FPC with stiffeners is taking over traditional PCBs regarding electronic development.
Surprisingly, you can bend and fold them to fit in tight spaces and compact devices.
Generally, a stiffener helps to improve the integrity, durability, and reliability of flexible circuits while simultaneously adding rigidity to certain areas.
Consequently, your final electronic device will experience better performance.
But what is an FPC stiffener? What are its uses, and how do you apply it?
This article discusses everything relating to FPC stiffeners to help you understand better.
Table of Contents
- What Is A Stiffener?
- FPC Stiffener Uses
- When Does A PCB Need Stiffeners?
- FPC Stiffener Materials
- Applying Stiffeners on PCBs
- Tips for Stiffener Selection Consideration
- Differences Between Rigidized Flex and Rigid-flex PCBs
- FAQ
- Conclusion
What Is A Stiffener?
Generally, a flexible circuit board is designed with highly flexible and soft materials.
Therefore, there is a need to add some materials to reinforce and intensify PCB areas so that it becomes easier to solder components.
These reinforcing materials are what we call stiffeners.
Surprisingly, stiffeners aren’t part of the circuit board’s electrical circuitry.
They offer mechanical support during the assembly process. Moreover, they enhance the PCB’s abrasive resistance, reinforcing the soldered joints.
FPC Stiffener Uses
(A flex PCB copper layer)
The main function of an FPC stiffener is to harden certain flexible circuit board parts.
As we highlighted, it’s not a critical component of the circuit board’s electrical circuitry.
Check out the uses of FPC stiffeners below:
- Firstly, the stiffeners support certain circuit board areas with plated-through holes and surface mount technology components.
- Secondly, they maintain the required thickness in electrical circuit boards.
- Thirdly, they support circuit board connects and components.
- Fourthly, they restrict the flexible circuit board areas to the hardened sections of the flexible board.
- Also, they enable you to handle and treat your circuit board better during the automated reflow processes.
- Moreover, the stiffeners will ensure the flexible parts of your circuit board remain flat and stable.
- They ensure your flexible circuit board meets the zero insertion force connector specifications.
- Furthermore, the flexible circuit board’s bending radius increases at the flexible and stiff section’s intersecting point. Consequently, your PCB won’t experience any stress during multiple twists and bends.
- Finally, it maintains and routes the flexible circuit board’s array.
When Does A PCB Need Stiffeners?
(A PCB with some components)
As highlighted, stiffeners find application in different designs to enhance the durability and reliability of flexible circuit boards.
Therefore, it’s required in the following situations:
- When you mount the component on the circuit board’s flexible section.
- When the mass or weight of the components you mount on the flexible section stresses the board’s flexible materials.
- When you need to make a flat or hard surface on your flexible circuit board to mount SMT pad components.
- When your connections require multiple insertions to minimize pad stress.
- When you want to amount PTH components, in this case, you should put the stiffer on the circuit board’s side where you place the PTH components. This way, you can access the solder pads directly.
FPC Stiffener Materials
There are three stiffener types according to the materials we use. Check them out below:
Polyamide Stiffener
These stiffeners find common applications in increasing ZIF connector thickness.
Additionally, they’re common in offering bending thickness tolerance having a bend thickness tolerance and width profile of +/- 0.002″ and +/- 0.003″, respectively.
You can use these stiffeners to limit your circuit board’s bending ability in preparation for installation in your final assembly.
Generally, you should enhance the hole locations to create more wear resistance when wear challenges arise.
Polyamide stiffeners offer the best option for high bond strength and solder resistance.
Therefore, we recommend applying them to insert the gold fingers’ backside.
FR-4 Stiffener
Without question, FR-4 is the most common stiffener material you’ll find in flexible circuits. The main reasons people use it include the following:
- It offers flat support when the flexible circuit board is in the reflow and pick-and-place processes.
- FR-4 is critical whenever you want to mount PTH (plated-through hole) components. However, in this case, ensure the stiffeners are on the same side as the flex circuit board that you insert the components to provide direct solder pad access.
