A Guide to PCB Stiffeners for Flex and Rigid-Flex
“The greatest achievement of humanity is not its works of art, science, or technology, but the recognition of its own dysfunction.”–Eckhart Tolle. Wondering why such a quote to start the article? Well, that’s because the topic of PCB stiffeners we will discuss is not a technology, but a result of human experience.
If we look into the history of PCBs, we will notice a gradual transformation of them according to market demands. But that’s not something new, right? What’s new are our market demands. While we thrive to indulge ourselves from better to best in terms of technology, we face a peculiar demand. A demand that rather than making outer cases based on PCBs, we need to build circuit boards according to case designs. Tough but we made ourselves capable enough to fulfil the demand.
One of the popular printed circuit boards now is flexible circuit boards. Flex PCBs allow you to outfit product circuit when the design of those products won’t admit a rigid circuit board. Therefore, they’re invaluable for many electronics applications.
Flex PCBs then have their own demons; they may be too flexible!
They may not be rigid in places that need stiffness. This is where human experience came into action. We created PCB stiffeners for these situations. You might not be familiar with this name or you have just heard it. You are not sure of what they are and their area of application.
What Are PCB Stiffeners?
First, it is important for you to understand that the PCB stiffener is not an integrated part of the electrical circuit board design. It exists just to offer mechanical support. We call a stiffener for when you need it. Here are some enlisted reasons to call for stiffeners:
- To strengthen any specific area of the board.
- To maintain proper thickness in the flex circuit.
- To support PCB components and connectors.
- To constrain the flexible parts to areas where they should be.
- To facilitate better handling of a thin flimsy circuit board.
- To keep certain areas of flexible circuits flat and stable.
- To meet ZIF connector specifications.
- To increase the bend radius of the circuit at the intersection of rigid and flex part. This will avoid the stress on flex part during multiple bending operations.
Basically, you use a stiffener when you require a rigid area in your flex circuit, maybe to protect components or connectors attached there. This will not let the circuit bend and protect the integrity of the part’s solder joint.
Why Do Flexible PCBs Need Stiffeners?
When routing and retaining arrays, the same material as the stiffener will simply be extended into the array. This rigidizes the array.
Stiffeners are required when:
- Components are near or in an active, or dynamic, flex zone
- Component size and weight will import stress onto the flex
- A large number of SMT pads make planarity critical
- Connectors that require repeated insertions or assemblies require a stiffener, or something to help alleviate the stress of pads.
Stiffeners are not required when:
- Small “static” components do not exert a large amount of strain on the flex of the PCB.
- There are no components in the flex region.
The PCB Stiffener Material
While talking of using something, the first thing to consider is the material that makes them out of. We usually make PCB stiffeners, like much of the flex PCBs of FR4. FR4 stiffeners may also have copper features, e.g. pads or plated holes used for component mounting, etc. Aluminum and polyimide are some other materials used.
The thickness of the stiffener can range from 0.008” to 0.059”. The most common thicknesses used are 0.020”, 0.031”, 0.039” or 0.059”. The thicker the PCB stiffener is, the more support it will offer. But every design seeks a different thickness. If you want your circuit board to be thicker, you will use polyimide stiffeners. You will put them at the contact fingers, as specified by the ZIF connector. It should be into that connector that the flex circuit plugs into. Some common thickness requirements are 0.3mm or 0.2mm. The stiffener thickness can vary between 0.002” and 0.008” depending upon the flex circuit material construction. If you use ZIF stiffeners to thicken your board, your board might end up being a lot harder than required.
In certain situations, you may need to use other materials, such as stainless steel or aluminum, for your PCB stiffener. Rightly, you thought these materials are more expensive. But you get some better rigidity and heat sinking properties. While there will be specific situations where you can put these expensive materials to your advantage. But most manufacturers feel the benefits don’t really merit the added cost with flex circuit boards.
How To Use PCB Stiffeners?
When we use a stiffener for plated-through hole (PTH) components, the stiffener(s) should be on the same side of the flex from which we insert the component. This allows access to solder pads on the flex circuit. You can also attach stiffeners to both sides of a part. But then the PCB assembly will require an array configuration review. Just to make sure you don’t run into anything complicating during the assembly process. When we require assembly array for flex circuit designs, it is common and cost-effective to include the FR4 stiffener throughout the array border. This concludes into a rigid array. This, in most cases, allows the parts to run through automated assembly in the same manner as a rigid circuit board design. This also eliminates the need for any additional tooling plates, etc.
Typically, you will use heat and pressure to attach the stiffener to the circuit. So basically, through thermal bonding. You can also attach PCB stiffeners with a pressure-sensitive adhesive. We have a wide variety of adhesives. The two popular types of adhesives generically fall under pressure-sensitive adhesives (PSA) and thermally bonded adhesives. We use these varieties to meet specific performance requirements. They are thermal exposure, chemical resistance and adhesion to various materials. The type of adhesive depends upon the location and or configuration of the stiffener(s). For example, if a stiffener does not extend out to the circuit outline it will, in most cases, require a PSA attachment and an additional stiffener outline added to the silkscreen to allow for precise placement.
We will now differentiate rigid-flex PCBs and rigid PCBs based on the FR4 stiffener.
- Rigidized flex is a flex PCB, but it’s bonded with FR4 stiffener to strengthen the rigidity during assembly. So, by nature, the circuit board with a stiffener is still a flex circuit. Rigid-flex circuits are known as hybrid flex circuit comprising rigid and flexible substrates. We laminate them into a single structure. From the definition, you can see that the rigid-flex circuit does not belong to the flex circuit.
- There are no traces on a rigid part of rigidized flex, even on pads. This rigid part only strengthens the rigidity of this location. That is to say, it’s just a mechanical connection between these two. However, for the rigid-flex PCB, both the rigid and the flex parts are designed with traces and we connect them by via holes. That is to say, it’s not a mechanical connection but an electrical connection for the rigid-flex PCB.
- As we discussed in our last point, if we talk about their connection, that is also different. Rigid-flex PCBs have electrical connections whereas rigidized PCBs are for mechanical support.
“Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution.” — Albert Einstein
What we basically want to convey is we can guide you, train you on certain topics, but what this world requires is your imagination. Whatever we discussed in this article that may be technical but is used out of human instinct. Take this guide to PCB stiffeners and make your PCBs stronger than ever.