Expert Tips for Designing Flex PCBs

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The electronics industry is a rapidly evolving world where designers and R & D teams are continuously researching ways to improve mainstream technology. One such development can be found in the field of Printed Circuit Boards (PCBs) where research and innovation led to the invention of flexible PCBs, more commonly referred to as flex PCBs.

Designing a flex PCB warrants a somewhat unique approach compared to what designers would use for designing rigid PCBs. The flex PCB design must be optimized and configured around the intended application and the material used for the PCB.

If rightly designed, a flex PCB can offer numerous benefits over its rigid alternative such as reduced weight, compact packaging, enhanced durability, and ease of installation. This makes flex PCBs suitable for industrious applications which include satellites, sensors, wearable devices and more. To assist you in designing a well-optimized flex PCB, this article will walk you through 6 expert tips for designing flex PCBs. So, without further ado, let’s begin!

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6 Expert Tips for Designing a Flex Circuit

Understand the Flex Requirements of Your Flex PCB

When designing a flex PCB, there are two key things you should be aware of before you proceed with the designing phase: how frequently will the PCB be flexing and to what extent will it be flexing. Based on the intended application of the PCB and how frequently it’ll have to bend or flex, a flex PCB can either be static or dynamic.

A dynamic board design will be robust and will undergo flexing very frequently, as such these boards are used in rigorous and harsh environments such as aircraft. A static board, on the other hand, will undergo less than 100 flex cycles during its life and is regarded as bend-to-install.

Bend radius is one of the main factors that influences the number of flex or bend cycles a PCB can bear without getting damaged. Therefore, knowing the flex requirements of your intended application for the flex PCB is crucial for designing suitable flex PCBs.

Additionally, avoid strenuous 90-degree bends while setting out the bend locations. Conductors that are smaller than 10 mils should be positioned on the neutral bend axis that is free from any compression or tension during flexing. Furthermore, conductors larger than 10 mils should be staggered in multilayered circuits (for improved effectiveness). Plated through-holes should be avoided in the bend area. Flex PCBs' flexibility is a key characteristic that allows PCB designers to employ them in small packages.

Implement Slots and Cutouts Where You Can

The bend radius of a flex PCB can be reduced by inserting slots or cutouts if there are no traces in the bend region. The amount of material needed to bend will be reduced by integrating cutouts and slots into the design. Sections from the flex PCB can also be cut away from regions where there is no circuitry, however this must be done longitudinally and requires routing afterwards.

Know Your Flex PCB Materials

Flex core layers and coverlay layers are primarily made of polyimide. When compared to rigid PCBs, flexible materials have superior material characteristics. Flex materials have homogeneous thickness and an enhanced dielectric constant (Dk) value that ranges between 3.2 and 3.4. Any variations in Dk are eliminated by the absence of the reinforcing woven glass. Thanks to its “cast” production technique, polyimide has a very consistent thickness that ranges between 0.5 and 4 mils.

Generally, there are two types of flex materials: Adessive-based and Adhesive-less. Adhesive based materials involve the use of acrylic adhesive to bond copper conductive layers to the polyimide material. However, this approach has a few shortcomings that are that the adhesive can cause crack formation in the copper plating through the holes. In addition, the adhesive layer increases the thickness of the board and is also prone to absorbing moisture from the atmosphere.

To counter these drawbacks, the Adhesive-less approach was introduced which involves casting copper directly onto the polyimide. This method had several benefits such as reduced thickness, improved flexibility, better thermal ratings, and better immunity to moisture absorption.

Making your pads as big as possible is vital since adhesive compounds tend to soften when heated. You can stabilize the outer layer and lessen stress by including anchors, spurs, or teardrops in your design. Another approach to get around adhesion problems is through multi-layer designs.

Knowing this, it is important that you select the flex materials appropriately that suit your application. Adhesive-based materials are generally adequate for laid-back applications, while the more demanding ones warrant the use of adhesive-less or multi layered designs.

Keep an Eye on Flex Trace Routing

A PCB's circuitry outline can make or break the PCB. Returning to the bend radius, short bending radii can shorten the life of PCBs, as such, a larger bend radius would be more suitable for trace routing. Additionally, in order to reduce the stress that can thin out copper circuits, it is recommended to avoid I-beaming.

Compared to angled ones, curved traces induce less stress. Additionally, if traces are placed on a flex PCB with two or more layers, they should be staggered on the top and bottom and kept perpendicular to the overall bend.

Mitigate Risk in Flex Vias

Peel flex PCB designs can, sometimes, have vias that crack or shatter. To make sure that your flex PCB design doesn’t have this problem, you must ensure that the annular rings are as large as feasible, tabs and anchors are added, and that the vias are tear-dropped.

Choose Your Flex PCB Manufacturer

Choosing the right manufacturer for your PCB is vital when it comes to flex PCBs. Make sure the electronics company you select is experienced in designing and manufacturing flex circuit boards and does so on a regular basis. The more experienced they are, the better they are at it. Make sure you inquire about the materials they use, the surface treatments they apply, etc. you could even ask to visit the facility before entrusting the company with your project.

Conclusion

Designing a flex PCB is an intricate process, one that requires detailed knowledge and expertise. If you need help designing and manufacturing a flex PCB, Hemeixin offers you the finest PCB solutions in the market. Our services extend far beyond flex PCBs and are backed by the highest standards of quality and field experience. To learn more about our services or to request a quote, call us now!

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