Flex PCB design requires a slightly different approach than rigid PCBs. While flex PCBs can provide major savings in manufacturing cost as well as reduced space consumption and lower weight compared with rigid, their design must be optimized for their materials and use cases.
A well-designed flex PCB will be lightweight, durable, easy to install, and suitable for demanding applications such as wearable devices and satellites. Indeed, the physical advantages of flex are that it offers improved resistance to vibrations and movement, and it is easier to prepare for harsh environments.
To know more about flex PCBs used in satellite applications read our article Flex PCBs in Satellite Applications: Lighter Than the Clouds.
In this article, we will have a look at a few must-knows to design a high-quality flex PCB:
There are plenty of ways to ensure a high-quality flex PCB design. Let’s look at five things you should know going into your first attempt at flex PCB:
It is vital to know two things about flexibility: how many times the PCB will be flexing, and to what extent the PCB will flex. The amount of times it can bend, or application, determines whether the board will be a static or dynamic board. A static board is considered bend-to-install and will flex less than 100 times in its lifetime. A dynamic board’s design needs to be more robust in nature, as flexing will be done on a regular basis—and will need to withstand tens of thousands of bends. These PCBs are used in very harsh conditions such as spacecraft and military applications.
To know more about military-grade PCB specifications, read our article: Military-Grade PCB Design Rules and Considerations.
Bend radius—the minimum amount of bendiness for the flex area—must be properly identified early in the design. This ensures your design can allow for the necessary number of bends without damaging the copper. The figure below will help determine how thick you can make your circuit. Calculating bend radius can be done based on how many layers are in the flex PCB, as explained in our Flex Design Guide.
Number of layersBend radius (mils/mm) 1 (single-sided)Flex thickness x 6 2 (double-sided)Flex thickness x 12 Multi-layerFlex thickness x 24To know more about flex PCB applications read our article Why Are Flex PCBs Used in Medical Devices and Wearables? When laying out the bend areas, avoid 90-degree bends that cause high strain. Plated through-holes should be avoided in the bend area, conductors should be staggered in multilayered circuits (for greater effectiveness), and conductors smaller than 10 mils should be placed within the neutral bend axis where there is no tension or compression during flexing. Flexibility is an important feature of the flex PCBs that enables the PCB designers to use them in tiny packages.
Heat-forming requires a steel jig, as it forces the board to lie a certain way. The steel jig is then inserted into an oven. One of the benefits of heat-forming is the tighter bend radius it provides. However, heat-forming is used primarily for ease-of-installation; tighter bend radius just happens to be one of the additional capabilities it provides.
If there are no traces in the bend region, the bend radius can be minimized through the insertion of cutouts, or slots. Using cutouts will reduce the amount of material required to bend. Another option is removing sections of the flex where there is no circuitry, although this must be removed lengthwise and will require routing afterward.
Polyimide is the primary material used for both flex core layers and coverlay layers. Flex materials offer better material properties when compared to rigid PCBs. The thickness of flex materials is uniform with an improved Dk value ranging between 3.2 and 3.4. The lack of woven glass reinforcement eliminates variations in Dk. Polyimide is also extremely uniform in its thickness due to its “cast” manufacturing process. The typical layer thickness ranges from 0.5 to 4mils.
Polyimide flex cores are cladded with either electro-deposited or rolled annealed copper. This copper is very thin and is suitable for both dynamic and static applications. 0.5oz (0.7mils) copper is commonly used in flex PCBs. The most common flex PCB copper weights are 0.5oz and 1oz. The maximum copper weight is 2oz. This gives the best combination of the thinnest possible construction.
There are two major types of flex materials:
Adhesives are used to laminate the copper layer with polyimide (core). The use of adhesives may cause cracks to form in the copper plating within via holes because acrylic adhesives can become soft when heated. Consequently, when designing for adhesive-based materials, it’s important to incorporate anchors and teardrops in the design.
Here are a few disadvantages of using adhesive based materials:
To address the use of adhesives in copper-clad flex laminate, adhesiveless construction was introduced.
Following are the features of adhesiveless materials:
Flex PCBs usually require looser outline tolerance than other boards. This is because their materials have less dimensional stability than rigid ones. Depending on the profile tolerance, you may also require a hard tool or laser cutting, which can be expensive.
Since the chemicals in adhesives can become soft when heated, it is important to make your pads as large as possible. Using spurs, anchors, and/or teardrops in your design can help stabilize the outer layer and reduce stress. Multi-layer designs are also a way around adhesive issues.
