A Complete Guide To Flexible Printed Circuit

Flexible printed circuit boards are PCBs with a flexible substrate material and a few copper layers to allow for bending.

This flexing brings forth several benefits, such as efficient space use and shock/vibration resistance, which are critical in several applications.

Let’s look at various aspects of this PCB, including its common construction materials, manufacturing process, benefits, and more!

What is a Flexible Printed Circuit Board
Types of Flexible Circuit Boards
What Are the Benefits of Flex Circuit Boards
What are the Disadvantages of Flexible PCBs
What are Common Flexible PCB Materials

Why Use PCB Stiffeners on Flex and Rigid-Flex PCB Boards
Are There Any Considerations While Designing Flexible Printed Boards
How to Manufacture Flexible Circuit Board: Step-by-Step Manufacturing Process
How to Choose the Right Flexible Circuit Board Manufacturer
FAQs
- Are flexible PCBs reliable?
- Why are Flexible PCB circuits expensive?
Conclusion

What is a Flexible Printed Circuit Board

As stated earlier, a flexible PCB is a circuit board with a bendable base material that is commonly made up of polyimide.

The materials that make up the other layers are also flexible and can withstand bending without breaking or cracking.

You can bend, fold, or twist flex PCBs to fit in tight spaces, which is important when you want the circuit to conform to the shape of the final product’s package, such as a wearable.

Another key advantage of Flex PCB is its resistance to vibrations, which makes it suitable for automotive, aerospace, and aviation applications.

Types of Flexible Circuit Boards

Here are the types of flexible PCBs:

Single-Sided Flex PCBs

They are the most common flex PCBs that you can find. They have one conductor layer with a flexible dielectric film.

Also printed on one side, making them cheaper to produce.

Double-Sided Flex PCBs

They have prints on both sides of the dielectric material. Consequently, they pack more components and perform better power-handling than other boards.

On the downside, they are costlier to manufacture.

Multi-Layer Flex PCBs

They have more than two layers of conductors, making them suitable for military and aerospace applications.

Moreover, they pack dense circuitry and are costlier to manufacture.

Rigid-Flex PCBs

From the name, they are a blend of rigid and flex circuit boards. More specifically, they are rigid circuit boards with flexible connection substrates.

Due to their construction, you’ll find them mostly in smart devices and military applications.

The HDI Flex PCBs

HDI is an acronym for High-Density Interconnect.

HDI flex PCBs have multiple microvias and fine structures that make it possible to have a higher wiring density per unit area than traditional circuit boards.

The high wiring density also increases the board’s functionality because you can mount more components on it.

Another defining feature of HDI flex PCBs is they feature thinner substrates than regular flexible circuit boards, which reduces their size and increases their electrical performance.

What Are the Benefits of Flex Circuit Boards

Compact and Lightweight Package: Flexible PCBs have thinner substrates, copper layers, and other materials than their rigid counterparts. Thin materials mean lightweight boards, as well, which results in a smaller package size and weight for the final product.
Flexibility: Flex boards can connect multiple planes during installation and can be flexed several times without experiencing failure.
High Reliability: Interconnection points are common when dealing with rigid boards and are prone to failure. Flexible PCBs reduce interconnection points in circuits, which increases the reliability of the product. Additionally, FPC boards can withstand shock and vibrations, making in-car electronics and other products highly reliable.

Allows High-Density Configurations: Since they allow extremely narrow spaces and lines, flexible PCBs free space for high-density device populations to allow for additional product features.
Design Freedom: Flexible PCB design is not limited to two layers. The boards can have multiple layers, some even combining rigid circuits in various sections. Therefore, these PCBs can have complex configurations to handle sophisticated circuitry.
Improved Airflow: The streamlined designs of these PCBs allow cool air to flow through the product easily and dissipate heat faster.

