Flexible electronics have bridged the gap between performance and miniaturisation, allowing tiny dimensions while simultaneously meeting functionality objectives in a device. It can be found in many important applications such as wearables, medical devices, mobile phones and smart TVs.
A flexible printed circuit board or Flex PCB is a mature technology consisting of flexible polymeric material that is capable of three-dimensional configurations due to its thin and flexible geometric profile. This is to address the limitation of rigid PCBs on design with space constraints.
In this article, we will talk about PCB coverlay, an important part of the flex and rigid-flex PCBs layer construction. We will learn more about its function, application methods, and types.
What is a PCB Coverlay?
A PCB coverlay serves as a mechanical and thermal protection as well as electrical insulation between the conductive traces on a flex PCB. It is a combination of polyimide and adhesive with controlled thicknesses, characterised by good thermal conductivity, sufficient dielectric strength and low outgassing.
Coverlays are allocated on both the inner layer and outer layer as protection of the copper foils. Solder masking used for rigid PCBs is not fit for flex PCBs, thus coverlays are applied to serve as the outermost layer of the flex PCB. Below are the key purposes of flex PCB coverlays:
- As copper easily oxidises when exposed to high temperature and moisture, flex PCB coverlays shield the metallic copper surface against oxidation, especially during PCB assembly processing and application when temperatures highly vary.
- The environment can carry foreign materials, dust and contaminants that get transferred to PCB surfaces, which can lead to shorts or degradation. Flex coverlays help protect the circuitry from these external factors.
- Coverlays maintain the flexibility of flex PCB and rigid-flex PCBs as they can be bent for flex-to-install or dynamic bending applications. This flexibility allows for versatile and dense designs.
- Flex PCB coverlays also provide insulation against adjacent conductors to prevent electrical shorts.
What are the Different Methods for Coverlay Application?
PCB coverlays can be applied based on the type of coverlay PCB material, intended precision level and cost range. Below are some of the popular methods by which coverlay is applied.
- Film-Based. A film-based coverlay is considered a more complicated and costly solution than the other methods. An adhesive layer in the form of acrylic or epoxy is first applied to the coverlay film. The coverlay PCB material is laminated together with the adhesive and flex PCB to achieve well-bonded layers. Laser processing is used to keep the coverlays from disturbing the circuitry.
- Screen Printable. In screen screen-printable coverlay method, a stencil or screen is used to form the PCB coverlay pattern. A liquid polymer is used to access the openings of the stencil or screen through a squeegee tool. After screen printing, the flex PCB is subjected to curing through UV or light to fully harden the coverlay PCB material.
- Photo-imageable. Photo-imageable coverlays are implemented using a technique known as the photolithography process. The PCB is first pre-treated by removing dust or foreign elements, then the photo-imageable film is applied using a vacuum laminator. The applied photosensitive film is exposed to UV with a photomask which contains the circuit pattern. After exposure, a development solution is used to further reveal the circuitry. A post-curing step is needed to achieve complete curing of the PCB coverlay. Photo-imageable coverlay is an effective method to attain a high resolution and intricate pattern.
What are the Different Materials Used as PCB Coverlay?
Flex PCB designers and engineers have several coverlay PCB materials to choose from. Here are some of them:
- Polyimide. Polyimide is the most popular choice for flex PCB coverlays as it offers several advantages such as high thermal stability, chemical and moisture resistance and good electrical insulation.
- Polyester. Polyester is another alternative as flex PCB coverlay, but with a lower temperature grade than polyimide. The application of polyester coverlay is less costly but is sometimes done manually due to potential planarity issues if not done correctly. This coverlay PCB material has good electrical insulation and flexibility, like other polymers.
- Acrylic. Acrylic coverlays are used in conjunction with polyimide films to establish strong adhesion. They are commonly used due to their high temperature resistance and durability. They also have good insulation properties, which are excellent for maintaining good electrical integrity in a coverlay flex PCB circuit.
- Silicone. Another option, yet with lower thermal and chemical resistance than its other counterparts, is a silicone coverlay. This material type requires less lamination temperature and has very good flexibility.
- Liquid Crystal Polymer. A popular choice for high-frequency applications such as aerospace, medical and microwave devices is liquid crystal polymer coverlay. It also has good thermal stability but is more expensive than polyimide and polyester coverlays.
What are the Different Industry Applications of Coverlays?
Let’s go through the different industries that benefit from using flex coverlays as protective layers on flex and rigid-flex PCBs.
- Consumer Electronics. The circuitry inside devices used for consumer electronics needs to be protected from external factors such as heat and stray contaminants, thus coverlays are required. PCB coverlays help to maintain the bendability of the tiny flex circuits in laptops and smartphones.
- Medical Devices. Diagnostic machines and wearables used in healthcare heavily depend on flex and rigid-flex PCB technologies. These PCBs have coverlays to maintain the reliability of the devices.
- Aerospace. With the harsh conditions in which aerospace devices are subjected to, PCBs need to have flex coverlays so that durability and performance are both intact, even with extreme stresses.
- Industrial. Industrial machinery and robotic systems require coverlay flex PCB. This is to guarantee long service life despite frequent flex cycles and movement when in use.
