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JULY 2024 I DESIGN007 MAGAZINE 69 ere is also a back-up material on the bot- tom that the drill goes through. Going back in time to high school wood shop again, drilling boards is like drilling wood. If there's no pres- sure on the back of the board when the drill comes out, the wood splinters and creates a volcano. It's the same for boards. Drill move- ment will blow out the back, so a sacrificial piece of material is critical. A guide on the top and a catch on the bottom makes for a crisp and accurate hole. It's a delicate balance between drill bit size and panel thickness. Drill bits are really small. An 8-mil bit is thinner than a human hair, so too much lateral pressure can break it or cause a deflection. e relationship between panel thickness and drill hole size is what we call aspect ratio. For exam- ple, a 10-mil drill on a 100-mil board creates a hole aspect ratio of 10 to 1. How far can you throw? e term throw refers to how deep you can plate into the aspect ratio called out in the design. How much copper can you get to the center of the hole vs. the knee or surface? If the board is too thick for your hole, there is an increased chance of weak connections or no connection at all. Copper expands and con- tracts a lot, and a thin spot can fracture with exposure to heat. If your board makes it to assembly with thin spots in one or more con- nections, as it thermal cycles through its use- ful life, there can be reliability issues and even field failure. Fewer layers mean there will be less risk of throw issues. With a board of four to six layers, there are rarely throw issues, but how many layers is too many? It depends on the manufac- turer. If you keep your design at or below the 10 to 1 example, most fabricators can accom- modate that using their standard drilling pro- cess. There is more to consider than just aspect ratio. e board complexity plays a role as well. For a complex design, there could be as many as 25,000 via holes on one panel and take as many as eight hours on the drill machine. As a designer, if you call out 8-mil and 12-mil holes, and they're spread out, that creates the need for a longer drilling process. If there is more dis- tance between holes, there is more time spent moving and less time spent drilling. e same can be said for multiple hole sizes. Every time we have to change a bit during drilling, everything stops. We encourage designers to consider the drilling process carefully and optimize their designs to make it more efficient. If, for exam- ple, vias can be combined to a singular size, we spend less time on drill setup and bit changes. Designs with fewer hole sizes and similarly sized holes grouped together are cheaper to produce and yield better results. Drilling is a critical part of the manufactur- ing process. Hole quality needs to be great and getting there starts with a design optimized for drilling. 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. You can view other titles in the I-007eBooks library. Fewer layers mean there will be less risk of throw issues.