Flexible printed circuit

Flexible Printed Circuit

Flexible Printed Circuit Meaning


A flexible printed circuit (flexi rigid pcb) also namedFPCB or flex PCB is a patterned assembly of printed circuitry and components made of flexible base material with or without flexible coverlay. These flexible electronic assemblies can be made with the same components as rigid printed circuit boards, but with the added ability to “flex” (conform to a particular shape) while being utilized.


Flexible Printed Circuit Applications


Besides high-end, complicated finished components, flexible printed circuit (flexi rigid pcb)  are commonly used in electronics and daily technology. Modern portable electronics, devices, hard drives, and desktop printers are some of the most famous examples of flexible circuit usage.

flexible printed circuit board (flexi rigid pcb) are widely applied in below industries :

  • Wearables
  • Transportation
  • Military
  • Medical
  • Industrial
  • Consumer Electronics
  • Communications
  • Automotive
  • Aerospace

Flexible Printed Circuit Advantages


Using Flexible printed circuit (flexi rigid pcb) has a lot of benefits. Flexible circuit boards are especially made to fit smaller and higher-density mount designs while also saving space. Additionally, they streamline the entire assembly process and improve PCB reliability. Below there is Flex PCB benifits:

  • Enhanced product looks.
  • less inductance cabling is necessary.
  • Increased system reliability.
  • Errors in assembly are decreased.
  • Circuits can be modified.
  • reduced expenses for assembly.
  • Reduced time for assembly.
  • Weights of packages are decreased.
  • Package sizes that are required are shrunk.
  • Airflow improvement inside the “box”
  • increased capacity to dissipate heat.


Disadvantages of Flexible Printed Circuit Board (flexi rigid pcb)


Although the FPC technology provides a number of significant benefits, it also has several limitations. First off, FPCs have a higher initial one-time cost than conventional rigid PCBs. The initial costs associated with circuit and prototype design are greater for flexible printed circuit  than for rigid PCBs since they are made for highly specialized purposes. It is preferable to employ the flexible printed circuit technology only for production volumes that are not too low if cost is a deciding factor in the type of PCB selection.


The difficulty of altering or fixing the PCB when it has to be reworked is another drawback. In this instance, it is actually required to carry out the intervention first, take off the protective film that covers the circuit, and then put it back in place. Since flexible printed circuits are a relatively new technology, not all manufacturers are able to provide their clients this kind of product. Furthermore, the circuit might be readily damaged if handled incorrectly or by unauthorized persons, so extreme caution must be used during the assembling process.

If you want to knw difference between Flexible Printed Circuit Board and Rigid PCB board, you can watch below video.

Flexible Printed Circuit Material


Flex circuit cores are made of a flexible polymer, unlike the majority of conventional PCBs, which have a fiberglass or metal basis. The substrate for most flex PCBs is a polyimide (PI) film. Although it remains flexible after thermosetting, PI film does not soften when heated. When heated, many thermosetting resins, including PI, become rigid, making PI a better material for flex PCB assembly. Standard PI film is not well resistant to moisture and rips, but upgrading PI film reduces these problems.


In order for the layers of a flex PCB to adhere, it also needs an adhesive or unique base material. Prior to now, manufacturers exclusively used adhesives, however this practice decreased the PCB’s dependability. They created adhesiveless PI, which adheres to copper without the use of an adhesive, to address these problems. Thinner designs are possible with less chance of through fracture because to this material. Manufacturers utilize a coverlay film also made with PI to cover and shield flex circuits instead of a solder mask. The manufacturer can laminate a stiffer to that piece of the flex pcb if you want it to be firm, but the signal cannot pass through both the flex and the stiffer.


Flexible Printed Circuit Types

  • Single-sided flexible printed circuit
  • Double-sided or back-bared flexible printed circuit
  • Sculptured flexible printed circuit
  • Multilayer flex circuits
  • Rigid-flex circuits

Flexible Printed Circuit Manufacturing Process

Flexible Printed Circuit Manufacturing Process words guideline, you can watch below video, it would help you under stand the Flex PCB fabrication process better.

