A Guide to Flex Circuit Stiffeners for Flex PCB

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Flexible printed circuit boards or Flex PCBs are some of the most popular printed circuit boards available in the market. They come in handy when working with rigid boards becomes difficult.

Types of PCB Stiffeners


It is vital to use the most appropriate material for PCB stiffeners because of the multiple benefits they offer during application. Following stiffener materials are most commonly used on projects requiring increased stiffness and heat dissipation:

Polyimide (PI) Stiffener
FR4 Stiffener
Aluminum/Stainless Steel Stiffener
Although they offer better heat sinking properties and rigidity, the aluminum/stainless steel stiffeners are an uncommon choice due to their very high prices. The PI stiffeners, however, are a cheaper alternative to the FR4 stiffeners.

1. Polyimide (PI) Stiffener
When formed into a thin film layer at high temperatures, a polymer is called polyimide. Multiple polyimide layers can be stacked on top of each other to increase the thickness. PI stiffeners are one of the most commonly used stiffeners for solder resistance and high bond strength. Using them allows the manufacturers to build tolerances in bending thickness and contour the component at the contact point.

They limit the bending ability of required PCB areas for installation in the final assembly process. The areas they’re used to target are the holes or the backside of the insertion gold fingers, which provide additional resistance for the wear issues.

A classic example of polyimide used in PCB stiffeners is Kapton. Kapton is the go-to polyimide when the flex ends need thickening and insertion into a ZIF connector.

2. FR-4 Stiffener
FR4 is a woven glass-fiber laminate suffused with epoxy resin. It is the most common material used for the flex circuits in PCB stiffeners. FR4 stiffeners are adopted because they provide flat support for the flexible PCB during the pick-and-place and reflow processes.

To save money while project assembly and to allow direct access for Plated-through hole (PTH) components to solder pads, an FR-4 frame is added on the border of the PCB to replace the SMT carrier.

3. Stainless Steel or Aluminum Stiffeners
Stainless steel or aluminum stiffeners are high-end quality stiffeners that are used for flexible circuit designs that require formability, anti-corrosion resistance, and robustness. The stainless steel/aluminum stiffener is added in flexible circuits to facilitate assembly by stiffening the flexible printed board.

What Is a Stiffener in Flex PCB?


A stiffener is a piece of the electrical circuit board design used to provide mechanical support to the PCB during assembly.

Flex PCBs on their own are too flexible. They require stiffness in some places to strengthen the attached components or connectors. This is where the stiffeners come in handy. They stop the circuit from bending and losing the integrity of that part’s solder joint.

Why Do We Use Stiffeners on Flex PCBs?


As we’ve established that PCB stiffeners are needed to harden the required parts of the flex circuit. Stiffeners are not part of the electrical requirements of the PCB; they are only there to provide a mechanical function for the flexible area which means that they let the board stay upright when using the solder joint to attach components to it.

Even though using the stiffeners might increase the cost and lead times due to undergoing multiple lamination cycles, they are a must in the following cases:

When the weight of components and connectors placed in flex zones starts bending the thin flex material of the circuit board.
When SMP pads components require a flat, rigid surface on the PCB.
When the connectors requiring multiple insertions need a stiffener for reducing the stress of the pads.
When you need to keep up the appropriate thickness in the flex circuit.
When you need to decrease the stress on the flex during multiple bending operations for enlarging the bend radius of the circuit between the rigid and flex parts.
When you need to comply with the Zero Insertion Force (ZIF) thickness specifications.
When you need to route and retain arrays, extending the stiffener material into the array.


How Thick is a Typical Circuit Stiffener?


The thickness varies between different designs. The typical stiffener thickness ranges between 0.002″ to 0.059″. The range differs between the kind of stiffeners being used. Kapton stiffeners range from 0.002” to 0.010” in thickness, while the rigid stiffeners range from 0.008” to 0.059”. This thickness directly affects the support it offers.

How to Apply the Flex Circuit Stiffeners?


The correct way of applying the stiffener on the board is to have it on the same side of the PCB from which the plated through-hole (PTH) components are inserted. This lets you easily access the solder pads on the flex circuit while easily feeding the component leads through the holes.

Another point to consider while stiffening the PCB is that the adhesive system and the registration process should be present in enough quantity to fulfill your set objective.

You can use an array configuration review to attach the stiffeners to both sides of the PCB. We add the FR4 stiffener throughout the array border for cost-effectiveness when the flex circuit designs require an assembly array.

Additional Details about PCB Stiffeners


Stiffeners are one of the last fabrication steps and are applied using either heat or pressure. Depending upon the performance requirements, such as adhesion to various materials, thermal exposure, and chemical resistance, the adhesives can be selected from either of these methods:

Pressure Sensitive Adhesives
Thermally Bonded Adhesives
Pressure Sensitive Adhesives Vs. Thermally Bonded Adhesives


More often than not, the pressure-sensitive adhesives are more cost-effective than the thermally-bonded ones. The traditional PCB prepreg (fiberglass strengthened by an adhesive layer) or thermal set on-flow adhesives require the flex PCB to go through another lamination press cycle to apply the heat and pressure to cure the adhesive. The adhesive must also be cut in advance to match the stiffener’s shape.

Conclusion
When designing the flex circuit PCBs, stiffeners are a dependable and often a necessary option to improve the durability and reliability of the flex circuit while assembling various flex circuit designs.

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