Introduction of FPC
Flexible Printed Circuit (FPC) is a circuit board made of flexible copper clad laminate as the base material, which is used as a signal transmission medium for the connection of electronic products, and has the characteristics of high wiring assembly density, good bending ability, light weight and flexible process. FPC can generally be divided into single-layer FPC, double-layer FPC, multi-layer FPC and rigid-flex PCB.
Features of FPC
● High wiring and assembly density, eliminating the need for redundant cable connections.
● Good bendability, high softness and high reliability
● Small size, light weight and thin thickness
● Circuits can be set to increase wiring layers and flexibility
● Simple structure and easy installation
The types of FPC
● Single-layer FPC
It has a chemically etched conductive pattern, and the conductive pattern layer on the surface of the flexible insulating substrate is calendered copper foil.
● Double-layer FPC
On each side of the insulating base film, there is a conductive pattern made of morning wonton engraving, which increases the unit area of the cloth thread.
● Multilayer FPC
The single-sided or double-sided flexible circuit of 3 or more layers is laminated together, and metallized holes are formed by drilling and plating, and conductive paths are formed between different layers, which has the advantage that the substrate film is light in weight and has excellent electrical characteristics, such as low permittivity. But it loses the excellent flexibility of single-sided, double-sided flexible PCB.
Rigid-flex PCB
The combination of FPC and PCB, the production of rigid-flex board should have both FPC production equipment and PCB production equipment.
Automotive wiring harness systems
For automobiles, whether it is fuel vehicles or intelligent vehicles, there are a large number of FPC applications, mainly in the automotive electronics sector. Automotive electronics is a general term for automotive electronic control devices and automotive electronic control devices and mainly includes engine control system, chassis control system and automotive electronic control system.
From the consideration of structure, space, etc., the future new energy vehicle will definitely use a large number of FPC instead of wiring harnesses, will be applied in many parts of the vehicle to achieve, so FPC technology in automotive electronics, especially intelligent vehicles is a very important trend, especially in battery BMS, vehicle lighting systems, door control systems, camera modules, etc.
Generally, there will be more than 100 FPC applications on an electric vehicle, among which the FPC application value in the battery BMS and the vehicle camera module is the highest, and it is also a key development area. The FPC of BMS is used in the battery, and cost and space are two important issues for the battery.
As far as the current technology is concerned, the capacity of the battery is basically to the extreme, and the size of the battery pack is almost fixed, so there is actually a limit to how many batteries can be loaded in the PACK. At this time, maximizing the use of space and improving efficiency has become an urgent problem.
The use of FPC instead of traditional BMS wiring not only ensures stable performance, but also reduces the risk of friction on the top cover caused by breathing, which is also the mainstream practice of various companies. Even in the long run, the circuit of the motherboard and slave board can be replaced by chips, and the chips can be installed on FPCs, which can maximize product stability, save space and reduce costs.
Application of FPC in new energy vehicles
The acquisition line is an important component required for the BMS system of new energy vehicles, which can monitor the voltage and temperature of the new energy power battery cells; Connect data acquisition and transmission with overcurrent protection function; Protect the car power battery cell, automatic disconnection of abnormal short circuit and other functions.
Previously, the power battery acquisition line of new energy vehicle adopted the traditional copper wire harness scheme. The conventional wire harness was made of plastic surrounded by copper wires, and each harness reached an electrode when connected to the battery pack. When there are many current signals in the power battery pack, many harnesses are required to cooperate, and the space is squeezed out.
In the pack assembly process, traditional wiring harnesses rely on workers to manually fix the ports to the battery pack, and the degree of automation is low. Compared with copper wire harnesses, FPC has outstanding advantages in safety, lightweight, and regular layout due to its high integration, ultra-thin thickness, ultra-softness and other characteristics.
In addition, the thickness of FPC is thin, the battery pack structure is customized, and FPC can be directly placed on the battery pack through the robotic arm during assembly, with a high degree of automation, which is suitable for large-scale mass production, and the trend of FPC replacing copper wire harnesses is clear.
Advantages of FPC in power battery modules
1. Highly integrated: self-embedded Fuse, connector, chip NTC, aluminum/nickel terminal. It not only provides excellent and consistent electrical performance, but also meets the design needs of smaller and higher density installations. The three-degree space wiring and the appearance can be changed according to the limitations of space, which is suitable for the development of high density, miniaturization and high reliability, so as to achieve the integration of component assembly and wire connection.
2. Automatic assembly can be realized: fast and accurate assembly, which is conducive to automation. In assembly and fault tolerance, many manual errors in the design of wiring harnesses can be avoided, and many opportunities for insertion errors can be reduced at the connector level.
Using FPC sampling, the complexity of the module integration process can be reduced, and the connection between FPC and battery busbar can realize automatic welding, effectively reducing labor costs. Even if the customer cannot mature to realize automatic welding, the traditional screw locking method can still effectively reduce the labor input.
3. Ultra-thin thickness: 0.34mm in the line area, 2mm at NTC.
4. Super softness: the line area can be bent and assembled at 90° and 180°.
5. Lightweight: When the whole vehicle is used, the weight can be reduced by about 1kg compared with the wiring harness solution.
6. Cost advantage: From the cost point of view, the cost of FPC itself is not high, and it is a great reduction in connection costs.
Compared with copper wire harnesses, FPC has outstanding advantages in safety, lightweight, and regular layout due to its high integration, ultra-thin thickness, ultra-softness and other characteristics.
