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Is the FR4 Semiflex the Right Choice for You?

Flex-rigid PCBs offer a vast array of uses as a result of the trend toward downsizing in the creation of consumer devices. Also, the PCB sector has been experiencing fiercer competition, which has increased the focus on flex-rigid Circuit Board manufacturing technology development. Several PCB manufacturers who specialize in rigid Circuit board fabrication are unable to match the expectations of flexible Board fabrication due to the complicated process of flex-rigid Circuit board manufacturing. The type of PCB is FR4 semi-flexible Circuit Board which we will discuss in this article. It comprises bendability, flexibility, and the ability to construct in three dimensions.

It is impossible to exaggerate the significance of flex-rigid Circuit boards in PCB manufacture. The single driving factor behind it is the expanding trend of product miniaturization. Also, the need for flexible-rigid Circuit boards is increasing due to their flexibility and ability to construct in three dimensions. Not all Circuit Board producers can, however, accommodate the challenging flexi-rigid PCB production process. Semi-flexible PCBs create in a way that reduces a rigid board’s thickness to 0.25mm or 0.05mm. The board can easily employ applications that call for flexing the circuits and putting them within casings as a result. The Circuits can easily use for both single-bend installations and setups involving several bends.

Properties Of FR4 Semiflex PLC

rigid-flex pcb fabrication
  1. The ability to adapt to the given area and be flexible are the most crucial qualities that make it suitable for numerous uses.
  2. The reality is that flexibility doesn’t affect the transmission of signals, which enhances its widespread use.
  3. Also, it is naturally lightweight.
  4. Semi-flex Circuit boards are frequently renowned for their lowest possible prices due to the fact that their fabrication process is in line with present production capabilities.
  5. Both throughout the design process and during assembly speed things up.
  6. They are a very trustworthy choice, especially since they prevent several problems like welding and entangling.

PCB Fabrication Procedure of FR4 Semiflex

The following is the general process for creating FR4 Semi-Flexible PCBs. Generally speaking, the procedure involves the following elements:

  1. Substrate Cutting
  2. Coating of Dry Film
  3. Automatic Optical Inspection
  4. Brownization
  5. Process Of Lamination
  6. X-Ray
  7. Electroplating
  8. Graphical Conversions
  9. Etching
  10. Printing Through Silkscreen
  11. Surface Finishing
  12. Depth-Control Milling
  13. Electrical Evaluation
  14. Quality Control Test
  15. Packaging

Issues & Solutions In The FR4 Semiflex Fabrication Process


The main challenge in manufacturing is to maintain precision and milling tolerance for depth control. It’s crucial to check for resin fractures and oil peeling, both of which might result in quality problems. This means keeping an eye out for the following things while using depth-control milling:

  1. Board Thickness
  2. Resin Material
  3. Milling Tolerance

Depth Controlling Milling Test A

For thicknesses of 0.3 mm, 0.275 mm, and 0.25 mm, the depth milling is carried out using the mapping approach. After this, the panel is tested to determine whether it can withstand the 90-degree bend. Generally, fiberglass is considered damaged if the remaining thickness is 0.283mm. So, when performing the depth milling, it is essential to take into account the panel thickness, the thickness of the fiberglass, and the dielectric condition.

Depth Controlling Milling Test B

Depending on the foregoing, it must be assured that the copper thickness between the L2 and solder mask range is 0.213mm and 0.188mm. Proper consideration must also be given to every warpage that could happen and affect the consistency of the overall thickness.

Depth controlling Milling Test C

When the panel has been prototyped, depth-controlled milling is crucial to guarantee that size is fixed at 6.3′′x10.5′′. Following that, the mapping point measurement is done to make sure the 20 mm horizontal and vertical spacing is maintained.

A unique manufacturing technique ensures that the deep-regulated thickness tolerance is within the limit of 20 m.

Composition Of A FR4 Semiflex Material

The purpose and final usage of an FPC often dictate its composition. The capacity of the material to sustain high electrical conductivity whilst resisting warping, breaking, and damage also plays a role in how flexible it is.

Because of this, manufacturers frequently create semi-flex PCBs by pouring a mixture of fiberglass and epoxy glue into fiberglass materials or FR-4.

But let’s look at some other components of a semi-flex PCB material.

1. Insulating Film

Polyethylene layers with high density are insulating films. These are created by forcing resin using a nozzle right before putting it on the FPC base.

Moreover, this HDPE coating prevents the substrate from being impacted by capacitive coupling. Electrically insulating connections on the upper layer also interfere with other circuits.

Moreover, an insulator film layer acts as a moisturizing barrier to shield the PCB from moisture as it cures.

2. Electrostatic Adhesion Layer

Well, after insulating the film, there is an adhesive layer to aid in holding electrical components on the board. This board layer is often acrylic or polyimide, allowing parts such as LEDs to connect directly to the flexible PCBs.

The electrostatic adhesive layer significantly reduces the amount of time and resources for assembly.

3. Conductor

Incorporating an electric conductive layer over the adhesion assists in avoiding circuit warping. Polyamide, epoxy resin, and the panel itself can also use as the coating.

