Cover Story: PCB Routers Cut Time and Copper
PCB Routers Cut Time and Copper
A computer-controlled router can produce prototype PCBs in almost no time.
by Jon Titus, Senior Technical Editor
At 3:00 PM on a Friday afternoon you devise a new design for a low-noise amplifier you've worked on for months. You want to try the circuit right away, but a quick-turn printed-circuit board (PCB) house cannot deliver boards until next week. You know another engineering group uses a PCB router to produce prototype boards in about an hour, so you decide to try it.
This type of "tool" uses an X-Y positioner under computer control to move a motor and a small router bit across the PCB surface to mechanically remove copper. The final PCB surface includes the same copper areas you would expect from a chemically etched board.
The benefits of the router approach include quick turnaround and local control. You can change a design and get a new PCB in an hour or so. And if by chance you mistakenly choose an older PCB version, you can quickly correct that error and create the newest layout.
Most PCB routers include a computer or at least software that works with Gerber files (RS 274D or RS 274X) from PCB-design tools. Some software also accepts Excellon drill files, Hewlett-Packard Graphics Language (HPGL) files, AutoCAD dxf files, and others. The software translates the provided information into tooling patterns that control the router mechanics and specify the proper router tool or drill to use.
For a typical PCB, the router would first use a small V-shaped tool to go around each signal path to isolate it from the surrounding copper. Then by using a larger tool it would quickly remove large swaths of unused copper. "On one side of a board you might need only pads and traces, but on the other side you might need large special ground planes under sensitive circuits," said Fred Schultheis, owner of North Bay Technical, an importer of MITS routers. "Engineers need to know their minimum air gap--the minimum pad-to-pad, trace-to-pad or trace-to-trace space. That dimension determines the tool to use first to isolate conductors."
According to John Taylor, president of T-Tech, his company's routers can create 4-mil-wide traces and spaces and drill 4-mil-diameter holes, which exceed the capabilities of many quick-turn board-etching shops.
Pins on a router's working table go through holes in a "blank" PCB to orient the board and hold it in place. The pins--which must fit tightly into the board's holes--also provide the proper reference points that align both sides of a board when you need a double-sided circuit. Some router suppliers offer copper-clad materials pre-drilled for their equipment. Or users can drill their own orientation holes. Then they use the computer to align one of the orientation pins with the board layout and produce their PCB--one side or two. (Router suppliers often sell through-hole connection products, too.)
"We have a customer who uses the entire 9-by-12-inch area available in our machine," said Brian Marshall, a sales associate at LPKF. "He doesn’t want large extra holes in his working area, so he uses an optional camera on his system to locate fiducial holes. Fiducial-drill software locates appropriate places on the outside edges of his board and drills four small holes. The camera locates those holes and then orients the router for each side of his PCB layout." This configuration also relies on a vacuum pulled through the working table to hold the board in place. Router vendors offer vacuum tables and cameras as add-on options.
By pulling the PCB material onto the table, a vacuum system also help overcome problems with slightly bowed materials. Better planarity means that routing occurs at a uniform depth. Manufacturers also use other techniques to maintain a uniform depth. "We use a milling head that freely floats over the PCB surface," explained LPKF's Marshall. Because it rides on the surface it keeps the router bit at a consistent depth. Before you start routing you run a 'scratch test' during which you adjust the router bit so it just contacts the copper. That contact provides the reference point so you can adjust the depth of the router's cut."
"For 20 years we have used a contact point that touches the copper surface and uses it as a reference for the router-depth setting," said T-Tech's Taylor. Now we offer an air-bearing pressure foot that places the routing bit at a constant height above the PCB material. Without depth control, when you route an RF circuit you could cut too deeply into the dielectric, which would change the impedance of the traces. Because the air-bearing pressure foot "rides" on the copper surface, it maintains a consistent cut as it moves across a PCB."
Because routers isolate traces, drill holes, or remove bulk copper you must change tools attached to the cutter motor depending on the needed operation. You can change tools manually when prompted by the software or use an automatic tool changer. Opting to change tools manually can reduce the cost of a machine but it can force you to stay near a machine for most of its operations. Manual tool changes, however, can offer better precision than automatic tool changers. On the other hand, automatic tool changers work well when you need to produce several identical boards in a step-and-repeat sequence and they can reduce the time needed to produce a prototype PCB.
In either case you have fiberglass and copper dust in your machine, so you might get foreign material in tool holders. These small bits can offsets drills or router bits slightly and increase mechanical error, called run out, in the spindle. A vacuum cleaner can remove most waste, though.
To isolate traces with a 4-mil spacing you don't use a 4-mil router bit. Instead, you choose a conical bit and adjust its depth to control the cutting width. So, you must precisely control the penetration of the tool into the copper and substrate. "The penetration should be precise to within 0.1 mil," explained Chris Ovtcharov, the owner of Accurate CNC. "Automatic tool-change tools usually have plastic rings that determine their position within a collet. Unfortunately, the ring-to-shank-end dimension can vary by as much as two mils. That height error translates into a significant routing-dimension error on your boards." So, an automatic tool changer is perfect when you need to drill, create cutouts, or cut a board to shape. But to isolate traces, I recommend you change tools manually to check penetration depth, particularly when you use a tool for the first time."
"Whether engineers change tools manually or automatically, aim for three or four drill sizes rather than, say, eight or 10," noted Schultheis of North Bay Technical. "A smaller number of tool changes reduces prototyping time. Ask, 'What can I do to simplify things?' After all, the difference between a 0.9-mm and a 1.0-mm hole is not much, so choose one or the other, not both."
Routing and drilling wears out tools, so you need to know when to replace them. "When an engineer feels the top of the board has begun to burr, or feels a bit rough, that's the time to switch to a new tool," said Schultheis. "Also, if the copper edges feel rough, change the V-shaped engraving tool." Software in the MITS machines will tell users the number of times a bit gets used, which can help determine when to replace it.
Acknowledgement: My thanks go to Alex Feng, vice president at Sun Equipment, who also contributed helpful information for this article.