What are the Disadvantages of Flexible PCBs

High Manufacturing Costs: The materials used to make flexible boards are costlier than the ones used to make rigid PCBs. Additionally, the manufacturing process is more complex, and there is a high probability of damage during handling.
Difficult to Rework: Reworking flexible boards (repairs or modifications) is tricky because you have to remove the protective film, fix the issue, and then restore this film.
Limited Component Densities: The thin and flexible nature of these boards limits the number and types of components you can mount onboard. High densities or large, heavy components can weigh down the substrate or even cause it to snap.

Multiple Design Complexities: Some design rules, like zero vias at bend areas, staggering conductors on multilayer boards, using specific cover lays, etc., make flexible PCB design complex.
Susceptibility to Damaging Environmental Conditions: Flexible boards are more susceptible to chemicals, heat, moisture, and other environmental factors than rigid boards. Therefore, they might not be the best option for some applications.

What are Common Flexible PCB Materials

Conductors: The most common conductor material used in PCBs is copper, but the one used in flexible boards must be thinner. So, it is either rolled annealed copper or electrodeposited copper. Other materials that can be used as conductors include aluminum, Inconel, silver ink, constantan, and cupro-nickel.

Adhesives: These materials bind the layers together, and they include epoxy, acrylic, or PSAs (Pressure-Sensitive Adhesives).
Insulators: Insulators separate the conductive layers and they include polyimide, polyester, PEN, PET, PEEK, or LCP. Solder mask, an epoxy liquid, is also an insulator.
Finishes: Surface finishes on the exposed copper areas can be ENIG, ENEPIG, Tin, OSP, hard gold, or hard nickel. There are several other options, but ENIG is the most popular.

Why Use PCB Stiffeners on Flex and Rigid-Flex PCB Boards

Stiffeners provide mechanical strength to the PCB, which improves durability and reliability. PCB stiffeners can be added to any side of the board, and their requirements fall under these usage categories.

Rigidizing connector sections to improve stress relief during repeated insertions or when carrying large, heavy connectors
To meet the ZIF (Zero Insertion Force) thickness requirements

Creating a flat surface for placing SMT pads and components
To help with localized bend constraints
Reducing component stress

To enhance heat dissipation (metal stiffeners)
Reducing the chances of failure during automated assembly

Are There Any Considerations While Designing Flexible Printed Boards

Before going into the manufacturing process, the flexible PCB must be designed properly, and it is important to keep these design considerations in mind.

Operating Environment: Consider the board’s final operating environment, which will determine if it needs protective layers for chemicals, high temperatures, or humidity.
Bend Ratio: This term refers to the relationship between the bend radius and board thickness. The ratio is important to consider because boards with a different number of layers have different bend ratios, and the tighter the bend radius, the higher the probability of failure when flexing.
Conductors and Routing: The copper traces and their transmission paths should be analyzed carefully to determine if they will be affected when flexing. Conductors should be routed perpendicularly to the bend areas to avoid snapping.

Pad Filets: These additions are necessary when the pad diameter exceeds the connecting strand width because they improve the etched yield and material strength.
Tear Relief: Consider a large corner radius, stiffeners, and relief slots to avoid PCB tearing.
Vias: Blind and buried vias should only be used when necessary for multilayer PCBs because they increase the fabrication cost significantly.

Plane Layers and Shielding: Ground or reference plane layers are critical for shielding, impedance control, and signal integrity. However, these solid copper layers make the board more rigid, so they should be included in the bend ratio calculation.
Signal Integrity and Controlled Impedance: Factors like the dielectric constant of the insulation materials, trace width, and signal trace distance from the reference plane determine the signal integrity and impedance.

How to Manufacture Flexible Circuit Board: Step-by-Step Manufacturing Process

This process is nuanced depending on whether the flexible board has one or multiple (2 or more) layers, but it generally follows the steps below after material selection and copper cladding.

Copper-Clad Lamination Cutting

The flexible PCB begins as a roll of a copper-clad laminate that is cut to a semi-finished copper foil sheet of the required processing size.

Drilling

Depending on the design files, holes are drilled through the laminate with specific diameters using mechanical or laser drilling. The latter is preferred because it does not exert pressure on the PCB. These holes are then cleaned and plated.

