In this webinar, we discuss flex PCB design guidelines for manufacturing. The flex PCBs offer many advantages for both the physical designer and the manufacturer. The ability of the flex PCB to be molded and bent without breakage can lead to smaller designs that are lighter and take up less space all while resisting vibrations and other disruptions from its environment. Furthermore, they allow better airflow, heat dissipation, lower assembly costs, and a reduction in assembly errors.
Flex PCBs are flexible circuits with very thin substrates and high levels of bendability, tensile strength, and physical flexibility. They can also be molded into complex three-dimensional shapes for use across a diverse range of applications, such as heads-up displays for aerospace piloting and wearable technology.
Flex PCBs materials are made of acrylic base. These materials offer a lower dielectric constant when compared to rigid PCB materials. The thickness of the flex PCB materials varies between 0.5 and 5mils.
Medical application: Flex PCBs are widely used in medical devices as they are compact and can have 360-degree bendability.
Automotive application: The inherent resistance of flex PCBs to vibration makes them ideal for the harsh environment inside a motor vehicle.
Space application: Most aerospace applications make use of flex boards to reduce the overall weight of the system.
Flex PCBs offer a variety of advantages for designers and manufacturers. In this section, let us have a look at a few advantages of flex PCBs.
Using flex boards can result in direct and indirect cost savings.
A single rigid-flex PCB with 6 rigid sections can replace the entire assembly of 6 rigid PCBs within the product. It eliminates wire harnesses and connector pairs. This inventory reduction leads to direct cost savings.
Since there is no requirement for wiring harnesses the assembly cost is cut down. Excluding wiring harnesses eliminates wiring errors hence the product is more reliable. This also reduces the overall installation cost.
Flex PCBs can be either static or dynamic. The amount of times a PCB can bend determines whether the board is static or dynamic. We will learn more about static and dynamic flex PCBs in the next section.
Flex PCB design requires a slightly different approach than rigid PCBs. Below are few guidelines to be followed when you design a flex PCB.
Knowing the number of times your flex PCB will bend is crucial for your design. If a PCB is bent more times than the design allows for, then the copper will begin to stretch and crack.
Static board: A static board is considered bend-to-install, and will flex less than 100 times in its lifetime. These boards will only flex during the installation process.
Dynamic board: A dynamic board is a flex PCB that is regularly flexed and twisted. These boards generally flex during their operation. The printer is one of the most common applications where dynamic flex PCBs are used.
The bend radius is the minimum amount of bendiness for the flex area. It must be properly identified early in the design. This ensures your design can allow for the necessary number of bends without damaging the copper. IPC-2223 specifies the standards for bend radius.
To help determine how thick you can make your circuit, you should calculate the bend radius. This can be done based on how many layers your design has. Bend radius can be calculated using the below table.
Number of layersBend radius (mils/mm) 1 (single-sided)Flex thickness x 6 2 (double-sided)Flex thickness x 12 Multi-layerFlex thickness x 24Consider the following points when you design bend areas:
There are two major types of flex PCB materials:
Adhesive-based material: The copper is bonded to the polyimide with acrylic adhesive.
Adhesive-less material: The copper is cast directly onto the polyimide.
The use of adhesives with the rigid areas 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 your design.
Here are a few disadvantages of using adhesive-based materials:
We recommend the below materials for your flex PCB:
9 Chapters - 30 Pages - 40 Minute Read
Basic properties of the dielectric material to be considered
Signal loss in PCB substrates
Copper foil selection
Key considerations for choosing PCB materials
Always opt for a larger bend radius.
There is a requirement of having sharp angles, it is always recommended to have a larger radius. This improves the reliability of the board.
Use curved traces instead of traces with corners. Curved traces cause lower stress than angled ones.
When designing multi-layer flexible PCBs, stagger traces on the front and back. Stacked traces will not only reduce the flexibility of your circuit, it will increase stress contributing to the thinning of copper circuits at the bend radius.
Traces should also be kept perpendicular to the overall bend.
To know more about flex PCB design, read our article 5 Must-Knows for Your First Flex PCB Design.
Request a stack-up from your manufacturer before designing begins. It is crucial that you know what stack-up you are designing. Rigid-flex is the simplest configuration that will allow you to reduce the number of connectors, which will also increase wiring density and reliability.
Having a face-to-face meeting with the supplier is the best way to ensure that you’re on the same page in terms of where the overall PCB process is headed. This meeting can also help ensure that flex PCB design guidelines and capabilities are well-understood.
For rigid-flex PCBs, we process the flex layer as a two-layer board, laminate it between the rigid layers. Placing 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.
Stack-up sent by a customer:
This was a four-layer flex board with ZIF connectors requiring controlled impedance. The high-speed ZIF connectors connected finger areas from the edge to the top of the board. This stack-up had a few issues. First, the board’s flex layers were located on the outside of the stack-up, which increased the possibility of manufacturing problems and issues. Second, we had to make sure the board would meet the impedance requirements.
Also read, How to Build a Flex Stack-Up with Controlled Impedance
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.
The flex layers are also protected by our surface plating. 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.
The stiffener is an additional mechanical piece that provides mechanical support to the PCB during the assembly. Single-sided, double-sided, and multilayered flex PCBs can be stiffened in specific areas by adding localized rigid material. The material can increase strength, thickness, and rigidity.
For assembly, stiffeners can add support for mounting components. You should consider adding stiffeners if components need to be close to a flex area. But depending on the component size, surface mount areas do not always require a stiffener. You should apply stiffeners to the opposite side of SMT components and to the same side as the connector or through-hole components.
Kapton and FR4 materials are commonly used for stiffeners and can be attached with thermally-cured acrylic or pressure-sensitive adhesive.
Stiffeners should overlap the bared coverlay by .030” to relieve stress.
Annular rings in rigid-flex and flex multilayers are often compromised, especially in places where tight hole-to-pad ratios are demanded. This is mainly due to the dimensional stability (1000ppm) of the flexible material. It is common to allow zero breaks out of the hole from the internal pad, and on some commercial parts, there is sometimes agreement to allow a 270-degree minimum contact ring.
When designing flexible printed circuit boards, allow for some misregistration between the internal pads and the drilled hole. Consider minimum space between the tracks and the drilled holes.
Vias are at a greater risk of getting peeled off from the flex layers. Consider the following points to reduce the risks associated with vias.
Understand that flex and rigid-flex design rules are different. Flex designs require button plating. For flex, annular rings need to be larger for flex rather than rigid. Each manufacturer has his own set of design rules and recommendations. PCB design and layout will also be affected by your planned circuit density and line spacing.
Another thing you should always work with your supplier on is material selection. The material should be suitable for the environment and the application in which the flex PCB will operate. Flex materials themselves are pretty durable, but flex laminates may be less suitable for certain applications.
To successfully design a flexible PCB, it is important for the designers to have a basic understanding of the flex drawing requirements.
The flex drawing requirements are:
To know more about flex drawing requirements, read our article: 9 drawing requirements for flex PCB design.
Below points should be covered in your flex fab notes
Below points should be covered in your rigid-flex fab notes:
In order to benefit from all that flex PCBs have to offer, you must have a clear vision of the printed circuit board’s functionality, familiarize yourself with the design rules, and follow strict guidelines.
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