What is Stiffener in a Flex PCB?
A flex PCB is electrically and mechanically very different from a rigid PCB. There are cases where a flex PCB is imperative, and in some designs, a rigid PCB will do just fine. For more advanced uses, there are rigid-flex PCBs that combine the traits of both of these PCB types.
But there is another configuration. It exists between a pure flex and a rigid-flex circuit, and we call it a rigidized-flex, which is possible, thanks to stiffeners.
So what are stiffeners, and why are they important for Flex circuits?
Stiffeners and Flex PCB
A flex PCB is, as the name suggests, a flexible PCB. It can be bent from any place (though it's not recommended as it may cause fractures in the copper or dislocate a component's connection). While this is a fantastic feature in a PCB, it has a few drawbacks. The biggest one is that a flex circuit is sometimes too thin or mechanically unstable to anchor components properly.
This is where a stiffener comes in.
A stiffener makes an area of the flex circuit hard (or 'rigidizing' a flex in a given area). This has a lot of benefits.
Uses and Benefits of Stiffener in Flex PCB
One of the most common applications of the stiffeners in a flex PCB is to create a hard area in the flex circuit where components can be mounted with relative ease. The stiffened (rigidized) part of a flex PCB mimics the mechanical characteristics of a rigid PCB. In that part, components and connectors are better anchored, compared to the rest of the flex.
Overall, this construction is very similar to that of a Rigid-Flex circuit, but there are characteristic differences between them.
Another use of a stiffener in a flex circuit is to establish bend constraints. This can be useful for two primary reasons. One is to prevent unintentional bending in certain areas of the flex circuits. This may be due to the component or via placement there, or any design implication. The second reason is to prevent fractures in the conductor layers that may appear as a result of sharp bends.
Another popular feature of the Flex circuits nowadays is the ZIF connector, which is created using stiffener in a Flex PCB. A ZIF connector eliminates the need for a mating connector on the flex, reducing weight and saving space.
Some other benefits of specific types of stiffeners are better heat dissipation, maintaining and controlling the thickness of the flex, and cost factor. In some simple cases, a rigidized flex can replace a costlier rigid-flex.
Types of FLEX PCB Stiffeners
A variety of stiffeners are available, each with its own application and price consideration.
FR-4 Stiffeners
One of the most common types of stiffener used with a flex circuit is FR-4. The material is constructed with a woven glass-fiber epoxy that laminates the pre-preg epoxy within the construction. It’s mostly used to provide mechanical support to the components and to control the bending area of a flex circuit.
It can be bonded to the flex using either a pressure-sensitive adhesive or thermally (using a flex adhesive). Commonly, the stiffener thickness lies somewhere between 8 and 59 mil. The width of the adhesive can also be as low as eight mil.
Polyimide Stiffeners
Polyimide stiffeners are a relatively high-end stiffener type. The stiffener film can be as thin as one mil. It usually doesn’t get thicker than nine mils, and the most commonly used size is five mil. They are the most diverse class of stiffeners and can be used for any stiffener requirement.
Polyimide stiffeners are exclusively used to ZIF connections. Their thin form helps achieve the precise thickness needed to make the right sized ZIF connection. For this type of construction, the stiffener is always thermally bonded.
Stainless Steel Stiffener
Stainless steel is a relatively more costly and less flexible stiffener material. It is used where a higher degree of rigidity and superior mechanical strength is required. A stainless steel stiffener layer of a certain thickness can provide more mechanical stability and rigidity than an FR-4 or polyimide layer of the same size.
Aluminum Stiffeners
Aluminum stiffeners are especially suited for high-heat generating flex circuits. Aluminum stiffener acts as a heat sink that collects and dissipates the heat from components and a conductive layer of the flex better than its native construction materials could. It’s also very costly, compared to FR-4 and polyimide, and provides better rigidity like stainless steel.
Few Design Considerations
There are a few design considerations that should be observed while using a stiffener with a Flex PCB.
- The stiffened (rigidized) part of the flex is a purely mechanical construction. It has no traces, and it’s not electrically connected to the flex part of the circuit. This is why stiffeners are usually only applied to one side of the flex (opposite of the component assembly).
- Adding stiffener on both sides of flex can significantly increase the complexity and cost of a flex circuit.
- Adding stiffener on the same side of the SMT component might not be possible, and it would be more useful to design the same PCB in rigid-flex.
- Stiffeners are also added to increase the thickness of the flex in certain areas, to better conform to the mechanical limitations and constraints of the device’s design.
Conclusion
Designing a flex with stiffener is usually seen as a relatively cost-effective alternative to developing the same circuit in rigid-flex. But it’s imperative to understand the abilities and limitations of stiffeners. An excellent way to look at a stiffener is that it allows you to modify your flex circuit from a mechanical perspective. It certainly has its uses, but it’s not a true alternative to a wholly different and more diverse-in-nature rigid-flex.
When you are working with us, you not only have access to some of the best-in-the-market technologies and processes but also to our experienced team. With our level of exposure, we can better guide you about your design’s limitations and cost implications. Its efficient use can make your circuit both effective and relatively cheaper. Consequently, dropping the ball when it comes to stiffener can be a very costly mistake.
It’s a powerful, and in some cases, unavoidable tool in designing with flex PCB. Therefore, you should always keep the design guidelines in mind. This way, you will know how far you can push your design, and you won't be designing something that is needlessly complex and unnecessarily costly.
