Flex Printed Circuit Board assembly serves at the frontlines of electronics and robotics technological revolution. It grants users the option to avail flexible alternatives for fulfilling modern PCB requirements rather than opting for conventional/traditional PCB boards. Conventional Printed Circuit Boards have been a foundational element in electronics / electrical circuits for the current era. To stay ahead of the curve, it is necessary to constantly keep an eye out for lighter, sleeker, more efficient, more versatile, and wide-application elements. Flex PCBs brought a halt to all the explorations. With their unique bendability and versatile functionality, these extraordinary Flex PCB boards revolutionized the global realm of hardware electronics once again and unlocked a new world of possibilities for all engineers and innovators. Compared with rigid PCBs with hard characteristics, flex PCBs are more likely to be damaged. They are used in the case of insufficient space when assembling and installed on the surface of the board, which requires the board to have a certain degree of bending and flexibility.
2. What is a Flex PCB board?
The flex PCB (full name Flexible Printed Circuit Board) is a circuit board that can bend, twist and even fold without worrying about breaking, with amazing flexibility. These characteristics make it suitable for different PCB assemblies, and important factors such as space retention and equipment flexibility need to be taken into account during PCB assembly. The flexible core materials that it uses are important for the above characteristics.
Flex PCB boards are constructed from flexible polymer-based dielectric substrates such as polyimide(Kapton), polyamide, polyester (PET), fluoropolymers (PTFE) and liquid crystal polymer (LCP) with the lamination of ultra-thin copper foil conductive paths on the substrate. The thickness of the copper foil is based on the current-carrying requirement of PCB board. Typically, in Flex PCB boards the thickness of chosen copper foil is 12µm to 35µm. For insulation and protection of copper traces, a layer of overlay – thin, flexible and dielectric, is coated over the whole board. Glass fiber, which is rigid and generally used in the fabrication process of PCBs, is not integrated with the board, in the case of Flex PCB boards.
3. Technical Considerations for Flex PCB assembly
Fabricating Flex PCB assembly involves employing unique construction and design methods to cater to its phenomenal flexibility, bendability and twisting properties.
Material and construction considerations confer the below-mentioned characteristics to flex PCB assembly:
1. Resistance: Substrates like polyimides equip Flex PCB assembly with high heat and
chemical resistance. Polyimides can withstand up to 400 degrees Celsius of heat and
work great with the soldering process. The substrates provide great chemical resistance against solvents and fluxes used during the soldering and assembly processes.
2. Frequency: All the di-electric substrates mentioned above, used in the Flex PCB assembly, offer great performances with high frequencies.
3. Ratings: The current carrying capacity of a PCB board depends on its copper-trace thickness. For Flex PCB assembly, typically the current ranges from 0.5A to 5A.
4. Flexibility: The main characteristic that sets Flex PCB assembly apart from other
conventional PCB assemblies is their flexibility property. Flex PCBs are manufactured to bend. Usually, they can bend up to 3 times their layer thickness. For instance, if a PCB
layer is 1mm thick, the bend radius will be 3mm thick, which is quite small. The smaller the bend radius, the more the flexibility. In some special cases, the bend radius can be as small as 10 times the layer thickness.
5. Layer thickness: Flex PCBs are designed to have thin and lightweight layers, allowing them to bend, twist and fold. For single-layer Flex PCB boards, the layer thickness is usually 12 micro-meters and for multi-layer Flex PCBs, the thickness can be reduced by up to 18 micro-meters.
Some of the design technicalities used in the process of flex PCB assembly are stated below. By sticking to these specifications, flex PCB assembly can endure many flexing cycles, leading to an increased lifespan of the product.
1. Bend radius: Keep trace routing orientation perpendicular to bending axes. Additionally, the foundation thickness of Flex PCB for static bend ought to be 3X, and for dynamic bend, it must be 10X where X is the thickness of Flex PCB material.