1. Material Preparation

To ensure appropriate film adherence, production panels are chemically cleaned before circuit forming photo resist film is applied. Conveyorized processes with thin core handling tools are used to prevent damage to the cores of extremely thin material.

2. Circuit Pattern Exposure

Circuit artwork patterns are applied on photo resist coated panels that have been exposed to collimated UV light in order to transfer the circuit image(s) to manufacturing panels. If necessary, both sides can be exposed concurrently.

3. Etch Process

Using conveyorized systems with specialized thin core handling equipment, circuit designs were chemically etched. If necessary, panels can have both sides etched concurrently.

4. Drilling Process

Drilling systems with high speed, high precision, and small hole capability make the necessary circuit hole patterns in manufacturing panels. Available laser-based devices can be used to create extremely small holes.

5. Copper Plating Process

To create layer-to-layer electrical interconnects, fully automated electrolytic copper plating methods deposit the necessary extra copper into plated through holes.

6. Applying Coverlay

Prior to the Coverlay lamination process, Polyimide Coverlays are positioned, aligned, and attached onto manufacturing panels.

7. Coverlay Lamination

To achieve adequate adherence, coverlays are laminated to production panels under pressure, heat, and vacuum.

8. Stiffener Application

Prior to the extra lamination process under heat, pressure, and vacuum, localized additional stiffeners (if necessary per specific design) are aligned and placed.

9. Electrical Test

100% netlist-driven electrical test in accordance with IPC-ET 652. testing for continuity and isolation of all circuits simultaneously. Systems for grid and flying probe tests are both used.

10. Final Fabrication

Individual parts are die cut with high precision male and female punch and die sets from production panels. Depending on the requirements of the design, further techniques include laser cutting, mechanical routing, steel rule dies, and chemically milled dies.


11. Flexible Printed Circuit SMT Assembly


11.1 Flexible Printed Circuit (flexi rigid pcb) Fixed


It is essential to precisely fix the FPC on the carrier board before beginning SMT. It is significant to note that after the FPC is fastened to the carrier, the shorter the storage period between printing, placement, and pcb soldering, the better. The carrier board lacks positioning pins and contains two different types of positioning pins. Use the positioning template with the positioning pin in conjunction with the carrier board without the positioning pin. The carrier board is positioned on the template’s positioning pin first, exposing the pin through the carrier board’s positioning hole when each flexible printed circuit (flexi rigid pcb) is added one at a time.


The carrier is then detached from the FPC positioning template for printing, placement, and soldering after the exposed locating pins have been taped. With the positioning pins, several spring positioning pins with a length of approximately 1.5 mm have been fastened to the carrier plate. The carrier plate’s spring positioning pins can accept the flexible printed circuit directly, which can then be secured using tape. The steel mesh used during printing may fully compress the spring positioning pin into the carrier without compromising the print quality.


11.2 Flexible Printed Circuit Solder Paste Printing


There are no specific specifications for the solder paste’s composition from FPC. The fine pitch IC on the FPC determines the solder ball’s size and metal composition. The solder paste must have exceptional qualities because the FPC has stringent standards for printing performance. Without issues with poor mold release, clogging of the steel mesh, or collapse after printing, thixotropy, solder paste should be able to print off the mold easily and firmly stick to the surface of the flexible printed circuit(flexi rigid pcb).

The printed surface of the FPC cannot be as flat as the PCB and the thickness is the same because it is loaded on the carrier and has a high temperature resistant tape for positioning. Therefore, it is not appropriate to use a metal scraper; instead, a scraper with a hardness of 80 to 90 degrees polyurethane should be used. The printing quality will be significantly impacted if the solder paste printing equipment does not have an optical positioning system.

Even though the FPC is connected to the carrier board, there will always be a tiny space between them, which is difficult with a PCB. The configuration of the device settings will have a significant impact on the printing effect due to the biggest variance between the boards.

The crucial station for keeping the flexible printed circuit clean is the printing station. The wearing of finger cots is required. To stop solder paste from contaminating the gold fingers and gold-plated buttons on the FPC, it is also vital to keep the station and steel mesh clean.

11.3 Flexible Printed Circuit Patch

It can be mounted by medium and high speed placement machines depending on the features of the product, the amount of components, and the effectiveness of the placement. The placement of SMDs on FPC and PCB varied little from one another because each piece of FPC has an optical MARK mark for positioning. Although the FPC is fixed to the carrier board, it should be noted that the surface could not be as flat as the PCB hard board.


The flexible printed circuit (flexi rigid pcb) and carrier board must be partially spaced apart. Therefore, it is necessary to correctly set the nozzle drop height, blowing pressure, etc., and to slow down the nozzle movement. At the same time, FPC is mostly used in boards and has a low yield. As a result, it is common for the PNL as a whole to have some poor PCS. The BAD MARK recognition feature on the placement machine is a must for this. Otherwise, when making such non-standard products If PNL is a strong board, the production efficiency will be significantly decreased.


11.4 Flexible Printed Circuit Reflow Soldering


It is recommended to utilize a forced hot convection infrared reflow oven so that the temperature on the FPC may be changed more uniformly, lowering the likelihood of subpar soldering.


The types of electronic components on the FPC vary because of the carrier’s various endothermic characteristics. During the reflow process, the temperature rises and the heat changes. As a result, the reflow oven’s temperature profile is carefully calibrated. large influence Putting two FPC-loaded carrier plates in front of and behind the test board in accordance with the actual board spacing used during production is a safer approach. In parallel, components are attached on the test carrier board’s FPC, and high-temperature solder wire is used to test the temperature.


While the probe leads are taped to the carrier with high temperature resistant tape, the probe is soldered to the test point. Keep in mind that the test spots are not covered by the high temperature tape. On either side of the carrier board, choose test spots close to QFP pins and solder joints. The test results are more accurate depictions of the actual circumstance.


Because the temperature uniformity of the flexible printed circuit (flexi rigid pcb) is poor, it is preferable to adopt the heating/heating/reflow temperature profile in furnace temperature debugging. This way, the parameters of each temperature zone are simple to manage and the FPC and components are less susceptible to thermal shock. some. Experience has taught us that the furnace temperature should be set to the minimum level necessary to meet the technical specifications for the solder paste. The reflow furnace typically operates at the lowest wind speed that it is capable of. There is no jitter and the reflow oven chain has good stability.


11.5 Flexible Printed Circuit Inspection, Iesting


It is preferable to install a forced cooling fan at the outlet because the carrier absorbs heat in the furnace, especially the aluminum carrier, so the temperature is higher when it is discharged. The operator must wear insulated gloves at all times to prevent burns from the high temperature carrier. The force should be uniform when removing the FPC that completes the welding from the carrier, and brute force should not be employed to keep the FPC from being torn or crumpled.


Under a magnifying lens with a magnification of five times or more, the removed FPC is visually examined with a focus on any surface residue of glue, discolouration, gold finger dipping tin, tin beads, IC pin blank welding, and soldering. The FPC is normally not suited for AOI inspection since its surface is not very flat, leading to a high false positive rate. However, with a special test fixture, the flexible printed circuit (flexi rigid pcb) can successfully complete the ICT and FCT tests.


Before testing for ICT and FCT, while the flexible printed circuit is mostly linked, it might be essential to complete the sub-board. Blades and scissors can be used to finish the tool, however doing so results in poor work efficiency and quality, as well as a high scrap rate. Making a specific FPC stamping and splitting die for stamping and splitting is advised if shaped flexible printed circuit is being produced in bulk because it can significantly increase operating efficiency. While the cutting board was being punched, very little internal stress was created, and the edge of the flexible printed circuit (flexi rigid pcb) was neat and lovely. It can successfully prevent cracked solder joints.


Flexible Printed Circuit Price


Different design of flexible printed circuit price may be far different from 0.5usd/pieces to 600USD/Pieces. Its price is so different, just become its price can be affected by a lot of factors such as layers, size, finishing request, test request etc. so if you want to know your new design flexible printed circuit price. You can contact us, we are flexible printed circuit manufacturer established in 2000 with 22 years of experience of manufacturing flexible printed circuit board. We can make high quality flex PCB at competitive price for you.