Flex Circuits: Design Tips and Assembly Consideration

One of the things that you will always hear when trying to research Flex circuit designing is the fact that it's both an electrical and mechanical task. The repeated insistence of this fact is simply to help you understand the repercussions of focusing on one thing and not considering the importance of others. If you manage to design a highly efficient and amazing flex circuit that doesn't "flex," because you didn't think about the placement of plated through holes and components and forgot about the bending radius, you didn't really create a flex circuit.

So, it’s a good idea to go through a few tips and keep in mind some assembly considerations when designing your flex circuit.

Tips for Designing The Flex Circuit

Before designing your flex circuit, make sure that you really need it. Especially if you are going to use flex with stiffeners. It's typically used as a cheaper replacement of a rigid-flex circuit, but it's not as efficient, compact, and durable. Some of the tips that can help you design an amazing flex are:

1. Make sure you understand the difference between a dynamic flex and a static-flex. A static-flex circuit will need to bend only a few times in its lifetime, mostly for installation. On the other hand, a dynamic flex will need to go through several bend cycles, probably thousands and hundreds of thousands of times. So you will be limited on the number of layers with the dynamic flex, and you will have to incorporate hatched polygons for copper layers, instead of a plane copper layer of high-speed circuits.

2. Account for and calculate the bend radius of the circuit. The bend radius depends heavily on the number of layers of the flex you have and the overall thickness of the board. Understand that you can’t make 90 degree bends because they will unnecessarily fatigue the copper and cause cracks in it. This will cause discontinuity and shorts and undermine the electrical integrity of the circuit. So the bends should be circular, and the radius should be calculated based on the number of layers and thickness of the flex. A general rule of thumb is the bend radius be ten times the thickness of the flex for a two-layer flex. For a higher number of layers, the bend radius can be somewhere between 20 to 25 times the flex thickness. For a single layer (this is something you usually see in dynamic flex circuits), the radius is relatively smaller, six times the flex thickness. But many manufacturers are capable of breaking convention with the right techniques and material choice.

3. Once you have identified the bend area, make sure that you don’t change the width or direction of the traces in that area. You can have different conductor thickness running side by side, but you shouldn’t change the thickness of any of the traces within the bend area. This will result in a stress concentration point. Also, when you do have to change the width of traces, it should be a gradual progression, not a sharp shift.

4. Some of the most common design aspects that you should consider before starting your flex circuit planning are minimum trace widths, hole sizes, space between traces and pads, and distances to design edges. Flex circuit’s outline tolerances are also an important point to keep in mind while designing, along with the distance of copper geometries from these edges. Most importantly, consider the overall thickness of the flex. There are ways you can reduce it, like efficiently using the surface real estate and go for a low layer count, or using an adhesive-less substrate.

5. Another major assembly/manufacturing consideration that you should keep in mind is when you are designing the overlay openings (for a typical 1-ounce copper design), oversize it by about ten mils. This is way more than you would do for the solder masking of a conventional Rigid PCB, but it’s needed in the flex. There are two major reasons for that. First is that because flex materials are less dimensionally stable than the rigid substrates (especially without the stiffener), they require relatively looser tolerances for the drill size and locations. This is in stark competition to how to fine-tune you can go with drilling in the solder mask of a rigid PCB. The second oversizing reason is adhesive. When the coverlay is laminated using pressure and heat, it squeezes out the adhesive into the openings. The adhesive must flow out of the pad but never get on top of the pad itself; otherwise, it will impact the size of the annular ring. After lamination is done, you can’t use etching on the flex.

6. If you want to check-out firsthand the mechanical compatibility of the flex circuit you are proposing, take a look at the sample circuits provided by the vendor. Make sure they have the same or a similar layer count. It will show you whether your circuit when finished, will have the necessary flexibility or not. A mechanical sample of your particular dimensions will also help you check out the fit and form of the flex and make sure it will be functional. The fit should be flexible enough and dimensionally stable enough for installation in the actual housing. The form of the circuit should be within the specs of your design scope. For example, if it’s too heavy or too large, it might offset the balance of the whole thing.

7. Make the right use of technology. Don’t just start designing from the electrical perspective. Based on where and in which position the flex needs to be installed, it's a good idea to use CAD software to visualize the mechanical design of the circuit. It will help you figure out the thickness and layer count limitations of your flex, as well as the smallest bend radii you can go for.

8. For multiple flex layers, make sure not all of them are bonded. An air gap between the layers (or a couple of layers will enhance its mechanical flexibility.

9. The need for shielding and the presence of SMT components in the circuit can drastically change your design. You also have to figure out whether you can get away with hatched polygon planer layers for grounding and signal, or will you need a solid plane. The presence and absence of through-holes can mean a significant difference in the surface real estate available to you. But the most challenging part will probably be reconciling specific electrical needs of the circuit (controlled impedance, high-frequency, noise), with the mechanical design.

Conclusion

One of the things that will solidify the possibility of designing the perfect flex will be the inclusion of your PCB fabricator early on in the process. As well as understanding the design guidelines, pcb capabilities, and limitations of the vendor. What trace widths and copper weights they can accommodate? What is their default material selection, and what can they get shipped? Via capabilities, hole-to-border, and pattern-to-pattern tolerance they can work with? And what are the value-added services they offer?

Answers to questions like this won’t only help you design the right flex circuit, but also get you the product you want.