2. Trace specifications: Narrower traces are more prone to cracks. To prevent thinner traces from cracking, flex spacing is increased in dynamic areas and is usually maintained in a 2:1 ratio of spacing to trace width.
3. Shielding and reinforcement: To enhance the overall structural authenticity of the Flex PCB board, reinforcements in the form of stiffeners are incorporated with the layers to avoid failure, bucking or wrinkling of the board under any environmental stress. Furthermore, to shield the structure from electromagnetic interferences and electrostatic discharges, flexible conductive films are incorporated with the multi-layers of Flex PCB board.
4. Vias and pads: Vias and pads are used across Flex PCB assembly to ensure the reliability and durability of the board. Vias are holes drilled through multiple layers of PCB for stabling connections. In the case of Flex PCB assembly, it is helpful to use tear-shaped vias as the shape tends to distribute mechanical stress evenly, thus minimizing the risk of cracks. Pads are also used for making electrical connections. Using rounded or circular pads also relieves mechanical and thermal stress around flexing areas.
5. Fillets: Fillets or rounded corners are advisable in the process of a Flex PCB assembly because sharp edges attract more stress concentration in comparison to rounded corners. The more rounded the corner is, the less the stress concentration around the corner is, which leads to a reduction in the probability of mechanical failure of board.
6. Coverlay: Bending areas or flexing areas in Flex PCBs are susceptible to more wear and tear than the rest of the board. Coverlay must be purposefully placed on the board
keeping in view all the dynamic bend areas to minimize mechanical stress on traces.
4. Applications of Flex PCBs
The flexible and dynamic characteristics of Flex PCB open numerous possibilities for their potential applications. A few of them are listed below:
1. Wearable devices: Wearable technological devices are usually compact in size and are perfect to embody Flex PCBs. Flex PCBs due to their flexible nature are easy to work with in space-reserved environments.
2. Robotics engineering: In robotics, the swift movements of the joints of robots or robotic arms are a major concern. The shape reforming property of Flex PCBs allows them to be used in such critical joints for controlling, sensing and data processing of robots.
3. Commercial electronics: As technology is advancing, the need for lightweight devices is also increasing. Flex PCBs are extremely lightweight in comparison to their counterparts, and thus are often preferred to be used in modern smartphones, watches, tables, systems and laptops.
4. Medical equipment: Pacemakers, hearing aids, vital sensors, ultrasound devices and wearable monitors all make use of Flex PCB due to its high reliability, durability and flexibility.
5. Automotives: Flex PCBs owe their second largest sales to automotive industries. Modern cars or luxury cars are full of electronic devices right from GPS dashboards to fully automatic electric vehicles. The flexibility and reliability of Flex PCB assembly make it compatible with the interior and technological features of vehicles.
6. Military and defense: Flex PCBs contribute to military applications due to their flexibility, reliability, durability and the efficiency to withstand harsh environmental conditions. They are commonly used in radars, missile systems and communication systems.
5. Advantages and Disadvantages of Flex PCB
Like all devices have their share of pros and cons, so do Flex PCBs. A few advantages and disadvantages of Flex PCB assembly are given below.
Advantages:
1. Flex PCBs are lightweight, which allows them to be a part of many consumers’ electronic and wearable devices.
2. Flex PCBs are small in size and can easily conform to any shape in space-reserved environments.
3. Flex PCBs are foundationally made up of flexible materials which allow for their extended use in many products.
4. Flex PCBs have high-temperature resistance and can withstand harsh environmental conditions, which allows them to be a part of military equipment and automobiles.
Disadvantages:
1. Flex PCBs are more expensive than conventional PCBs due to their unique design requirements.
2. Flex PCBs are compact in size, but they are difficult to store because they are extremely sensitive to starches and require extensive storage methods.
3. The assembling process of Flex PCBs is quite complicated and thus requires skilled craftsmen.
4. If damaged, Flex PCBs are difficult to repair owing to their small size.
