What Are Flexible PCB Stiffeners

30 October 2022
New

The biggest advantage of flex PCBs can also be its biggest shortfall - and this is why stiffeners are used. Without a stiffener, flex circuits will not be rigid without a stiffener in areas requiring stiffness.

Nearly all flex circuit designs utilize stiffeners for mechanical support. Stiffeners are also used to ensure that the flex circuit goes through the assembly line without any errors.

Let's take a look at some of the main uses of flex PCB stiffeners.

The Primary Uses of Flex PCB Stiffeners

Stiffeners are used in areas of the board that require a certain degree of rigidity. It should be noted that the stiffener is not part of the electric circuit board design. Let’s take a look at a few reasons why stiffeners may be used on flex circuits:

Provide support to PCB connectors
Provide more thickness to the flex PCB
Improve handling of the flexible circuit board
Keep the board stable in certain areas and flexible in others
Meet the connector specifications for Zero Insertion Force (ZIFE)
Increase the circuit’s bend radius at the area where the rigid and flex part of the circuit meet. This is done to minimize the stress on the flexible part of the circuit.
Stiffener material can be extended into arrays for retaining and routing purposes
Provide mechanical support to areas of the flex circuit that contain PTH or SMH components

Stiffeners may be used in situations when:

Connectors require several insertions that could increase the stress on the circuit
The components are placed in the flex zone of the circuit
The weight of the components in the flex zone is providing too much stress
There is a need to create a rigid surface on the flex zone, especially when placing SMT pads

Which Material is the Flex PCB Stiffener Made Out of?

Choosing the most appropriate material for flexible PCB stiffeners is crucial to maximize its benefits. Stiffener materials used in flex circuits are mostly made out of Rigid Polyimide (PI), FR4, and aluminum stiffener. The materials are selected after considering the circuit’s applications and heat dissipation.

Let’s take a look at the materials.

Rigid Polyimide (PI) Stiffener

Polyimide stiffeners are mostly used to increase the thickness of ZIF connectors. This increases the circuit's tolerance in bending thickness. PI stiffeners are also used to limit the bending ability of certain areas on the PCB. It is common to use stiffeners in hole locations that are often prone to wear resistance.

Polyimide stiffeners are recommended for high bond strength and solder resistance. They are often used on the backside of insertion gold fingers.

FR-4 Stiffener

FR-4 stiffener, also known as G10, is a glass-fiber laminate board that is impregnated with epoxy. They are bonded using a flex adhesive with the help of heat bonding. The typical thickness for FR4 stiffeners is in the 0.008” to 0.059” range. FR4 stiffeners provide flat support to the flex PCB, especially in the reflow process.

They also play a role in providing support to plated through hole components. The stiffeners may be placed on the same side of the circuit where the components are inserted to allow more access to solder pads. Manufacturers may also add FR-4 stiffeners on the opposite side of SMP flex boards as a cost-saving measure.

Aluminum Stiffener

Aluminum stiffeners are a useful alternative to polymer stiffeners because of their excellent heat conductive properties. Manufacturers may add aluminum stiffeners to flex boards to dissipate heat generated from components. This allows the connectors and components to remain relatively cool because the aluminum draws heat away from them like a heat sink.

Aluminum stiffeners are more expensive than polyimide and FR4 materials.

The Best Way of Applying Flex PCB Stiffeners

In the case of plated-through hole (PTH) components, stiffeners should be installed on the same side of the flex boards where the components are inserted. As discussed earlier, this provides direct access to solder pads. Manufacturers can also attach stiffeners on both sides of a part, but the board may require an array configuration review.

It is common to use FR4 stiffeners for assembly arrays. This saves manufacturing costs because it allows the parts to run through an automated assembly. It is common to use heat and pressure bonding to attach stiffeners to a circuit. Designers can also attach PCB stiffeners with the aid of a pressure-sensitive adhesive.

The table below summarizes the differences between heat bonding and pressure-sensitive adhesives.

Heat Bonding	Pressure Sensitive Adhesives
The stiffeners are applied to the flex circuit using heat and pressure.	Stiffeners are attached using only pressure.
Used in military-grade applications.	Used in consumer-grade electronics such as TV, laptops, and smartphones
Takes relatively more time	Takes relatively less time
Will damage the circuit if removed from the flex circuit	Can be removed without damaging the circuit if care is taken (but the risk of damage is still there)

A Few Things to Note About PCB Stiffeners

The thickness of flex PCB stiffeners is an important factor because it can affect the specs of the entire board. Polyimide stiffeners are available in the range of 0.001” to 0.005”. FR4 stiffeners vary from 0.003” to 0.125”. Aluminum and steel stiffeners are thicker at 0.1mm to 0.45mm.

Here are a few things to consider when using flex PCB stiffeners:

An important design tip is for the stiffener and coverlay termination points to overlap a minimum of 0.030” to keep all the stiffeners at the same thickness and reduce the processing time in fabrication.
The stiffener thickness should be the same when using multiple stiffeners

Wrapping Up

To summarize, stiffeners are used in the flex part of the circuit to keep it rigid and ‘less flexible’. This will protect the integrity of the circuit and prevent it from bending.

We hope this blog answered your questions about flex PCB stiffeners and which type of material you should choose when designing a circuit. Feel free to contact Hemeixin PCB here if you have any other questions. You can discuss the specifications of your project for the best results.