With some flexible PCB designs, the FR-4 stiffener’s hole diameter is the same as that of FPC. Consequently, the holes become difficult to attach, resulting in dis-alignment.
Therefore, ensure the holes on your FR-4 stiffener are 0.3mm bigger than related holes on your FPC to guarantee proper alignment.
Aluminum/Stainless Steel Stiffener
These two stiffeners are more expensive and durable than the other options.
These stiffeners find applications in design that require robustness, formability, and anti-corrosion resistance.
More specifically, we use aluminum stiffener for heat dissipation.
Moreover, we use stainless steel in situations with limited stiffener space, and the design needs more component support area than an equivalent FR-4 thickness could offer.
However, you’ll have to drill, mill, register, and pre-stack the prepreg with the aluminum stiffener bonding process.
For the stainless steel option, you need drilling, registration, laser cut, and pre-stack of prepreg.
Applying Stiffeners on PCBs
(An optical sensor on PCB)
Ensure you apply circuit board stiffeners with plated-through hole components.
Also, the stiffener should be on the same side of the board where you insert your components.
Later, you’ll do some configurations to ensure the circuit board doesn’t run into complications.
Generally, we have two approaches you can use to apply stiffeners to your circuit board. Let’s explore them below:
Pressure-Sensitive Adhesive
This approach involves attaching stiffeners to your circuit board using pressure alone.
Surprisingly, it is the method we use for bonding IPC in class 2 products like laptops, televisions, and consumer electronics.
The process is more affordable and will take less time to accomplish.
However, the bond is not very strong. Therefore, you can easily remove stiffeners without damaging your circuit board.
Thermal Bonding
This method involves applying stiffeners to your circuit board using pressure and heat. You’ll usually use it to bond in IPC class 3 products like avionics and military applications.
Unlike the pressure-sensitive adhesive option, this method takes more time and is more expensive.
However, it offers stronger bonds than the pressure-sensitive adhesive approach.
Therefore, removing stiffeners from your circuit board will be difficult.
Tips for Stiffener Selection Consideration
(An orange flex PCB)
Keep in mind the following while selecting your flex circuit board stiffener:
- First, the stiffener material should be compatible with the PCB material. This way, they’ll bond well and cause no problems during use or assembly.
- Secondly, the stiffener’s thickness should be perfect for your intended application. Surprisingly, a thick stiffener can make the PCB too rigid, while a thinner one won’t offer reliable support.
- Finally, the adhesive you use for attaching the stiffener to your circuit board should be durable and strong. Moreover, it should be easy to remove should need to.
Differences Between Rigidized Flex and Rigid-flex PCBs
The main difference is that a rigidized is designed to offer mechanical support while a rigid-flex offers circuitry and electrical connections.
A rigidized flex PCB has stiffeners that strengthen different circuit board areas to ease the assembly process.
Moreover, the rigidized flex’s rigid party and pads don’t have traces.
Therefore, its purpose is to strengthen the board and has nothing to do with functionality.
However, the rigid and flexible parts in a rigid-flex circuit board are all designed with traces.
Manufacturers connect these traces via holes and offer electrical connections for the circuit board.
FAQ
Do stiffeners improve circuit performance?
Yes! Stiffeners improve circuit performance by offering mechanical support and strength to reinforce and intensify PCB areas. Without a stiffener, you’ll find it difficult to solder components.
Is it easy to apply an FPC stiffener?
Yes! Applying an FPC stiffener is easier, especially if you have the required skills. Surprisingly, applying stiffeners is simpler than designing a PCB from scratch.
Conclusion
As we have highlighted, a stiffener is simply a material that you add to your circuit board to reinforce and intensify different areas so that you can easily solder components.
Surprisingly, stiffeners aren’t part of the circuit board’s electrical circuitry.
Their main purpose is to offer mechanical support during use and assembly.
To get the best results, ensure the stiffener material is compatible with those on the circuit board.
Also, the adhesive should be durable, strong, and easy to remove. Finally, the stiffener thickness should support your intended application.
