This article mainly introduces flexible circuit boards (FPCs). First, it describes the characteristics, advantages, and disadvantages of FPCs. Second, it introduces the types of FPCs. Finally, it discusses whether FPCs can be repaired. Let's take a closer look.

What is a Flexible Circuit Board?
Flexible circuit boards, also known as flexible printed circuit boards (FPCs), are a type of circuit board that, compared to ordinary rigid resin circuit boards, have advantages such as high wiring density, light weight, thinness, less wiring space constraints, and high flexibility.

Characteristics of Flexible Circuit Boards
1. High flexibility, allowing for three-dimensional wiring and changing shape according to space constraints.
2. Resistant to high and low temperatures, and flame retardant.
3. Foldable without affecting signal transmission function, preventing electrostatic interference.
4. Chemically stable, with high stability and reliability.
5. Facilitates the design of related products, reduces assembly time and errors, and extends the service life of related products. 6. It reduces the size and weight of applied products, increases functionality, and lowers costs.

Advantages and Disadvantages of Flexible Printed Circuit Boards (FPCs)
Flexible printed circuit boards (FPCs) are printed circuits made of flexible insulating substrates, possessing many advantages that rigid printed circuit boards lack:
(1) They can be freely bent, rolled, and folded, arranged arbitrarily according to spatial layout requirements, and moved and stretched arbitrarily in three-dimensional space, thus achieving integrated component assembly and wire connection;

(2) FPCs can significantly reduce the size and weight of electronic products, meeting the needs of electronic products developing towards high density, miniaturization, and high reliability. Therefore, FPCs have been widely used in aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras, and other fields and products;

(3) FPCs also have good heat dissipation and solderability, as well as advantages such as easy assembly and lower overall cost. The combination of rigid and flexible design also compensates to some extent for the slight deficiency of flexible substrates in component load-bearing capacity. Disadvantages of FPC

(1) High initial cost: Because flexible PCBs are designed and manufactured for specific applications, the initial costs for circuit design, wiring, and photolithography are high. Unless there is a special need for flexible PCBs, it is generally best not to use them for small-scale applications.

(2) Difficult to modify and repair flexible PCBs: Once a flexible PCB is manufactured, any modifications must begin from the original drawing or a pre-written photoplotting program, making changes difficult. Its surface is covered with a protective film, which must be removed before repair and restored after repair, making this a challenging task.

(3) Size limitations: Since flexible PCBs are not yet widely used, they are usually manufactured using intermittent processes, thus limited by the size of production equipment, preventing them from being made very long or wide.

(4) Prone to damage due to improper handling: Improper handling by assembly personnel can easily damage flexible circuits. Soldering and rework require trained personnel. Types of Flexible Circuit Boards

1. Single-sided board
Using single-sided PI copper-clad board material, after the circuit is completed, a protective film is added to form a flexible circuit board with only a single layer of conductor.

2. Ordinary double-sided board
Using double-sided PI copper-clad board material, after the double-sided circuit is completed, a protective film is added to both sides, resulting in a circuit board with double-layer conductors.

3. Single-sided board with substrate fabrication
Using pure copper foil material, a protective film is added to both sides during the circuit manufacturing process, resulting in a circuit board with only a single layer of conductor, but conductors are exposed on both sides.

4. Double-sided board with substrate fabrication
Using two layers of single-sided PI copper-clad board material with adhesive that has windows in specific locations for lamination, resulting in a double-sided conductor circuit board with a structure where the two layers are locally laminated and locally separated, achieving high flexibility in the delamination areas.

Can Flexible Circuit Boards be Repaired?
Flexible circuit boards can be repaired. We will explain in detail the repair of a broken circuit in a keyboard FPC flexible circuit board. 1. After removing 21 screws and uncovering the keyboard, a clean white flexible circuit board is revealed, along with a small green main control circuit board in the upper left corner. It is said that the keyboard's flexible circuit board (FPC) is made of polyester film (PET, a type of polymer plastic film). It has three layers: the top and bottom layers are printed with circuitry using silver paste or silver foil, and the middle layer serves as insulation (separating the top and bottom layers to prevent short circuits; the holes at the top allow for a momentary short circuit between the two printed circuit boards when the keyboard is pressed, thus inputting the corresponding characters or commands).

2. Only a few keys are malfunctioning, indicating the keyboard's main control board is working properly. A quick glance at the bottom layer of the keyboard's flexible circuit board revealed that the malfunctioning keys were on the same circuit. Using a multimeter in diode/buzzer mode, tracing the FPC's circuit to the pin connecting to the main control board, the multimeter stopped beeping. Removing the main control board revealed that this pin was several millimeters burnt black (other pins were also blackened, but not as severely), and it was an open circuit.

3. On a regular PCB, an open circuit is easily fixed with a soldering iron. However, this is a flexible PCB, made of plastic film, so soldering won't work. A quick search online suggests drawing a few lines at the break with a pencil, but this won't last long. It's best to repair the keyboard's flexible circuit board with conductive silver paint. Baidu Baike states:

Conductive silver paint allows current to flow through unsoldered points, and is widely used for repairing open circuits (broken copper circuits) on circuit boards, automotive windshield heating wires, television remote controls, capacity switches, and audio/video tapes. It also dissipates static ions, reduces high-voltage contact resistance, seals plastic casings, and blocks high-frequency magnetic fields. It can also be used to repair flexible circuits on various plastic circuit boards, such as those in desktop and laptop keyboards.

Conductive silver paint generally has strong adhesion, adhering to plastics, wood, and interior walls. It dries quickly (approximately 3 to 24 hours), has low resistance and strong conductivity, and can adhere to any plastic fiberboard and metal. Before use, it must be thoroughly mixed. After mixing, it can be applied with a stiff-tipped brush or injected with a syringe. After drying, it can be sealed with Electrolube base film or contact protectant. The bottle cap must be tightly closed after use to prevent it from drying out completely.

A friend on Baidu Knows mentioned that it's important to note that the silver paint will slowly spread, so be careful not to apply too much (a little won't hurt). Domestic conductive silver paint takes approximately 12-24 hours to take effect (depending on temperature and humidity; imported paint takes 3-12 hours). You can also use a heat gun or hairdryer (temperature controlled at 40-50 degrees Celsius) to accelerate the process. Conductive silver paint has penetrating power, so it's best to place a piece of paper on the back of the printed membrane inside the keyboard; this can be removed after the paint dries. Also, conductive silver paint is not conductive normally; it only becomes conductive after it's completely dry.

4. Conductive silver paint seems quite expensive; 0.3 ml (3 grams) costs around 20-30 yuan. It doesn't seem worthwhile to buy it specifically to repair a broken keyboard; I need to find another way. I tried tracing it with a pencil; it conducts, but the connection isn't great. It's said that sticking a thin copper wire with transparent tape will work, and that's true! I didn't have any transparent tape, so I had to use double-sided tape. It took me ages to get it right, but after confirming there wouldn't be a short circuit, I checked with a multimeter—it worked! I connected it to the computer, and the eight keys are working perfectly now. From now on, I can use Ctrl+Alt+A to take screenshots, Ctrl+A to select all, and Alt+Tab to switch windows.