Remember to spray the conductor with a 100C solvent to prevent warping effectively.

4. Board Enhancement

A second adhesive improves the FPC further. Also, it lessens the possibility of experiencing fracturing or bending issues. Manufacturers frequently use cellulose or acrylic to create the coating.

5. Coverlay

The last plate to shield the underside of the FPC material is the coverlay. Because it comprises acrylic or polyamide, the coverlay can also serve as an insulating layer.

It thus shields the FPC substance from dampness.

Moreover, the coverlay offers high-temperature tolerance, enabling the use of FPC boards in heater and oven applications.

Difference Between FPC And FR4 Semiflex

rigid flex pcb circuit board

The most typical sort of circuit board is a PCB, although even this form is not suitable for all purposes. FPCs, in contrast to PCBs, can flex and bend, making them ideal for contemporary technologies.

Also, relative to their conventional counterparts, Flexible PCBs provide greater heat dissipation, increasing the durability of the devices you create with them.

PCBs come in three different varieties: single-layered, double-layered, and multilayered versions. FPCs, whereas dividing into flat flex boards and semi-flexible boards.

Application Of FR4 Semiflex PCB

· Medical Imaging Instrument

Medical devices need a great deal of semi-flex Circuit Boards. Medi-care equipment, including endoscopes and imaging tools, employs a semi-flexible PCB. A flexible tube called an endoscope has a lens and a light at both ends. It is put inside the human body to observe internal organs or carry out treatments. These include tissue biopsies, draining fluid clumps, removing foreign objects, and widening congested channels like the esophagus. Low cost and superior electrical performance are two benefits of semi-flex PCB. The semi-flexible PCB is appropriate for this use since it can be twisted into the correct form.

Further uses for semi-flexible PCB exist in the medical industry. For instance:

  • Circuit board for protecting injection needle tips (ETC)
  • PCB for temperature control in medical equipment
  • Circuits for a mobilized blood pressure monitor

· Automotive Applications

 Automotive devices with vibrations and high temperatures employ semi-flexible PCBs. The flexible architecture guarantees that the PCBs won’t crack from vibration or mechanical shock when operating at high temperatures. To suit the needs of the majority of applications, they are manufactured from stiff FR-4 fiberglass epoxy composites with extra copper layers bonded on each side of the substrate.

Due to their inherent flexibility, Semi-Flex PCBs are perfect for serving automotive applications with high-temperature requirements, such as Digital Control Systems or DCU, Electronic Control Units or ECM, etc. These boards are resistant to breakage and distortion at high temperatures of 150 °C.

· Aeronautics

With the rising demand for weight reduction and miniaturization, the usage of rigid-flex circuits for aerospace devices has expanded. As interconnect modules, rigid-flex Circuits are an efficient way to decrease size, weight, and energy consumption while enhancing dependability.

Several aircraft applications, including avionics, communications, and military systems, have employed rigid flex Circuits. Rigid-flex PCBs are useful in aviation applications as connectors between PCBs and additional parts like switches, displays, or cables. They aid in reducing the number of connections needed on Circuits, which aids in reducing system weight. They also improve dependability by cutting down on the number of linked components.

· Wearable Devices

The most well-known examples of wearable technology are fitness trackers and smartwatches, which are gaining popularity. Small, battery-operated gadgets that one wears over the body are wearables. In wearable technology, rigid-flex circuits and semi-flex circuits are both employable, with rigid-flex circuits being more popular in watches & semi-flex in fitness trackers. This is due to the fact that semi-flexible materials are more suited for wearables that must bend on the body. At the same time, stiff flex can sustain bending pressures, which is vital for wearables that require to endure impacts while falling.

· Smartphones And Laptops

PCBs that can flex and not bend are frequently useful in applications that need semi-flexible Circuits. For instance, semi-flexible boards are frequently useful in high-end tablet and smartphone systems. With these gadgets, the back of the screen is directed to a very thin layer of a semi-flex board. This enables the display to bend or fold without breaking. When only a little amount of flexibility is required, a semi-flexible Circuit can easily be employed. These may use, for instance, on the back of a gadget with a stiff front panel, like many computers and tablets.

· Gaming Devices

The primary benefit of adopting rigid flex Circuit in game consoles is that it offers superior protection against physical shock damage. They are better able to endure powerful external pressures like collisions and drops due to having lesser layers than conventional rigid PCBs. When compared to a conventional rigid PCB, the layers of rigid-flex Circuits are more resilient to bending. Therefore, if you somehow drop the gadget, it will sustain the greater force. For instance, the stiff flex PCB may readily twist or bend without cracking or breaking while using a gaming system, which frequently demands bending.


Due to certain of its characteristics and qualities, rigid or semi-flex PCBs will perform better than their competitors in a number of areas. They compete fiercely with other PCBs on the market since they perform comparable tasks and give end consumers a more affordable choice. Some of these uses are with one another, while others do not fit well into any one category. Whatever one you choose to develop will depend on your own demands and preferences. 




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