A PCB is an essential component of any electrical circuit, having connectors that serve as a base layer for other working course elements. The manufacturers use PCBs to support the wirings and surface-mounted components. These are of different types.

Let’s analyze one of the types: Flexible PCBs, and highlight the strategies for its accurate design.

What is a Flexible PCB?

A flexible PCB is a unique printed circuit board that replaces the stiff FR4/metal/PTFE/ceramic substrate with a thin, bendable polyimide (PI) substrate and a PI overlay in place of the solder mask. Plating through holes or buried vias insulates and safeguards the conductive copper traces that link various flexible PCB layers with non-conductive PI materials. A flexible circuit board, flex PCB, or FPC, is another name for a flexible PCB. The non-conductive flexible polyimide materials are etched with copper circuit traces, and plated holes (PTH) or buried vias connect the various circuit layers. Electronic components can be surface-mounted on the PCB pads on the two sides of a flexible PCB. Flexible PCBs may occasionally have PCB stiffeners added to the ends for connection to the system, device, or other parts to provide better mechanical support. Flexible PCBs can be single-sided, double-sided, single-layer, dual-layer, or multi-layer, and they can also be custom designed and made. One PCB layer equals one circuit layer. But everyone can quickly tell a flexible PCB apart from a standard rigid PCB since they are often thin, light, and yellow. Its thickness ranges from 0.05mm to 0.6mm. It is also PCBONLINE’s completed flex PCB thickness range. Typically, flexible PCBs have two sides. To comprehend them properly, you must be familiar with flexible PCBs’ fundamental structure.

Difference between polyimide(PI) and polyester(PET) material

The flex PCB raw materials and stack up determines the flexible circuit’s quality. Depending on the market demands, three different types of base raw materials are used to make flexible circuit boards. Polyester (PET), Polymide (PI) with adhesive, and Polymide (PI) without adhesive. The benefits of PET material include a more straightforward production method, lower costs, and shorter lead times. PET material’s drawbacks include the inability of polyester to pass through the soldering flow. Then it can’t be operating in a hot climate. The benefit of polyimide (PI) is that it is flexible. Can function in a hot environment. The cost of raw materials is significantly higher than that of Polymide (PI), which is one of its drawbacks.

Why do flexible PCBs have to be unique, and can you not make every PCB flexible?

Although flexible circuit boards are helpful, stiff circuit boards will still be necessary for some applications. The most significant barrier to using an utterly flexible circuit board design in a consumer product is cost-effectiveness. In a typical automated high-volume fabrication factory, rigid circuit boards are less expensive to produce and install. To reduce manufacturing and assembly costs, the optimal solution for an innovative product often uses wooden circuit boards whenever possible and flexible circuitry only when essential. For this aim, some manufacturers even use hybrid rigid-flex printed circuit boards. Laptops and medical equipment to connect rigid circuit boards generally employ flexible circuits resembling ribbons. By concentrating on the unique capabilities of each circuit board base technology, these boards are combined and created to fulfill a wide range of technical needs.

Types of flexible PCBs

• There are single-layer, two-layer, and multi-layer FPCs, depending on how the circuit layers differ.
• There are single-sided and double-sided FPC, which vary based on the side of the component mounting.
• There are three types of flexible PCBs: clear, flexible PCBs, flexible aluminum PCBs, and yellow polyimide PCBs.
• There are HDI flexible PCBs and standard flexible PCBs, which differ in terms of FPC vias.

How to choose the right assembly technology for a Flexible PCB?

Following are ways you can ensure that the Flexible PCB you design or use has been designed correctly for you:

• Choose the appropriate material for your PCB

The process of creating your flexible PCB begins with selecting the appropriate materials. We are referring to choosing the suitable substrate material, conductor, adhesive, and cover layer. It would help if you considered using a film with solid insulating qualities as the primary material. It must perform mechanically and electrically following standards.

For this reason, producers typically choose polyester or polyimide. FR4 is used in standard boards, although its thin variants may also be used, particularly in flex-rigid panels. 

