Flex PCB design requires a slightly different approach than rigid PCBs. While designing a flex board, you must consider the board outline, bending requirements, optimum material selection, stack-up, placement of copper features, and cost factors.
A well-designed flex PCB will be lightweight, robust, durable, reliable, and easy to install. Hence, it suits demanding applications like aerospace, satellites, IoT, medical, and wearable devices. Flex boards offer improved endurance to vibrations and high temperatures, making them more sustainable against harsh environmental conditions.
Flex PCBs can significantly save manufacturing costs, reduce space consumption, and be lighter. However, their design must be optimized for their materials and use cases. This article provides several useful pointers to ensure maximum reliability, manufacturability, and economy when planning your first flex printed circuit.
Before we begin with the flex design essentials, look at our infographic 5 design guidelines to build a reliable flex PCB. This can be your visual guide to craft a reliable circuit board.
The flexibility feature of flex PCBs enables designers to implement them in tiny packages. It is vital to understand two things about bendability:how many times the board will flex and to what extent it will flex. The number of times it can bend determines whether the PCB will be static or dynamic. A static board is considered bend-to-install and will flex less than 100 times in its lifetime. It is generally bent during the assembly process. A dynamic board‘s design needs to be more robust in nature, as it flexes regularly and will need to withstand tens of thousands of bends.
The thickness of these boards depends on several factors:
The bend radius is the degree to which the flex area of a circuit board can flex. The minimum angle at which the flexible region can bend must be identified early in the design phase. This ensures that your design allows the necessary amount of bends without damaging the copper. It is calculated based on the number of layers in the PCB.
The bend ratio is the ratio of the bend radius (R) to the thickness (T) of the flex circuit. The failure probability is higher if the bend radius is tighter. For a reliable flex PCB design, the minimum bend ratio for different types of circuits according to IPC is mentioned in the table below:
Bend radius of a flex board
Design tips when laying out the bend areas:
The thickness of the flex circuit directly influences its flexibility. The lower the thickness, the more the pliability. Reducing copper trace thickness can increase bendability. It becomes difficult for the board to bend in multilayer flex PCBs with a large layer count. Copper thickness can be reduced by cross-hatching the ground planes on both sides of the signal layers.
The tighter the bend radius, the higher the flexibility. However, the chances of damage are also higher. You should understand the optimum trade-off between having a smaller radius and the extent of bending required for your flex PCB design.
If there are no traces in the bend region, the bend radius can be minimized by inserting 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.
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IPC 4202, 4203, and 4204 standards prescribe different materials and their specifications. You need to know the PCB materials and the characteristics that suit your design criteria. The properties that you need to consider are:
Read IPC standards for laminate selection to know more.
Flex materials offer better material properties than rigid PCBs. A flex board will have the following materials:
Dielectric insulator and coverlay:Polyimide is used primarily for flex core and coverlay layers. The thickness of this dielectric flex material is uniform, with an improved dielectric constant (Dk) value ranging between 2.5 and 3.2 at 10 GHz. The lack of woven glass reinforcement eliminates variations in Dk. Due to its cast manufacturing process, polyimide has an extremely uniform thickness. The typical layer thickness ranges from 0.5 to 4 mils.
Conductor: Copper foil is a good conductor, so it is extensively used in making PCB circuits. Polyimide flex cores are clad with either electro-deposited or rolled annealed copper. This copper is very thin and suitable for both dynamic and static applications. 0.5oz (0.7 mil) 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.
Flexible copper-clad laminate (FCCL): It is a flex PCB’s core component comprising copper foil and polyimide layers. These boards generally use rolled annealed copper. Rolled annealed copper is created by subjecting electro-deposited copper to the rolled annealed process. It provides a tighter bend radius. The grain structure is transformed from a vertical to an elongated horizontal structure. This improves the ductility of copper, making it suitable for dynamic applications. Fully annealed or low-temperature annealed copper gives better flexing characteristics.
Bondply: These composites are polyimide films coated with B-staged acrylic adhesives on both sides. To encapsulate etched details in heavy copper multi-layered constructions of flex/rigid-flex boards, bondply is used between two conductive layers from different FCCL cores.
Adhesives: Different types of adhesives are available for flex circuits—acrylic adhesives, epoxy adhesives, and pressure-sensitive adhesives (PSA) are a few of them. PSAs are flexible, have superior bond strength, and are easy to work with. They can adhere to the substrate or other surfaces directly. PCB adhesives can be acrylic or epoxy-based and supplied as flexible tape.
They are known as thermosetting adhesive films. Applying enough pressure and heat makes the film tacky to secure the components. After this, they are placed in an autoclave/pressure, and additional heat and pressure are applied to finalize the bond.
Acrylic adhesives stay malleable even after they are cured and are a good choice for dynamic applications. Epoxy adhesives are not preferred for dynamic operations as they cure hard. In a rigid-flex PCB design, the flexible region can use acrylic adhesive, and the rigid region can use epoxy adhesives.
Stiffeners: Single-sided, double-sided, and multilayer flex circuits can be stiffened in specific areas by adding localized rigid material called PCB stiffeners. This can add mechanical support for mounting components, increasing strength, thickness, and rigidity. Kapton and FR4 materials are commonly used for stiffeners.
Sometimes, aluminum or stainless steel is used. These materials can be attached with thermally cured acrylic adhesive or pressure-sensitive adhesives. Stiffeners can relieve strain, balance weight, and promote heat dissipation. They reinforce solder joints and increase abrasion resistance.
Surface finish types: Many different types of surface finishes provide a solderable surface and prevent copper oxidation. Based on the application, choose what best suits your flex PCB. The board surface is protected using coverlay, covercoat, photo-imaged dry film, and liquid photo-imaged polymer.
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 in the copper plating via holes because acrylic adhesives can become soft when heated. Consequently, incorporate anchors and teardrops when you choose adhesive-based materials.
Adhesive-based materials are prone to absorbing moisture from the environment. Hence, it is suitable to use this type of material in a system exposed to the outside environment.
Adhesive-based materials have many manufacturing disadvantages, such as via cracks, adhesive squeeze-out, and dimensional errors. To avoid these common errors, adhesive-less construction was introduced.
Material Selector
Flex PCB layout design and routing considerations
Consider the placement of preliminary components (SMT or TH) and determine whether those components require stiffeners. Circuitry layout makes or breaks a PCB. Important layout design and routing considerations during flex PCB design include:
Plating considerations in flex PCB design
For more flex design tips, see avoiding the common errors in flex PCB design.
It is important to understand the risks of using vias when designing flex boards. Vias can crack or break in flex designs.
To mitigate this risk:
