BoardSurfers: Bending the Flex Boards

When you design a rigid-flex board, the focus is, of course, on the bend. Your design might be bend to install (stable flexion) - it will be bent only a few times while installing. Or it might be dynamic - it will be bent regularly. It's important to know because you have to design accordingly. Once you know, you can decide on the number of flex layers, the materials used, the trace widths and spaces because you're concerned about the bend. You worry about the compression of inner layers and the tension in the outer layers. Will the inner layers delaminate or the outer layers tear?

Your concern is valid and quite commonsensical. Think of any hardbound book or the many planners we get as gifts at the start of each year - after all, the headband, case cloth, and liner, all form a multi-layer bendable area just like the bends of your flex circuits. The first thing that happens, if we read the book or use the planner quite often, is that the liner comes out loose. You can see it actually, the liner being compressed, even when you open the book or planner for the first time. And, slowly, with time, the headband starts showing creases that will eventually tear apart (you don't lose much though - usually, the headband contains the author's name and the title of the book). Well, in principle, this is what happens with the flex circuits too.

And, there are quite a few things you can do about it. For one, you can ensure that the thickness of the flex circuit is the minimum necessary. You will also take care of the trace width and space - place wide traces with large spaces. If possible, you will ensure conductors are perpendicular to the bend. However, when it comes to a bend, the most important aspects to concentrate on are the radius and angle of a band, along with the circuit thickness. Oh, you might also want to ensure the bend does not contain any vias or symbols - you don't want any cracks or splits, do you? That's not to say the materials used, the coverlay design, etc. are not important. It's with the assumption that you have taken care of all these already. But if you are worried about these aspects, you can read the blog post on designing rigid-flex PCBs.

What all would you want from any EDA application to easily and accurately create bends? You will want to define bend areas in the flex zones of a board to generate manufacturing outputs needed to properly manufacture your rigid-flex boards. You will then want to view and edit bend information for existing bend areas on the flex zones of a board to change parameters, such as bend angle, location, and direction.
That's where the Bend Editor feature of Allegro® PCB Editor comes to your rescue. Using Bend Editor you define a bend line that represents the center of a bend zone arc. You assign bend line parameter attributes that are attached as a property to the bend line. Once the line is defined, a bend area is created that visually displays the extents of the bend based on the bend values.

What all attributes do you assign? To start with you assign the radius and angle of the bend. These two are important because you definitely want to optimize the angle. You will want to flex by only what is necessary to minimize damage. You will also want to increase the radius as much as possible, again, to minimize damage.

Do you have multiple bends in your design? Then, you need to define the order in which the bend will happen.

Won't you want to add via keepout and package keepout geometries relative to the bend area’s outline geometry? You definitely will because these areas prevent the placement of symbols and vias in the bend area where vias may cause cracking or splitting, and the bending may cause components to pop off the design.

Once you have created a bend line, editing, of course, is as easy.

So, if you are onto a rigid-flex design and want to quickly and accurately define bends, just choose Setup – Bend – Create (and, then Edit when needed) and you are ready to go!