CAD systems for flex circuit design: when it comes to designing for flex, some tools are more adaptable.

Traditional PCB design tools are not easy to use for designing flex circuitry--and not just because they try to draw everything at 45[degrees] angles. So how do flex designers make these tools work? The better question is, what tools do flex designers use? This article will take a quick peek at the popular CAD tools used to create flex circuits and then look at how they fare at designing flex.

Usually flex circuits are electrically simple but mechanically complex. As a result, many flex circuits are designed using mechanical CAD programs rather than PCB CAD programs. In fact, many flex circuits are created by mechanical engineers rather than PCB designers. They draw the circuit outline and the traces in AutoCAD or Pro-E, then export a DXF file and send it directly to the flex proto shop that will create the Gerber files. The flex business has an army of very talented CAM people who perform this kind of alchemy all the time.

Some engineers take the extra step of using a DXF-to-Gerber translator and a Gerber viewer to create their own Gerber data. CAM350 by DownStream Technologies does a pretty good job of importing DXF files and turning them into Gerber data, as does ASM500 by Artwork Conversion Software Inc. With a little practice, an engineer or draftsman can create DXF files and convert them to Gerber data that is good enough for fabrication.

But what if you have a more complicated flex circuit that does require some netlist capability? ProFLEX from Infinite Graphics is very capable for designing flex circuits with complex curved traces. It does not use a netlist to begin, but a netlist can be extracted once finished so it can be checked against the customer's netlist. ProFLEX has been around a long time and flex manufacturers swear by its capabilities to create very intricate flex designs.

Personally, I design many flex circuits in a wonderful program called Electronics Packaging Designer--EPD for short. EPD, developed by CAD Design Software, is an add-on program that runs on top of AutoCAD to make AutoCAD do PCB design functions. You can import a netlist, begin without a netlist and export a netlist later, or draw a schematic in AutoCAD that will export a netlist. With a netlist imported, EPD will draw a ratsnest, run DRCs, complete netlist checks and many other functions that you would associate with any PCB design program. Because EPD runs on top of AutoCAD, drawing complex curved traces is quickly and easily routed, something that would take forever in a standard PCB design program. It also easily imports and exports DXF files for communicating with mechanical engineering. I've used it for almost 15 years; it's a good tool.

We all know that sometimes the customer insists on a specific program. How do the more popular PCB CAD programs, such as Cadence Allegro, PADS or Expedition, add up? I spoke with other flex designers who use these programs and here is their feedback.

All the designers I talked with use either an add-on module or a separate mechanical CAD program, usually AutoCAD or IGI ProFLEX, in addition to the PCB CAD program. Some only need to clean up the board outline DXF file that they receive from the vendor, while others use the mechanical CAD program to create the curved traces and use them as a template from which they can pick the beginning and end points of the arcs and the radius. The extent to which the designer needs a "helper" CAD program depends on the PCB CAD program's ability to create and modify arcs.

Mentor Board Station has a very hard time drawing arcs, but Expedition has some ability to add or modify arcs. Mentor offers an add-on module called ATP Flex, which will give Expedition some mechanical CAD and trace modification capabilities useful for flex circuit designing. Another designer using Board Station told me that for complex curved routing he uses ProFLEX to create a template from which he can pick the beginning and end points of the arcs.

Cadence Allegro is very powerful program but it is not very easy to make or modify curved traces in it. Some folks use a program called Harmony by PTC that runs on top of Allegro to give it much of the mechanical CAD capabilities of AutoCAD. Unfortunately, PTC has decided not to support or enhance this product. You can still buy it from PTC; it sells for about $7,500 and comes with a training video. It's not on the company's Web page--you have to call and ask for it. When designing in Allegro I export the trace layers and use EPD to clean up the Allegro trace routing. CAD Design Software is working on a module that will allow EPD and Allegro to swap files back and forth. Right now it is customized for the Allegro Package Designer but they are working on a version that will interface with Allegro PCB Performance.

Designers who use PADS told me that creating arcs is pretty easy, as PADS will allow you to add a radius to a trace that has a 45[degrees] or a 90[degrees] angle. It also has the ability to do bus routing, but it won't naturally follow the board outline. You have to configure the constraints for the bus routing, lay down the first trace and then the other traces will follow.

Another feature to consider is the ability to handle netlist changes and whether you can begin a design without a netlist. As mentioned above, many flex circuits are designed without a netlist. Sometimes only the pin out of one connector is defined and the designer has the freedom to choose the pin out of the other connectors for the most efficient trace flow.

Allegro, Board Station and Expedition are netlist driven--you can't create a design without a netlist and it won't let you route a trace that violates the netlist. However, PADS does not need a netlist to begin a design and it has the ability to modify the netlist on the fly. It is very flexible (bad pun intended).

Another CAD program that is useful for designing flex circuitry is Zuken CR5000. Zuken has been around for as long as I've been designing flex, but until recently, it had very few users outside of Japan. However, several large North American companies, including some military/aerospace folks, have purchased seats of CR5000. The program has the capability to route multiple traces that hug complex board outlines with concentric radii. It also has the ability to begin a design without a netlist and the ability to change the netlist on the fly.

TOM WOZNICKI is president of Flex Circuit Design Co. He can be reached at [e-mail].