What is flexible printed circuit board (PCB)?

Flexible printed circuit board (FPC), also known as soft circuit board and deflection circuit board, is very popular with excellent characteristics such as light quality, thin thickness, and free bending folding. It mainly relies on artificial visual inspection, with high cost and low efficiency. With the rapid development of the electronics industry, circuit board design has become more and more high-precision and high-density. Traditional artificial testing methods can no longer meet production needs. FPC defect automation testing has become an inevitable trend of industrial development.

Flexible printed circuit boards have physical flexibility. These PCBs are widely used in mechanical sensitive equipment because flexibility allows circuit boards to resist vibration. Flexible PCBs are more durable than traditional circuit boards, but the flexible PCB manufacturing process is sensitive and complex.

Flexible PCB type

1. Single flexible
As the simplest type, a single-sided flexible circuit has a single conductive layer on one side of the media layer (polyimide or polymer thin film).

2. Both sides flexible
The flexible double-sided PCB has a conductive copper layer on each side of the dielectric material. Metal-tonged holes are connected to two copper edges.

3. Multi -layer elasticity
This type comprises a plurality of copper layers separated and packaged by a dielectric layer. Connect each metal layer to the hole.

4. HDI Flexible PCB
High density interconnection (HDI) circuit is an efficient and reliable circuit board with multiple electronic components. Due to thin, non-conductive substrate materials, they provide better electrical performance with smaller packaging sizes.

5. The flexible circuit carved
The flexible circuit has a circuit trace line with different thicknesses throughout the circuit design. Donc, they may have different copper thickness in certain areas.

6. Polymer thick film flexible PCB
Polymeric thick film is a simple and economical method of producing reliable FPCB by using templates to print through silk mesh. You can find this kind of PCB in low pressure applications, because their functions are not enough to process high pressure.

7. Dual channel/backplane flexible PCB
This is a special single-sided PCB that can be accessed from any side of the circuit board.

8. Single -layer flexible PCB
Single-layer FPCB has a simple layer, including a flexible base layer (usually polytamide), then the adhesive layer, and then copper. The protective layer (coating layer) of the adhesive and polyimide is covered with metal. Cependant, the conductive portion may not be exposed.

9. Dual channel/back panel FPCB
Just like a single-layer circuit, there are five layers of this circuit, but there is a difference. The outer layer of the polyamide on the base has a laser opening that can access the copper circuit.

10. Double -layer soft board
PCB uses a flexible layer as a base to separate the conductive layer. Each metal layer has a protective circuit covering layer and copper plating holes are connected to both sides.

11. Multi -layer flexible PCB
The multilayer flexible circuit combines a single-sided and double-sided flexible circuit design. Inside, the middle part has a double-sided flexible circuit, and the surface has a single-sided conductive design covered with a protective film.

Flexible Conception de PCB skills

1. Understand the working principle of flexible circuit

Understanding the types and functions and applications of flexible circuits will provide guidance for their use in design. It may be important to know what does not work. If possible, contact the famous manufacturer of flexible circuits for guidance on material characteristics and limitations.

2. Create a flexible circuit model

The best way to determine the feasibility of the design is to create a physically flexible circuit model. This involves a system point for electrical connection via the deflection circuit and end connection method (such as connector, pin, or ZIF). Suivant, determine a similar circuit “empreinte”, which will provide conductor wiring for each terminal position. See the detailed information in the schematic or grid table and the special electrical requirements (such as the flat layer) to determine the approximate number of layers. Check the sampling circuit with similar layers to see if the proposed design will provide sufficient flexibility. If no sample circuit is available, free samples can be obtained from flexible circuit manufacturers.

Then check the mechanical requirements to ensure that the bending radius falls within the acceptable value of the circuit thickness and number of layers. For acceptable bending radius criteria, see IPC-2223. Construct the “paper doll” outline with thick paper to construct a flexible circuit and check if it is appropriate. Modify as needed. Continue to build a paper prototype and modify it until it is suitable. Finally, use 0.010 inches. (0.25 MM) Polyester film to rebuild the prototype to make representative models. Install it in the prototype housing and adjust the size as needed.

3. Get mechanical sample

Before investing a lot of time and money to create a flexible circuit prototype, test mechanical samples to ensure that the flexible circuit has the correct shape and position. The shape refers to the physical size, shape and quality of the parts, and the fitting refers to its environmental interface. Mechanical samples help avoid installation problems or potential mechanical problems that can cause failure.

4. Minimize circuit cost

The main cost driver of the flexible circuit is the size, number of layers and characteristics of the overall circuit. The cost of the wire width, the size of the pad, and the size of the small hole. Follow the recommended tolerance as much as possible, and design no adhesive areas in absolutely necessary places. If there are no SMT components, or they are only on one side, consider using hardcock parts instead of rigid curved parts. The ribs can be much cheaper than the rigidity circuit. Use standard materials when possible.

5. Do not ignore the minimum bending radius

When the circuit is bent too sharply, there will be some problems. Compression on the inner diameter may cause wrinkles to cover the cover layer. Stretching on the outer bend may tear the covering material and destroy the conductor. Start mechanical design by establishing a curved radius. If the radius is at least ten times the thickness of the material, the circuit is likely to work reliably. In order to improve reliability, please reduce the overall thickness of the bending area so that it can be bent.

Selection of Flexible PCB Materials

1. Selection of copper foil substrate
Copper Clad Laminate ( CCL) consists of three layers of “copper foil+glue+substrate”. En outre, there is also a combination of glue substrates, c'est, le “copper foil+ substrate”, which is high in price and is suitable for products with a bending life of more than 100,000 fois.

(1) Copper Foil
Copper foil has two categories: Rolled Anneal Copper Foil and ElectrodePosited Copper Foil. Pressed copper has a good bending performance compared to electrolytic copper. There are four thicknesses: 1/3oz, 1/2oz, 1OZ, 2OZ.

(2) Substrate
Commonly used media for flexible circuits include polyamide (abbreviated PI, divided into glue and gum) and polyester (abbreviated PET). The price of PI is relatively high, but its combustion resistance is better. PET is low in price but not heat resistant. Donc, when there is a welding demand, most of them are made of PI. There are generally 1/2mil, 1mil, and 2mil in thickness.

(3) Adhesive
There are two commonly used glue for flexible boards: acrylic (Acrylique) and Epoxy. The most commonly used is epoxy resin glue. It is available from 0.4 à 2 mil thick, et 0.7 mil thick glue is generally used.

2. Selection of covering membrane

The coating film is composed of “substrate+glue”. Its substrate is divided into two types: PI and PET, and the thickness includes 0.5~1.4mil.

3. The choice of reinforcement materials

Rôle: In order to weld the parts on the soft plate, the local area increased the reinforcement materials for installation to compensate for its soft plate thickness.

Material: PI/PET/FR4/SUS

Flexible PCB production process

1. Double -panel processing process

Open → Drilling → PTH → Electroplating → Pre -processing → Dry Film → Pickup → Exposure → Graph Electroplating → Dalf Meeting → Putting Dry Film → Displaying Disposal → Etancular → During Mask → Surface Treatment → Paste the cover film → suppress → curing → nickel gold → printing characters → cutting → electrical test → final inspection → packaging → shipment

2. Single -panel processing process

Open → Drilling → Patching dry film → Division → Exposure → Evidence → Decoration → Surface treatment → Patch covering film → curing → surface treatment → sinking gold → printing character → electrical test → rushing → rushing Cut → final inspection → packaging → shipment