Circuitry layout makes or breaks a PCB. Going back to the bend radius, a large radius is preferable here to the sharp angles that shorten a board’s lifespan. Moreover, it is best to avoid I-beaming so as to minimize the stress that can thin out copper circuits.
Curved traces cause lower stress than angled ones. Traces should also be kept perpendicular to the overall bend and, if placed on a flex PCB with two or more layers, staggered on the top and bottom.
For rigid-flex PCBs, Sierra processes the flex layer as a two-layer board, laminates it between the rigid layers, and mills it so the flex is visible. Putting flex layers on the inside of the stack-up provides protection from exposure to outer-layer plating. This placement also simplifies manufacturing and improves impedance and control in the flex area.
The flex layer can be etched away from the design as part of a separate process, allowing for more protection. Below is a mini-case study.
The Issues:
The Solution:
We embedded the flex layers in the center of the stack-up. This protected the layers during the manufacturing process and ensured that the less-durable flex layers were not exposed to outer-layer plating. This is how most rigid-flex stack-ups are designed. When the flex layers are on the outside, panels are harder to handle and harder to process. This made the board more durable and easier to manufacture. It also allowed for better impedance and better control around the flex finger area.
Because the flex layer is a separate process, putting the flex layers inside allows flex manufacturers the ability to etch away from the design while protecting the flex layers. Putting the rigid material on the outside also allows us to manufacture what is essentially a rigid panel. The flex layers are also protected by our surface plating because it should brittle the material. The material used also played a large part in making this board rigid-flex instead of flex. Rigid AP material was used, allowing for better impedance and reliability. It was a much better option than the original FR-4 material.
Also read, How to Build a Flex Stack-Up with Controlled Impedance
Sierra Circuits’ rigid-flex manufacturing process is as follows: first, we process the flex layer as a two-layer flex board. Then we laminate the flex layers in between the rigid layers. The last step is milling the layers so the flex becomes visible.
It is possible for vias to crack or break peel flex PCB designs. To prevent them from doing so, make sure that vias are tear-dropped, that anchors and or tabs are added, and that the annular rings are as large as possible.
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To learn more about flex design guidelines, watch our webinar Flex PCB Design Guidelines for Manufacturing.
Testing is an essential step in ensuring the integrity of your flexible printed circuit boards. While there are many possible approaches to verify the quality of both your raw materials and your finished products, guidelines from the Association Connecting Electronics Industries (IPC), such as IPC-6013, IPC-2223, and IP-FC-234 and their respective successors, are a good place to start.
IPC-6013C was first published in December 2013. It provides the qualification and performance specification for flex PCBs. This standard supersedes a number of previous IPC standards, including the original IPC-6013 from 1998. It specifies many different test methods for flex PCBs, including thermal, bend, and impedance testing. This standard also includes quality assurance provisions, such as sample test coupons and guidelines for acceptance and quality conformance tests.
IPC-2223C provides guidance on selecting adhesive materials and rigid-flex interface. This document also provides tips on plated-through holes and flex vias.
This document provides information regarding the usage of pressure-sensitive adhesives (PSAs) for the assembly of flex PCBs. This guide provides information on adhesive types available and processes suggested for their proper use, highlights strengths, weaknesses, or limitations, how to start implementation and where to find additional information.
IPC-2221 establishes the standard component mounting and PCB interconnecting structures. This document also provides the test coupon design standards used for quality conformance testing.
Generally, this document is used in tandem with the document IPC-2221. IPC-2223 establishes the design specification of the flex PCB. It also provides information regarding component mounting and interconnecting structures.
Circuit board testing is an application and environment-specific process. For example, aerospace and space applications require Aerospace and space applications, for instance, require a formidable process than consumer-oriented applications.
The IPC guidelines are always a good foundation in order to design reliable flex PCBs. IPC-2223 and IP-FC-234 are the documents recommended for getting started with flex PCB testing. These documents ensure the best possible reliability for your flex PCBs. Following these standard guidelines gives you a solid framework for designing a wide range of flex PCBs (from single metal flex to multilayer boards to multi-layer boards such as 6-layer flex PCBs.
When it comes to flex, you have to choose the right manufacturer. Make sure that the PCB shop you pick produces flex circuit boards on a regular basis: the more they do, the more they master flex. Do not hesitate to ask about the materials they are used to work with, what their surface finishes are, etc.
It is even better to visit your PCB manufacturer and see their capabilities for yourself. If you come to Sierra, we will give you a “behind-the-scenes” tour of our facilities in Sunnyvale, CA. You will learn all you need to know about our manufacturing and assembly process.
Check our flex capabilities and call us or chat with us!