Issue link: https://iconnect007.uberflip.com/i/1523825
68 DESIGN007 MAGAZINE I JULY 2024 so we need a method to align the holes with images now hidden by the copper. We use a test board called a coupon to cre- ate drill targets for each layer, anticipating that the coupon holes match what is in the rest of the board. e drill is programmed to aim for those targets on every layer and, using an X-ray, calculate best fit for all locations on each layer, hitting all connections and missing clearances. e drill team, like the designer, has to bal- ance constraints. at is why we consult design notes that can help us with tolerances and drill bit sizes. e more help the designer gives us in the notes, the better we can assess tolerances during the drill process and make good con- ducting vias. If it seems like a lot of things can go wrong when drilling a multilayer board, that is because it can. Which brings us to the most important part of the drilling process: Prep. Sound preparation is at the heart of drilling. As with much of PCB design and manufac- turing, drilling is both a science and an art form. e process requires more than just cutting-edge equipment. To get the job done right, you need experience and expertise in every aspect of production. Both are key to properly preparing to drill holes in boards. Take the hole sizes on the print as an exam- ple. We can't just drill at the size on the print because, aer plating, the hole will be smaller. Adding copper decreases the diameter of the hole so we need to know the amount of copper and surface finish applied at end of the process. A good CAM department will account for any- thing that goes in the hole and adjust the hole size to account for this, thus ensuring the drill- ing process finishes with the proper tolerances. ere should also be an open and ongoing conversation with the designer to remove any uncertainty about instructions in the notes. Designers should be prepared to answer ques- tions about hole sizes, vias, and through-hole components, just to name a few. In addition to the complexities related to design, the drilling process itself offers intangi- bles. Books aren't 100% rigid. e inner layers will skew and stretch, creating movement. e drill team has to predict how panels will move so when we drill, it will be accurate. ink of it like a layer cake with frosting in the middle. ings squish around when you mash them together. is is where the cohesion between the CAM team and the drill team comes into play. e CAM team is responsible not just for iden- tifying the right size bit for the drill machine, but also for predicting layer behavior. e drill team uses the information provided by CAM to align the drill to the inner layers to ensure a functional circuit leaves the drill department. Now we go ahead and make holes in the board. But before we start drilling, we have to make a sandwich. When panels are piled atop each other in drill stacks, we place an alumi- num entry material on the top and a phenolic material on the bottom of the stack. e top of the sandwich helps dissipate heat and, at the drill point, gives the device a foothold to drill straight. If the material is slick or hard, the bit can skate on the surface before it goes in the board, meaning it can deflect from the target point or go in diagonally, which is clearly not optimal.