In addition, the thickness of FPC is thin, and the FPC can be grabbed by the robotic arm and placed directly on the battery pack when the battery pack structure is customized and assembled, which has a high degree of automation and is suitable for large-scale mass production, and the trend of FPC replacing copper wire harnesses is clear.
At present, FPC solutions have become the most important choice for most new energy vehicle models. FPC is integrated into CCS (Cells Contact System).
CCS products are composed of FPC, plastic structural parts, copper and aluminum rows, etc. The copper and aluminum busbar connects multiple cells in series and parallel by laser welding, and the FPC is connected with the copper and aluminum busbar and plastic structural parts to form an electrical connection and signal detection structural component.
Materials of FPC
Insulating film
The insulating film forms the base layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In a multi-layer design, it is bonded to the inner layer. They are also used as a protective cover to keep the circuit insulated from dust and moisture, and to be able to reduce stress during deflection, where the copper foil forms a conductive layer.
There are many types of insulating film materials, but the most commonly used are polycumimine and polyester materials. At present, nearly 80% of all flexible circuit manufacturers in the United States use polyimide film materials, and about 20% use polyester film materials.
Polyimide materials are non-flammable, geometrically stable, have high tear strength, and have the ability to withstand welding temperatures.
Polyester (Polyethyleneterephthalate or PET), whose physical properties are similar to polyimide, has a lower dielectric constant, absorbs little moisture, but is not resistant to high temperatures.
Polyesters have a melting point of 250°C and a glass transition temperature (Tg) of 80°C, which limits their use in applications that require extensive end welding. In low-temperature applications, they exhibit rigidity. Nevertheless, they are suitable for use in products such as telephones and other products that do not need to be exposed to harsh environments.
Polyimide insulating films are usually combined with polyimide or acrylic adhesives, and polyester insulating materials are generally combined with polyester adhesives. The advantage of combining with a material with the same characteristics is that it can be dimensionally stable after dry welding or after several lamination cycles. Other important properties in adhesives are low dielectric constant, high insulation resistance, high glass transition temperature and low moisture absorption.
Conductor
In some flexible circuits, rigid components made of aluminum or stainless steel are used, which provide dimensional stability, physical support for the placement of components and wires, and stress relief. Adhesives bond rigid components and flexible circuits.
Another material that is sometimes used in flexible circuits is the adhesive layer, which is formed by coating both sides of the insulating film with an adhesive. Bonded laminates provide environmental protection and electronic insulation, as well as the ability to eliminate a layer of film, as well as the ability to bond multiple layers with fewer layers.
Copper foil is suitable for use in flexible circuits, it can use Electrodeposited (ED). Copper foil with electrodeposited has a glossy surface on one side, while the surface being processed on the other side is dull and dull. It is a material with compliance and can be made into many thicknesses and widths, and the matte side of ED copper foil is often specially treated to improve its adhesion. In addition to its flexibility, forged copper foil also has the characteristics of hard smoothness, which is suitable for applications where dynamic deflection is required.
Adhesive
In addition to bonding insulating films to conductive materials, adhesives can also be used as overlays as protective coatings as well as covering coatings. The main difference between the two is the application method used— overlay bonding covering insulating film is to form a circuit with a laminated structure, and screen printing technology used for overlay coating of adhesives.
Not all stacked structures contain adhesives, and stacks without adhesives create thinner circuits and greater compliance. It has better thermal conductivity than adhesive-based lamination construction. Due to the thin structure of adhesive-free flexible circuits and the improved thermal conductivity due to the elimination of the thermal resistance of the adhesive, it can be used in working environments where flexible circuits based on adhesive stacked structures cannot be used.
Structure of FPC
Copper Film
Copper foil: basically divided into electrolytic copper and RA copper (Rolled & Annealed) copper. Common thicknesses are 1oz 1/2oz and 1/3 oz
Substrate film: Common thicknesses are 1mil and 1/2mil.
Adhesive: The thickness is determined according to customer requirements.
Cover Film
Covering film: for surface insulation. Common thicknesses are 1mil and 1/2mil.
Adhesive: The thickness is determined according to customer requirements.
Release paper: avoid adhering foreign matter before pressing; Easy to work.
PI Stiffener Film
Reinforcement film: Strengthen the mechanical strength of FPC, convenient for surface mounting operations. Common thicknesses range from 3mil to 9mil.
Adhesive: The thickness is determined according to customer requirements.
Release paper: Avoid adhering foreign matter before pressing.
EMI: electromagnetic shielding film protects the lines in the circuit board from external interference (strong electromagnetic area or vulnerable to interference).
Production process of the FPC
Single-sided FPC production process
Double-sided FPC production process
Conclusion
The wiring harness system of traditional cars is complex and messy, multi-interface to the vehicle electronic system brings complex and high costs, in the era of intelligent cars, the algorithm ability of vehicle computers has far exceeded the current connection technology, and a large number of automated applications of FPC on the battery are only a matter of time. In the long run, many low-voltage wiring harness units in vehicles are likely to be gradually replaced with technological upgrades, especially in the future, vehicles are becoming more and more intelligent. An era in which the requirements for signal immunity are becoming more and more stringent.
Flexible Printed Circuit (FPC) is a circuit board made of flexible copper clad laminate as the base material
1. Highly integrated
2. Automatic assembly can be realized
3. Ultra-thin thickness
4. Super softness
5. Lightweight
6. Cost advantage
Insulating film
Conductor
Adhesive
