Issue link: https://iconnect007.uberflip.com/i/1505220
38 DESIGN007 MAGAZINE I AUGUST 2023 cal features help govern the size of the stack, panel thickness, smallest drill size, thickness of the copper, registration requirements, and even material type. e complete stack also includes a panel of entry material (such as alu- minum) and a panel of backup material (phe- nolic material is a common backup), both of which help improve the drill quality and accuracy. Once all the boards are sandwiched between the entry and backup, the actual drill- ing can begin. Precision dr illing requires specialized machinery for accuracy and consistency. e drill machinery makes a huge difference in through-hole quality, and the best holes make for the best quality boards. Great drill machines are capable of high aspect r at io dr i l ling , small via sizes, and tight posi- tional accuracy. Once the drilling pro- cess is finished, individual boards can move on to the next stages of manufacturing. Electroless copper is added to the newly drilled through-holes, panels are imaged, cop- per electroplated, and then the boards move on to the etching process. When Drilling Goes Wrong When something goes wrong during the drilling process, it can result in one or more boards being damaged and discarded. e most common causes are material problems (image transfer from stacked pads/ground plane) and preparation errors which can lead to burrs and other hole quality issues. Burring is a fairly common issue. If drilling happens too quickly, the stack isn't sandwiched correctly, or a drill bit gets too dull, burrs can form around through-holes. is can lead to short circuits, faulty connections, or uneven application of copper in through-holes. e bottom board can receive the worst kind of burr: the "volcano effect." If you have ever drilled a hole too quickly through a piece of wood, you've seen the volcano—where the bit emerges on the other side, a large, conical burr can form. While minor burrs can be fixed with a sander or by scrubbing, volcanoes are usually too severe, and will result in scrapped boards. To prevent burrs and volcanoes, sandwich the stack between entry and backup layers. Designing for Efficient Drilling Designers can take a number of steps to improve the efficiency of drilling and help cut down on errors. One of the most important improvements is to reduce the variety of through-hole sizes on the PCB design, which allows for fewer tool changes. is can be accomplished by paying close attention to the allowances for vari- ous through-h o l e r e q u i r e ments. If mult i p l e a l l o w a n c e ranges overlap, select a through-hole size that fits in as many allowances as possible. Keep in mind that tool changes take time. By cutting down on drill size requirements, the designer will not only greatly improve the speed of manufacture but reduce the number of extra steps needed to finish the manufactur- ing process. Some design elements can increase the chances for burring. Among these are higher copper weights and anything that can keep lay- ered boards from sitting flat against each other. Sometimes burring can be fixed with a hand sander, but that takes a lot of time and can slow down the manufacturing process. By minimizing the number of hole sizes, a designer can reduce the amount of material Precision drilling requires specialized machinery for accuracy and consistency.