How Is a Flex Rigid PCB Made?

Rigid-flex circuit boards are special printed circuit boards that combine soft and hard boards. They are created by laminating polyimide (PI), a soft board material, with hardboard material FR4. So far, they have not been favored by many designers because most of their designs use rigid boards. More and more designers are searching for printed circuit boards that can actually bend because the designs of electronic devices are often high-density.

Ordinary rigid boards are connected in three dimensions through connectors and cables. This type of connection takes up space, is unstable in structure, and has unstable performance. It is not appropriate for the current trend of products that are lightweight, high-performance, and precise. Rigid-flex PCB boards combine the advantages of the flexibility of flexible boards and the rigidity of rigid boards so that products can easily achieve three-dimensional layouts without any connectors and cables, change shapes according to space limitations, and achieve small size and high performance.

Our stiff-flex PCB technology outperforms standard rigid boards in this regard. It can be difficult for clients who are unfamiliar with this technology to design and use rigid-flex PCB boards. Understanding the structure of rigid-flex PCB boards is very helpful for properly handling these circuit boards in design and application.

1. Rigid-Flex PCB Substrate Materials

All printed circuit boards require a substrate to provide a certain rigidity. Traditional circuit boards use fiberglass and epoxy as the substrate. Even though it is made of fiber, the epoxy lends it stiffness, especially as it hardens. However, many contemporary items cannot be used with classic circuit boards due to their lack of flexibility. However, conventional rigid printed circuit boards are appropriate provided the board does not need to flex or move after construction.

Polyimide

The circuit board’s substrate has to be more flexible than epoxy for a variety of uses. Most PCB manufacturers use PI or polyimide instead of an epoxy-glass fiber combination because it is soft but strong and does not tear or stretch easily. It can survive the high temperatures of the soldering reflow process because of its strong thermal stability. Polyimide has almost no expansion and contraction deformation due to temperature fluctuations.

Polyester

Polyester, usually referred to as PET or polyethylene terephthalate, is a flexible material that has a little lower temperature deformation and heat resistance than polyimide. This material is used by PCbandassembly for inexpensive electronic devices that need to be wrapped in soft film. Since polyester cannot withstand the high temperatures associated with the soldering process, we use a cold-press process to form this soft and flexible circuit board.

Pcbandassembly uses several common materials to build the core of flexible circuits. These materials include PI film, PET film, thin epoxy resin, and fiberglass. Additionally, we employ protective PI or PET film as solder masks. The protective film separates the conductor traces from the outside, much like the solder mask we use for rigid boards. They are shielded from corrosion and harm as a result. We employ protective films that are between 1 and 3 mm thick. Since fiberglass and epoxy resin are typically 2-4 mil thick, we also utilize them for circuit boards that are less flexible.

2. Rigid-Flex PCB Conductor Materials

The many components that assemblers put on circuit boards are connected by printed wires or traces. Copper traces are used in conventional rigid PCB boards. Copper is also frequently used, whereas rigid-flex boards use carbon film or inks based on silver to create traces. On rigid-flex boards, we use several copper types based on the application. Electrolytic copper foil is among the best and most popular options. By merely adding copper weight, we may raise the trace’s current carrying capability. Different copper widths can be achieved in high-frequency circuits by increasing copper weight, particularly for applications like board-mounted planar inductors.

For flexible circuits, for example, copper performs poorly when work hardening and stress fatigue are present. When flexible circuits are frequently bent or folded, rolled annealed copper, or RA, is a superior option. Although rolled and toughened copper adds cost, the extended life of the flexible circuit can offset the additional cost. This is because RA copper foil can withstand multiple bends and folds before failing from fatigue fracture. RA copper is more flexible in the Z direction, which can withstand more bending and rolling and extend the life of the flexible board. Ordinary copper’s grain structure is lengthened during the rolling process, which increases its elasticity and flexibility in the Z direction while strengthening it in the planar direction.

3. Rigid-Flex PCB Adhesive Materials

When making copper-clad laminates, manufacturers must bond the copper foil to a core board, such as PI film or PET film. For standard stiff FR4 boards, this is not an issue because the copper foil’s surface is already rough. After rolling and tempering, the surface of RA copper foil becomes very smooth, and even high temperatures and high pressure cannot provide good adhesion.

Corrosion-Resistant Copper-Clad Laminates

Pcbandassembly uses corrosion-resistant copper-clad laminates to make single-sided and double-sided rigid-flex boards. These copper-clad laminates only need acrylic or epoxy glue that is 1-2 mil thick. Flexible circuit boards can be used with this adhesive.

Glue-Free Laminates

Pcbandassembly also uses glue-free laminates to make flexible circuit boards. We use new processing technologies such as direct deposition and coating of copper on PI film. For flexible HDI circuits that need finer pitches and smaller through holes, this approach is especially helpful.

4. Rigid-Flex PCB Supportive Materials

The area where the flexible PCB meets the rigid component often requires support to reinforce it. Typically, we utilize hot melt adhesive, silicone, or epoxy to improve the joint’s tensile strength. This guarantees that stress exhaustion and tearing won’t happen even if the flexible PCB is bent repeatedly.

Knowing the materials that manufacturers use to make rigid-flex boards is important because it can help designers correctly select and appropriately choose the materials needed for a specific application. This can assist in avoiding unspoken risks that result in disastrous field failures.

5. Summary

Understanding the properties of rigid-flex PCB materials also helps designers design, evaluate, and test the mechanical components of electronic products. Products for automotive applications, for instance, must be able to distribute heat and resist moisture, chemical corrosion, shock, and many other typical circumstances. Similarly, rigid-flex circuits require careful evaluation to use the right materials for the highest reliability and the smallest allowable bend radius. Particularly in low-cost consumer electronics applications, rigid-flex PCB boards may also be subjected to severe environments. For example, personal electronic devices are often subjected to vibration, drops, sweat, etc.