Flexible PCB wiring and manufacturing steps

The method required for flexible PCB design is slightly different from the rigid PCB. When designing the flexible board, you need to consider the outline, bending requirements, the best material selection, stacking, copper special placement, and cost factor.
The carefully designed flexible PCB weight is light, strong, durable, reliable and easy to install. Therefore, it is suitable for harsh applications such as aerospace, satellites, the Internet of Things, medical and wearable equipment. The flexible board has higher vibration resistance and high temperature resistance, making it more sustainable under harsh environmental conditions.

1. The bending of flexible PCB

Bending number
Before designing, we must generally understand the number of bending times of design flexible boards and the severity of bending. The number of curves determines that the circuit board plays a role in static or dynamic.
The static board design is less than 100 times in its service life, and it is considered to be bending the dynamic board. It needs to bear thousands of bending, which is suitable for applications without failure space.

Bending radius
The curved radius is also known as the minimum bending amount of the circuit flexible area, which will ensure that the circuit can adapt to a certain degree of bending without affecting his structural integrity.
In terms of, the minimum curved radius is ten times thick. For one to two layers, the bending radius of the flexible PCB can be 6 times the thickness of the final layer. For 3 or more layers, the curved radius can be 12 times
In dynamic bending applications, copper can be placed along the neutral axis of the curved radius, and the neutral shaft is neither under pressure nor tension. The flexible PCB bending radius in dynamic flexibility can be 100 times the thickness of the final layer.

2. Wiring and layout of flexible PCB

Consider the placement of preliminary components (SMT or Th) and determine whether these components need to be strengthened. The circuit layout determines the success or failure of PCB. Important layout design and wiring precautions during flexible PCB design include:
1. The large curved radius is better than the sharp corner, because the sharp corner will shorten the service life of the circuit board.
2. Bending wiring is lower than that of angle wiring.
3. The wiring of the wire should also be vertically bending vertically. This eliminates the stress point that may cause trace copper to break
4. There are two or multi -layer flexible boards on the top and bottom staggered line. This will avoid the workbench beams caused by stacking copper traces, which will minimize the stress that may damage the 4 copper circuit that may damage the 4 copper circuit.
5. Use a tear protective device in the circuit, strengthen the flexible material along the interior bending radius and eliminate tear.
6. Traces are gradually transitioned from width to narrow, and traces gradually become thinner from thick to thin
7. Avoid non -continuity such as excessive hole, incision, slit, and holes in the bending area. The bending hole will produce stress and cracks, which eventually leads to faults and faults.
8. According to the PC standard, the incision/slit is terminated in the flexible area, and a circular cross section (release hole) is carried to prevent the corner from cracking. The radius should be greater than 0.75mm.
9. Enhanced reinforcement in areas that are prone to overly stress. Use anchor and bone spurs encapsulated with a covered layer to avoid tilting lines and pads peeling off.
10. Always keep the distance between the special copper characteristics, perforated and plating holes with the transition area of PCB with PCB. The region between the flexible part of the rigid and rigid parts is called the transition area.
11. Flexible materials are prone to exercise and contraction during the manufacturing process. This makes copper drilling a key factor when designing flexible boards. The distance from the drill to copper is always at least 8 dense ear.
12. Understand whether the plating of the surface of the plate or only pad -only pad (button electroplating) is more suitable for your flexible circuit. In the buttons, the copper is deposited only on the hole/pad. Due to the small amount of copper, the buttons provide greater flexibility, which allows manufacturers to control the thickness of copper and increase the etching production of small etching patterns. Because the conductor traces have consistent copper thickness, width and spacing, it also helps control impedance at a higher speed. Because additional processing steps are required, the price is expensive.

The three major manufacturing steps of flexible PCB

Step 1: assembled flexible PCB
This is the initial stage, and the main focus is on saving basic materials. The main materials for flexible circuits are polyimide. Compared with FR-4, this material is expensive and needs to be used correctly. In order to properly use polytide, you need to use nested technology to make the circuit get closer as possible. The prototype PCB manufacturing includes the following process:

Cycle: It is acceptable to add a small amount of additional materials that exceed the designer. This extra material is called a service circuit, which can realize the service length and circuit component.

Wire size: It provides the greatest flexibility, so you need to choose the thinnest copper, especially when you use the circuit for dynamic application.

Eclipse: Complete this process to compensate for any different oriental losses in the manufacturing process. In this process, the line width loss is almost twice the thickness of the copper foil. There are several factors that affect the width of the line, such as different types of copper, etching mask, conductivity, etc.

Routing: The route of the conductor can be easily completed. Just in the vertical position of bending and folding. This will improve folding and bending by reducing stress.

Grounding plane: If the electrical distribution is sufficient, create the ground area of the cross -film line. This helps improve the flexibility of the circuit by reducing the weight of the circuit board.

Step 2: Flexible printing circuit board (PCB) manufacturing process

Now, let’s pay attention to the board of directors. It starts with conductive spacing and width. The polymer film requires a standard conductor width, that is, 375 microns. At the same time, nominal polymer thick film and silver -based polymer membrane carry the percentage of circuit current required. The diameter of the hole in the flexible PCB may vary from design and application.

The size of the hole: The manufacturer can create a small hole or the PCB layout flexibly. With advanced technology, you can make the hole as small as possible (that is, 25 μm).

Corporal: rounded angle is a technology that can multiply with pads and divides stress. All pad pads and pads on the flexible circuit require round corners. The electroplated hole is suitable for forming reliable solder joints.

Buttons: Here, you can create alternative electroplating holes. Today, the manufacturer uses copper to prepare through the holes and holes.

Step 3: Focus on physical constraints
In the process, the manufacturer handles the coverage layer and coverage. We bring you some common coverage layers used in the process:

Back glue film: suitable for dynamic flexible circuit applications because it consists of raw materials. Back glue film is mainly used to cover custom PCB.

The outer coating of liquid printed on the silk mesh: The outer coating of the liquid printed on the silk mesh is convenient for carriers and is usually used with polymer film.

A sensor and film polymer: This is the most advanced outer coating method, with some surprising features, such as:

It acts as a welded layer and prevents welds from forming circuit traces.
It protects the printing circuit from external and internal damage.
It can prevent the circuit from charging from the outside.