Flexible PCB Coverlay: All You Need To Know

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Oakley Mae

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You’re likely here because you want to understand a flexible PCB overlay. Without a doubt, you have come to the right place. 

Generally, a flexible PCB overlay is just a cover we use to protect and encapsulate the external circuit board circuitry.

Therefore, it’ll serve the same function as a solder mask that we employ in rigid printed circuit boards.

However, a flexible PCB overlay offers more durability and flexibility than a solder mask. 

For more on flexible PCB overlay, keep reading. 

Table of Contents

Introducing Flexible PCBs

Two flexible PCB

(Two flexible PCB)

Flexible PCBs, otherwise known as flex PCB or flexible printed circuits, represent improved circuit boards that can twist, bend, and fold.

Surprisingly, most circuit boards we are used to are traditional circuit boards that have a solid substrate.

However, improved technology has given rise to flexible PCBs that you can easily bend to fit your desired shape. 

Generally, flexible circuit boards are lightweight and occupy less space. Therefore, you can use them in tight spaces to create compact electronic devices.

Moreover, they find applications in wearable products, automobiles, telecommunication equipment, military devices, and consumer appliances.

However, you’ll find these circuit boards more expensive than traditional ones. 

Check out their features below: 

  • They’re easy to customize
  • The circuit boards are lighter
  • They comply with specific flexing requirements
  • Impedance control
  • Thermal management
  • Improve aesthetics
  • They have better reproductivity and reliability
  • They have a low assembly and production cost
  • Minimal noise reflection thus improved signal integrity and quality
  • Provides uniform electrical features for high-speed circuits
  • They’re flexible for easy installation and services such as satellites, power generators, and solar panels
  • Has better expansibility and scalability, thus offering better component interconnections

What Is A Coverlay?

A sensor on a PCB

(A sensor on a PCB)

We use this cover to protect and encapsulate the external circuit board circuitry. Surprisingly, it is the solder mask or resistor for flexible circuit boards.

Generally, we know that conventional solder resist and solder masks have limited flexibility and elasticity. Therefore, the overlay is the best option for flexible circuit boards that needs even better flexibility. 

The overlay helps you protect the wirings and copper foils on your circuit board. And we like it since it offers a more robust and durable solution with high dielectric strength and excellent flexibility. 

An overlay normally has two parts:

  • A single layer of polyimide
  • Another layer of either epoxy-based or acrylic flexible adhesives

The function of the adhesive is to bond the polyimide to the flexible circuit and its circuitry. You can then align the overlay to the circuit surface under specific pressure and temperature.

In most cases, the overlay is usually available in film and adhesive thickness combinations. A 1:1 ratio representing 1 mil film and 1 ml adhesive is the most common combination.

However, the specific film and adhesive combinations depend on the below items:

  • Cost
  • The dielectric resistance voltage
  • The external copper layer weight
  • The minimum bending requirements

How A Flexible PCB Coverlay Differs from A Flexible PCB Solder Mask

Although many mistake solder mask for overlay, the two have unique differences that we’ll consider below: 

Material Differences

Generally, the overlay is made up of a solid sheet of polyimide. Also, it withstands the same dielectric voltage as polyimide flex cores.

Furthermore, it offers an impenetrable barrier and is available in different thicknesses to comply with the tight minimum bend requirements. 

However, flexible solder mask contains epoxy-based liquid that undergoes curing to harden.

We usually add a flex agent and limit their thickness to ¾ to 1 mil.

Surprisingly, the material we use to develop solder masks makes them less flexible and robust than an overlay. 

These differences will affect the following:

  • Flexibility
  • Electrical insulation properties
  • The material processing
  • Stiffener application and requirements
  • Long-term reliability and robustness
  • Ability to define fine PTH features and pitch SMT

Design Considerations

Solder masks and overlays come with unique capabilities thanks to the different materials.

Generally, you’ll need SMT and PTH openings to create an overlay mechanically using laser cutting, routing, and die sets.

Also, there is a need for laminating the overlay to your flex.

Therefore, it’ll limit the minimum material web and opening sizes between adjacent openings. You should note that overlay comes with unique design rules. 

On the other hand, a flexible solder mask comes with a photo-imaged liquid that offers the capabilities of rigid boards.

However, the solder mask isn’t perfect for the thermal lamination of EMI or RF shield laters or stiffeners.

Doing so will compromise its bond strength. 

If you want to use a solder mask and stiffener simultaneously, you must apply a double-sided tape (PSA).

Color and Thickness Options

We noted that overlay is available with various polyimide and preattached adhesive layer thickness.

Generally, polyimide and adhesive thickness range from ½ mil to 2 mils.

However, for both cases, 1 mil is the most common thickness. 

Surprisingly, the adhesive you need depends on the copper thickness, with 1 mil of adhesive going with 1 ounce of copper. 

However, the solder mask has a thickness of ¾ to 1 mil and is thinner directly over your electronic circuitry. 

As for the color, the overlay is available in white, black, and natural (light brown), with the natural being the most common.

However, solder mask is available in several colors, including black, white, green, and yellow. 

Selective Application

Generally, the overlay is applicable in sections prone to bending with large shield layers and pitch components. However, solder mask is applicable in high-density component sections. 

Surprisingly, you first apply the overlay and then follows with a solder mask with a slight overlap to eliminate gaps between the two.

We recommend involving your flex supplier to help you ensure a reliable and functional design that complies with the requirements.

Coverlay Materials & Construction

Flexible PCB Coverlay:  An engineer handling circuit boards

(An engineer handling circuit boards)

Generally, the most common materials for constructing overlay are silicones, epoxies, polyesters, and polyimide. However, while using some of these materials, consider their toxicity.

Surprisingly, how rigid your flex overlay is will depend on the material’s flexibility and thickness. 

Normally, after you assemble and test your circuit board, you must perform a “burn-in.”

During this process, all defects show up, and you get a chance to remove them.

Also, you can break down any existing solder joints. 

After the burn-in process, you should test and measure the electronic components to guarantee the absence of defects.

Choosing The Correct Coverlay

Flexible PCB Coverlay:  Someone holding a flex PCB

(Someone holding a flex PCB)

  • To begin with, choose a trimmer overlay to meet the tight bend requirement.
  • Second, use 1 mil of adhesive per OZ to ensure complete encapsulation. This means using 1 mil of adhesive for 1 oz of finished copper.
  • If cost concerns you, avoid thicker or thinner adhesives since they’re expensive.

FAQ

Flexible PCB Coverlay: What’s the difference between a PCB coverlay and a stiffener?

Generally, a coverlay works like a solder mask; we use it to protect circuit boards.

However, a stiffener is a material that we include on the circuit board to provide mechanical support. 

Flexible PCB Coverlay: Are flexible PCB coverlay necessary?

Yes! As we highlighted, these materials offer the same functionality as solder masks that we use on rigid boards.

Without it, your electrical circuitry will be exposed.

Therefore, they provide more flexibility and durability to flexible circuit boards. 

Conclusion

As highlighted, a coverlay protects and encapsulates the external circuit board circuitry.

We can equate their functionality to the solder mask or resist in rigid circuit boards.

Without coverlay, you’ll expose your circuit board wirings and copper foils to damage. 

Moreover, a coverlay has a polyimide layer and an adhesive to bond the polyimide to the circuit board.

And make no mistake of confusing a coverlay with a stiffener since we have given the differences above.

We hope you now understand what a flexible circuit board coverlay is.