Polyester has the lowest solderability temperature when you compare the different materials. The lowest maximum continuous operating temperature is 110C, lower than 150C for FR4, and 220C for polyimide. When it comes to absorbing humidity, polyimide is a preferable option because of its excellent tensile strength and elongation. Polyimide has a dielectric loss factor that is twice as low as polyesters.

In general, polyimide is a non-flammable material with excellent electrical and physical properties. Polyester, on the other hand, is less expensive and provides adequate performance, although it is vulnerable to damage when soldered.

• Manufacturing- Blanking and Drilling: The key to assembling a PCB

The preparation of the materials is the first step in the assembling process. You should list all the supplies required for the method, also known as “blanking.” That also contains a laminating paste, a separating film, and other required auxiliary materials. Dealing with thin and delicate materials throughout the drilling process might be difficult. The experts advise stacking the boards first because drilling them is difficult. Consider the cladding layer and stack all of its components, for instance. They might resemble books after you combine them, but they will be thicker when combined than when used separately.

• Assembly technology- Pitting and Desmearing

If the drilling process has been successful, some leftover drilling debris may be on the material. Because of this, you should attempt to clean off any dirt before moving on to the following action. However, it calls for extensive vigilance because you work with small and thin components. PTH (plated-through-hole) and pattern plating are the next steps in the procedure. Here is an example of that procedure:

• For electroless plating, experts advise using acidic colloid palladium as a pre-treatment solution. They predict that it will function more effectively than alkaline ionic palladium. The rationale is you don’t want the materials to expand or create unneeded holes. Plus, do concentrate on speed and reaction time as fast as possible. 

• Electrical plating: The layer with electroless copper plating has poor mechanical qualities, including flexibility. That renders it vulnerable to thermal shock damage. Electrical plating aims to thicken the panel because doing so will ensure the integrity of the hole wall coating later in the procedure.

• Imaging – clean the board surface before moving on to this phase. For standard PCBs, the assemblers use a comparable technology. However, since flexible boards could deform, applying electrolytic and chemical cleaning methods would be beneficial. Stick the dry film now and watch it develop. It will become fragile and lose some of its copper foil binding force due to polymerization.

• Etching

Etching is the next assembling technology that we employ. At the points of bending, you’ll see that substrates have many parallel wires. Focus on preserving the direction of the etching fluid throughout the procedure if you want to maximize the etching results. Think about the transmission direction, pressure, and position as well. The rigid substrate is to be joined to the flexible substrate. You want to draw the flexible substrate forward, so do that in front of it. When the etching process is complete, you need to modify the surface to increase the binding potential. Then place the cladding layer. Moreover, ensure baking the flexible board and the cladding layer because they have differing prospects for absorbing dampness. Please bake it for approximately 34 days, but make sure the stack-up height is at most 25mm.

• Laminating the PCB

Knowing that you must select the process’s particulars based on the substrates you chose is crucial. The materials will determine the time, pressure, and heating rate required for lamination. Some general tips to consider are:

• Heating rate: The temperature should reach 173C between 10 and 20 minutes.

Use a pressure of 150N to 300N/cm2. It should take you five to eight seconds to reach maximum pressure.

• Time – according to the experts, the net pressure time will last roughly 60 minutes at maximum pressure.

You will need professional laminating equipment for any sophisticated uses and professional applications. Additionally, make sure you select the proper gasket material. Take into account low fluidity and outstanding formability, and choose a material that won’t shrink after cooling.

• Finalizing the PCB- The assembly

Baking, hot melting, and sharp processing make up the flexible PCB assembly process’ final steps. Professionals use these techniques to remove the extra moisture, dry it out, and prevent the board from getting wet. Finally, it would be beneficial if you took packaging into account. Since wrapping paper alone cannot separate the boards, producers pack numerous boards using vacuum packing technology. The most trustworthy businesses will take every precaution to package the PCBs safely and dispatch them via a dependable service.

Conclusion

The PCB world is ever-growing and exciting in a way that sets it apart. Flexible PCBs are one example of how PCBs can be modified and used to make newer inventions, which opens the world of PCBs to a whole new area of usage. So, it’s high time to get to work. We just handed you the knowledge about choosing the technology and accounting for the different variables.