Flex PCB Design Best Practices

Estimated reading time: 7 minutes

David Lackey, a flex circuit expert at American Standard Circuits, speaks about the advantages and challenges of designing with flexible and rigid-flex PCBs. He also discusses how consulting before the design phase can save time and costs by avoiding manufacturing issues and highlights the reliability and packaging benefits of flex technology, especially for compact designs in sectors like aerospace.

Brittany Martin: Thanks for taking the time to speak with me about all things flex PCB. Can you share a little about your experience in the industry?

David Lackey: I have been in the printed circuit board industry for 43 years and with American Standard Circuits for 16 years. I spent most of my career in high-end shops like Northrop Grumman and many defense companies. A lot of my work was in the circuit board flex market, so I'mthe resident flex expert. When they have questions, they contact me to talk about their designs. Beforehand, we had written a couple of books about flex to help designers.

Martin: When your customers work on a flex design, what is the advantage of reaching out to you in advance?

Lackey: The advantage is they don’t spend all this time on and then submit the design only to find out it’s not manufacturable. The board may have many issues, and then we have to go back to the drawing board. We encourage our customers, particularly when they're doing more complex designs (not just flex), to reach out to us in advance to make sure that what they're doing is right. For example, they may think a material is readily available, but it could be several weeks before it is. It could be something in the design that can't be achieved. We have to figure out a way to fix that. It's always good to be upfront when they are doing a design. It's more beneficial for them than for us because they'll lose time if we have to go back and fix some things.

Martin: Do they often come with designs that can't be used?

Lackey: Not completely, but sometimes we'll find a few things we may have to tweak along the way. It's usually not a typical redesign, but we may find some things in the data that we recommend they fix. It’s also not just about whether it can be manufactured. Sometimes, it's about cost. We want to save them money. That's part of our job. We say to our customers, “Hey, if you can do this instead of this, you'll save some money.”

Martin: What is the difference between flex, rigid-flex, and rigid PCBs?

Lackey: Basically, the difference is the word flex. The flex board can be moved, twisted, and turned. There are some packaging advantages. If you have to fit your hardware into a small box designed by a mechanical engineer, you would not want to use a rigid PCB because that can't always be done. If the PCBs have to talk to each other, they usually do so through some sort of wiring harness and connectors. That takes up space, so by integrating a flex and rigid-flex, they can bend the board after assembly. It's easier for them to fit it into a box that contains all the components.

Regarding packaging, there are definite advantages to using a flex or rigid-flex versus something with a bunch of connectors, especially when getting things into smaller areas. Think about the first computer and its size. Your iPhone is much more powerful than that first computer. Packaging is an issue, but you can get more into a smaller area using a flex board, and there's more reliability. They use wire harnesses to connect two circuit boards, and then they have to plug those into a connector.

There could be some reliability issues when you have a rigid-flex board. You don't have those connectors and wire harnesses; it’s all integrated. It's a little more costly now, but when it comes to reliability, it has an advantage over the current way with wire harnesses and connectors.

Martin: Is there more reliability with flex for wire harnesses and connectors?

Lackey: Yes. As I mentioned, it's a much more reliable process. Everything's integrated. You don't have these external components that must be plugged into different boards. Everything is continuous. You don't have to worry about whether the operator put the connector on properly or if the wiring harnesses kinks or a wire breaks inside. It is a much more reliable process, which is attractive to many who use the components.

Martin: The cost of a flex board is typically more expensive. How do you see that evolving?

Lackey: One of the first things people say is, “I've got these two circuit boards, and I'm using this connector and this wiring harness to put them together that costs X, but what would it cost to use a rigid-flex?” It is typically higher, but you don't have the labor of installing the components, which somebody has to do to connect the two separate boards because it's all integrated, and you have much more reliability.

If it's in a device that may be in a harsh environment, shaking constantly, or experiencing extreme temperature variations, it is certainly more reliable from that aspect, which is why you see more of it now, and the packaging is so much smaller.

Martin: How does the use of flex boards help with packaging and designs?

Lackey: You may have a box the size of a Rubik's cube, maybe 3x3 inches, and at one point, you had to package three circuit boards together. You would have to hook up all three rigid boards so they could talk to each other—several connectors and wire harnesses. That meant that at one point the Rubik's cube was the size of a shoe box, but when you start developing with rigid-flex, you can make things smaller; you have less space and you can use less weight, which is important in aerospace. There are huge benefits from a packaging perspective, such as trying to fit smaller and lighter things into areas and better reliability.

Martin: How is technology evolving?

Lackey: We more opportunities everyday in our business. Sometimes, they're very simple. You're just connecting a couple of rigid pieces: one flex circuit, one layer. Sometimes it's very complicated. You may have a rigid-flex board like an octopus with a base, and all these tentacles coming out. Each one can be a different length and have different layers with different functions. It's a growing technology, even in the rigid-flex world. Some of the flex materials, cover layers, and bond materials coming out are attractive to rigid-flex engineers for signal integrity. You're also starting to see some growth in that market using flex circuitry.

Martin: Are there other industries or new technologies you think will be more reliant on flex?

Lackey: In almost any market, there's a certain amount of opportunities for flex where there may not have been before. Packaging is a big one because you can use less space. Any device—our phones, everything—could have some sort of flex circuit because it's easier to manage and assemble inside because of its flexibility. You don't have to be precise about the location. Most of the time, it's flexed to install. It is in every market segment and very popular right now.

Martin: What types of flex technology does ASC currently deal with, and is it becoming more challenging?

Lackey: We see some very simple one-layer flex boards, but we're seeing some more challenging designs coming our way. It's the growth of the technology. We are currently running a 28-layer rigid-flex. It'll be the most we've ever done here. Before that, the most we had done was 22 layers. It has a very large footprint and is a very large panel that we have to run. The chips are faster, and everything is more complicated, like AI with the speed at which it has to operate. Integrating flex circuitry into those types of designs when the chip packaging gets so much smaller makes it that much more of a challenge, and flex circuits are very difficult from a handling standpoint. So much of our business requires manual operation, where people touch things, and because it's so flexible, it's easy to damage. There’s always a risk. We're running up against some very complex designs now, but we strive to meet the challenge.

Martin: Do you like the challenge?

Lackey: Yes. There are good days and bad days, which is why collaborating upfront is important. We want to make sure we can make this happen; that we're not going to waste any time, and that we can hopefully work out some bugs right up front.

Martin: Thank you, Dave.

Lackey: No problem. I appreciate it.