Mark Finstad

The shorter the length, the stiffer the flex.

I have an application requiring two rigid PCBs mounted at 90° to each other. I would like to connect them with a flex zone and use rigid-flex construction. How close can rigid areas be to each other; i.e., what is the shortest flex length between rigid areas?

Most manufacturers can form 0.250" flex sections with no issues, and many can get down to less than half that. There are manufacturing and final-use implications of short flex sections between rigid areas on rigid-flex circuitry. The manufacturing issues affect cost, and the final-use issues could cause premature failure if the specified flex length is too small.

Manufacturing Issues

Alignment/misregistration. How a flex area is created in a rigid-flex circuit is important to understand to grasp the challenges. You probably know that copper-clad polyimide substrates are present in both flex and rigid areas of a rigid-flex, not just in the flex areas. The way the manufacturer makes an area “flexible” is by eliminating all rigid materials in those areas. This is done many ways, depending on the manufacturer, but virtually all include a punching operation to form “windows” in the prepreg adhesive and FR-4 layers. These rigid layers then are aligned to the flex layers, so the future flexible areas are prepreg-free. The smaller these areas, the more critical alignment is. For example, consider an application where the distance between rigid areas is 0.1". If the top prepreg layer is skewed to the right by 0.015" and the bottom prepreg is skewed to the left 0.015", the flex zone is now 0.07", not 0.1". This is a 30% reduction in flex area, which could have a significant impact in the final application. However, if the flex zone is an inch wide or more, a 0.03" reduction is inconsequential. Very short distances between rigid areas can have a significant impact on yields and therefore cost.

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Read more: What Is the Shortest Flex Length Between Rigid Areas?

Nick Koop

If the clearance is not at least 10 mils, yields may drop.

One of the often-overlooked aspects of a board design is the moat. Perhaps this conjures up images of Monty Python’s Holy Grail, but moat does not refer to the ring around a castle. Instead, this is the clearance between pads and a surrounding copper plane, sometimes also referred to as embedded clearance.

These clearances often are 0.004" to 0.005" wide. This may seem like plenty of room, but Pareto analysis tells us this can lower overall manufacturing yield. These clearances often lead to unexpected yield loss, depending on certain design and processing factors. Believe it or not, etching these moats or clearances is difficult, due to the closed-ended, circular nature of the clearances. They do not image or etch well and are prone to shorting.

One reason is that driving the energy into the resist can result in bleeding and create an imaging short. But etching is also more difficult, as the etchant flow is trapped in a dead-end donut. These can conspire to create unintended image/etch shorts.

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Read more: How Big is Your Moat?

Mark Finstad

It might be factually correct, while also being completely impossible.

Question: I am looking to add a flex circuit supplier to our vendor base and requested its technical roadmap. After review, it appears almost exactly as the two vendors we currently have. Is this a coincidence, or do most (or all) flex suppliers have the same technical capabilities?

A technical roadmap is basically a document that outlines what a company can and cannot do from a technical standpoint (e.g., minimum trace/space, layer count, pad and via size, overall circuit size, etc.). I have never been a fan of technical roadmaps. Virtually every flex (and rigid) PCB supplier is compelled by their customer base to provide their capabilities, and therefore also their limitations. The problem with roadmaps is twofold. First, every supplier advertises the absolute best it has ever done in every single category. This is true even if it only did it one time, on one circuit, and in a beaker. This is not a fair representation of what the fabricator can or cannot do on production quantities. The second problem is the real answer for what a supplier can or cannot do is “it depends.” Let’s look at a few examples.

Read more: How to Really Read a Technical Roadmap

Nick Koop

Yes, but there are performance and cost tradeoffs.

You have started a new design. The chassis is defined, and you are thinking about how everything could be connected. Unlike the past, you are thinking about the interconnect strategy early in the design process, rather than at the last minute. Now you must decide what will be connected as an integrated rigid-flex and what might need to be done separately. So many design options are available right now. Here is where the question comes up: What if my layer counts are not consistent everywhere?

When faced with this, don’t worry. Rigid-flex allows us to design in almost any configuration. Each has performance and cost tradeoffs. Let’s review few of the more common design styles.


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