the panel again to remove all the remaining solvents. Once it comes out of the oven and cools, the solder mask and legend are hard and durable, and all solder- able pads are exposed. Solder Mask Design Considerations As with any phase of production, designers should be aware of what can go wrong during solder mask and know how to prevent issues before they occur. Avoid Missing or Thin Dams As HDI designs become more common, solder mask becomes a more critical part of the manu- facturing process. In tight pitch areas of the board, designers need to be sure they have enough solder mask web or dam between fine pitch pads to avoid solder shorts while assembling. Most manufacturers can resolve and maintain a 23 mil-wide web between pads with green solder mask. Other colors are less capable, especially white and black, and may require a slightly larger dam. Keeping as much of the spacing between pads to accommodate a smaller dam or web be- tween features is important. If pad spacing leaves something like a 1-mil web, most manufacturers will struggle to produce boards that meet design requirements. Narrow pad spacing can also cre- ate solder bridging during assembly and result in shorting defects. Mind Your Solder Mask Thickness Thickness can have an impact on the manufactur- ing process and so can the type of solder mask. When we get down to an impedance tolerance of ±5%, every little thing can have an impact on im- pedance, including the thickness and pigmentation of the solder mask. Solder pad shapes, sizes, and locations should provide sufficient room for solder mask webs and dams, accounting for the solder mask's swell. Knowing your tolerances and ensur- ing design consistency will be key to a smooth solder mask process. PCB Legend Design Considerations The PCB legend is typically printed over the solder mask layer. It puts all the identifying text, compo- nent labels, symbols, and patterns onto the surface of the board. The legend layer serves as a visual guide for people working with the board during processes such as assembly and testing. As with every stage of production, designers can do a lot to ensure their PCB legends are well or- ganized and clear to the manufacturer. Some best practices include: • Collaborate with your manufacturer about their specific legend guidelines. Their capa- bilities and processes will inform the legend design rules, covering everything from text size to clearances to text orientation. • Make your legends readable. Pay attention to contrast, font readability, and overcrowd- ing. An easy-to-read legend will help avoid production delays and ensure smooth manu- facturing all the way through assembly. • Double-check your legend for accuracy and functionality. Spelling errors can happen, so try to catch them before they go to produc- tion. Your manufacturer will thank you. The solder mask and legend process occurs near the end of the manufacturing process, but we still have to apply a surface finish. Keep an eye out for my next article on that subject. If you can't wait, listen to the Designing for Reality: Surface Finish episode of On the Line With… To hear our in-depth conversation about solder mask and legend, please listen to episode 10. I-CONNECT007 Matt Stevenson is vice president and general manager of ASC Sun- stone Circuits. To read past columns, click here. Read Matt's book, The Printed Circuit Designer's Guide to … Designing for Reality C O N N ECT T H E D OTS