Rigid vs Flexible PCBs: How to Choose Your PCB Type

Tense about choosing the perfect circuit board between Rigid vs Flexible PCBs?

This article will be an excellent support for you to distinguish between these PCBs. Let’s make a move.

Rigid and Flexible PCBs

Rigid PCBs, consisting of robust materials placed on a glass-reinforced non-conductive substrate, don’t allow bending or stretching. Here, manufacturers use conductive tracks to connect the PCB components electrically.

Rigid vs Flexible PCB

On the contrary, flexible PCBs are formed via flexible materials. Flex circuit materials are arranged on a non-conductive substrate, but their features differ from rigid circuits.

Rigid vs Flexible PCBs: a head-to-head Comparison

Advantages of Flexible PCBs

• Flexibility of Design

Flex circuits are free from unnecessary components and wires. Besides, FPCs are allowed to bend precisely under a specific limit. So, That’s why designers have huge flexibility while designing these circuits.

• High Compatibility

As flex PCBs offer high stretchability and bendability, manufacturers can use these PCBs in numerous applications, including military, aerospace, gaming, etc.

• Improved weight-to-performance Ratio

Flex circuits have a low weight. That’s why these circuits deliver an improved weight-to-performance ratio in harsh environments or under heavy vibrations.

• Thermal Resistant

Flexible printed circuits offer excellent thermal resistance because of their effective materials that handle high heat superbly. 

Types of Flexible PCBs

• Single-sided Flexible PCB
• Double-sided Flexible PCB
• Multilayer Flex PCB
• Rigid-flex PCB

Flexible Printed Circuits

Notable Applications of Flex Circuits

• Gaming devices
• Laptops
• Computer products
• Military instruments
• Communication equipment
• Medical equipment

Applications of Flex Circuit

Rigid PCB Benefits

• High durability
• Economical, compared to flexible circuits
• Mechanical stability
• Withstand vibration and environmental hazards
• Excellent dielectric properties

Types of Rigid PCBs

• Single-sided Rigid PCB
• Double-sided Rigid PCB
• Single-layer Rigid PCB
• Multilayer Rigid PCB

Multilayer Rigid PCBs

Applications of Rigid PCBs

• Mobile Phones
• GPS
• Tablets
• Automotive devices
• Hand Help Devices
• File serve
• mobile phone repeaters
• Satellite
• Scanning device
• Testing Equipment, X-Ray, Heart Monitor,
• Military and Defense Equipment
• Aerospace

Rigid vs. Flex PCBs- the Most Significant Factors

Unlike rigid PCBs, flex circuits can be bent and stretched to a large extent within the flexibility limit. But do you know why?

Let’s find out.

Materials of Rigid and Flex PCBs

Manufacturers construct both of these circuit boards by using the same process. In the case of a single-layer PCB, you can find a base layer, adhesives, conductive layer, overlay or solder mask, etc. But you can find some differences in terms of materials used in these layers.

Base Material Layer

In the case of flexible PCBs, the base material needs to offer enough bendability and temperature resistance. Usually, you can find Polyimide in most flex circuits. Polyimides are reputable for their excellent flexibility with sufficient robustness.

Flex PCB Construction with Effective Materials

Besides, their thermal control and the way Polyimides handle chemical interactions are pretty impressive. Polyimide comes with successive Imide linkage and is basically a polymer made of Dianhydride and Diamide. These groups ensure flexibility that keeps the circuits compatible with modern stretchable applications like gaming modules, laptops, aerodynamic equipment, etc.

Except for Polyimides, Polyamide flex circuits have also become popular in recent times. Polyamides consist of Diamine and Dicarboxylic acids as monomers which are also effective in bringing out flexibility in PCBs.

On the other hand, rigid circuit boards demand a certain rigidity with adequate dimensional stability. FR4 is the kind of PCB material to fulfill these criteria. This material has a firm reputation for improving circuit board materials’ flammability and mechanical strength.

FR4 material is also usable in flex circuits requiring a sharp stiffness with bendability. But it depends on the type of application and the way you intend to meet your requirements.

If you emphasize the dielectric properties rather than flexibility and thermal drainage, you can remain on the side of FR4. Moreover, FR4 is comparatively less-expensive than flex circuit materials. This property blurs the lack of durability in many cases.

Here, we are going to share a head-to-head comparison chart between these two base materials used in rigid and flexible circuit boards. It will help you have a better understanding of these materials in a quick note.

Adhesive

The application of adhesives in rigid and flexible PCBs is slightly different. As rigid circuits require minimum bendability and stretchability, adhesives are chosen here to improve only chemical resistance and mechanical strength.

Usually, flex circuit adhesives are a product of Epoxy and Acrylics. These materials offer fantastic flexibility, along with creating bondage between the base layer and conductive layers.

Besides, adhesive-filled flex circuits have superb stability, durability, and, most importantly, a cost-to-performance ratio.

Manufacturers rely on these adhesives for flex circuits having a lower layer count. In 10-layer or 14-layer flex circuits, polyimides deposited with adhesives offer low reliability due to a lack of desired mechanical strength.