Dry Film Lamination

Dry films are photosensitive resists that help with the circuit image transfer to the copper layer. This photoresist material is applied using a heated roller with the appropriate pressure to melt and form it evenly above the copper.

Pattern Plating

Pattern plating can be done using UV light (photolithography) or LDI.

Etching

This process requires a corrosive chemical solution to remove copper from the uncured dry film areas. Pattern plating creates a hardened positive film on the photoresist material to form the circuit. Etching corrodes the other sections to leave the circuit.

Dry Film Stripping and Inspection

The hardened positive dry film is removed to leave the exposed copper circuit, which undergoes inspection for short or open circuits using AOI.

Coverlay Lamination

An overlay is a film applied to the PCB to protect it from oxidation and mechanical damages, such as scratching. After application, the board undergoes lamination using heat and pressure using specific heat and pressure parameters to avoid damage.

Surface Finishing

Surface finishing is important over the exposed copper areas to protect them from oxidation. It also simplifies soldering.

Silkscreen Printing

Silkscreen indicates the relevant information on the surface (test points, warning symbols, logos, etc.), and it is printed with the solder mask for protection.

Electrical Testing

Electrical testing requires techniques like flying probes or a bed of nails to check for shorts or open circuits in the PCB.

Flying probe testing on a PCB

Punching

Punching refers to cutting the sheet into individual PCBs that match the customer’s requirements (design files).

Inspection

Visual inspection is necessary to ensure the flexible board has no scratches or contaminations.

Packaging and Shipping

The complete PCBs are packaged according to the customer’s needs before shipping. If the customer requires assembly, the boards head to these plants to form PCBAs before shipping.

How to Choose the Right Flexible Circuit Board Manufacturer

Flexible PCBs are delicate electronic boards that need proper design considerations and handling when manufacturing them. Therefore, you should consider the three E’s when picking your flexible board assembly manufacturer. These are expertise, equipment, and experience.

At FlexiPCB, we have an in-house team of engineers who use our software and state-of-the-art equipment to help you with the following.

Concept development
Design optimization

Material selection
PCB stackup design
DFM analysis

Signal integrity analysis
Power integrity analysis
Thermal analysis

Mechanical analysis

Since these boards are delicate to handle, we can handle the assembly process for you to eliminate any complexities that may arise on your end.

Our turnkey PCB assembly solution also covers material sourcing and procurement, quality control, testing, packaging, shipping, after-sales support, and maintenance, and we recommend this service because it lets us do all the risky and difficult work for you at reasonable costs.

FAQs

Are flexible PCBs reliable?

Generally, most flexible printed circuit boards are reliable and durable. This is because manufacturers make these PCBs using flexible substrate materials like polyimide.

Afterward, they use them in devices that need to be flexible.

For example, we use flexible PCBs in wearable devices like watches.

On the downside, the reliability of your flexible PCB will depend on the quality of its construction. Poorly made ones won’t be as reliable as high-quality PCBs.

Why are Flexible PCB circuits expensive?

Some flexible PCBs are costlier than rigid PCBs for several reasons. First, they are more complex to design and manufacture.

Consequently, manufacturers allocate more resources to manufacture a flexible PCB than they would with traditional circuit boards.

Second, flexible PCB substrates are more expensive than the materials used to make rigid PCBs.

For instance, polyimide substrates cost more than rigid materials manufacturers use in traditional PCBs.

Third, the flexible nature of the PCBs makes the manufacturing process intensive and time-consuming.

Overall, it reduces the number of boards a manufacturer can make in a given time.

As a result, manufacturing costs increase, translating to higher selling prices.

Conclusion

In conclusion, flexible PCBs require careful planning, consideration, and care when designing, manufacturing, and assembling them to achieve their intended benefits.

These benefits are critical for current and future generations of electronics, where compact, lightweight, and reliable circuits will be required in various applications.

With a reliable manufacturing partner like us, we can assure you of durable and high-quality flex circuit boards because our commitment to excellence and continuous improvement has made us remain at the forefront of the industry.