But these adhesives are not always compatible with flex applications. As flex circuits tend to bend and stretch a little bit, there is a possibility of cracking when rigid adhesives are used in flex circuits.

That’s why you must select adhesives offering certain flexibility, proper thermal management, and moisture resistance.

Conductive material Layer

When you have to pick one between Rigid vs Flexible PCBs, you must consider the conductive copper layer with top priority. Both of these circuit boards are compatible with different types of conductive layers of copper.

Electrodeposited copper brings rigidity whereas Rolled annealed coppers offer flexibility in circuits. That’s why manufacturers prefer RA copper as the conductive layer for flex printed circuits.

But ED (Electrodeposited) coppers are comparatively more robust than rolled annealed coppers. This property makes ED coppers effective for robust applications. You must take a different approach if you want to use ED coppers in flex circuits.

HDED (High Ductility Electrodeposited) coppers have a certain flexibility in maintaining all properties of ED coppers. That means you have an opportunity to ED conductive copper layers in flex applications. Isn’t it pretty good for professional flex PCB makers?

We think you must agree with this.

Flex Circuit Overlay

Have you ever thought about how a flexible circuit gets protected from external hazards?

This is a coverlay (Flex circuit overlay) that acts as a protective shield for flexible circuit materials placed in the outer region of the PCB surface.

Construction of Coverlay

Coverlay is basically a lamination technique during PCB fabrication. Here, a Polyimide film deposited with adhesives creates the actual coverlay film. You can set the film thickness in different ranges. It depends on the circuit design and stack-up of layers.

Flexible PCB Overlay

But the most used coverlay film thickness is 0.0005”-0.005”. Nowadays, manufacturers use 0.001”-0.002” thick overlay films for flex circuit manufacturing. But how can you attach the adhesives and Polyimide materials perfectly?

Applying a certain amount of heat and pressure to laminate these elements is required. Here, flex circuit adhesives smoothly flow through the middle of the pads and PCB traces and fill-up all gaps so that there is no air found trapped between flex circuit layers in the case of multilayer PCB fabrication.

After being laminated with Polyimide films, adhesive gets discharged from the openings. This property is known as “Adhesive Squeeze Out” and indicates the lamination process’s success for the flex PCB overlay.

Importance of Flex Circuit Overlay

Do you remember what the solder mask does for rigid circuit boards? Though coverlay is not as robust as a solder mask, flexible circuit overlay, being flexible in nature,  performs precisely the same as solder masks.

Importance of Overlay in Flexible PCBs

Overlay in flexible printed circuits (FPCs) creates an encapsulation around the external flex components so that these elements remain safe under extreme thermal and mechanical pressures or other environmental pollutants.

Things to Remember about Flex PCB Overlay

• You must eliminate excess copper from the flex circuit board before coverlay lamination. Etching hampers the lamination to a certain degree. That’s why you need to apply this trick.
• Coverlay doesn’t allow you to use the components placed via drilling, routing, or laser cutting.

Coverlay Design in Flexible PCBs

Is There Any Difference between Designing Rigid and Flexible PCBs?

Of course,  You must experience sharp variations while designing flexible and rigid circuits. If you aren’t adequately aware of design guidelines, it will be a mess in the long run.

3 Questions on Designing  Flex Printed Circuits

How to lay the bend radius safely?

• To minimize strain during bending, it is recommended not to bend vertically.
• Try to increase the conductor stumbling effect to improve functionality in multilayer PCBs.
• Never place PTH (Plated Through Holes) on the bend region.

Flexible PCB Design

How to set the maximum bendability of flex circuits?

• Calculate the bend radius that is allowed without disturbing the copper layer. Usually, multilayer flex circuit boards require 24 times the bend radius compared to the flex thickness.
• Check the bending density on the circuit, bending angle, and bending period
• In the case of the static flex circuit board, bending should be done upon installation.
• Dynamic flex circuit boards require constant bending

How to handle the bend area?

• As cutouts decrease materials for the bend, lessen the bend area while there is a lack of traces by incorporating cutouts into the bend region.
• Conduct the decrease of bend area lengthwise.
• Don’t forget to route the flex circuit after lessening the bend area.

Check the Flex Trace Routing

• Choose a larger bend radius for sharp bend angles
• Traces need to be placed at vertical orientation concerning the general bend
• Avoid i-beaming. It increases the thinning stress in flex circuit coppers
• Bend PCB traces are more stress resistant than angular PCB traces.
• Stagger the traces on the top and bottom layers in the case of FPCs with high layer counts.

Essential Considerations for Designing Rigid Circuits

• Layup the rigid PCB materials considering the robustness, dielectric properties, loss tangent, and thermal stability.
• Don’t place the PCB components and vias close to the bending area.
• A high layer count is effective for rigid circuit boards.
• Incorporate plated through holes as much as possible. PTHs cover the PCB pads adequately.

Rigid PCB Design

IPC Standards for Rigid and Flexible PCBs

Final Words

Both Rigid vs Flexible PCBs have extensive use in the electronic world. Before choosing between these circuits, you must focus on your device’s specifications and demands. MV Flex Circuit, based in China, is a trustworthy rigid and flex PCB manufacturer. Contact us immediately if you want to experience premium manufacturing and assembly services.