In terms of manufacturing cost, if the PCB area is the same, the cost of multi-layer flexible circuit boards will be higher than that of single-layer and double-layer circuit boards. As the number of layers increases, the cost will continue to increase. In PCB design, if there are stacks of odd layers, the following method can be used to increase the number of layers.
Flex Pcb (Flexible Circuit Board), in terms of structure, can be divided into single-layer board, double-sided board, and multi-layer board. The structure of different boards is different, but the most basic structure is copper substrate + cover film. The most commonly used substrate copper is rolled copper and electrolytic copper, and the cover film is generally PI, that is, polyimide.
The structure of the single-sided board is single-sided substrate + cover film, that is, the circuit is made on the copper surface of the single-sided substrate, and the finished product is made after the film is covered; The structure of the double-sided board is a double-sided substrate (both sides are copper sides), and the lines are made on the front and back copper sides. post-processing;
The most common structure of a three-layer board is a double-sided substrate + a single-sided substrate, and the lines are made on the three-layer copper, and the lines of each layer are selectively conducted through the via holes, and then covered with a film;
As the name suggests, the four-layer board has four layers of circuits. Its structure is the inner layer (double-sided substrate) + the outer layer of the front and back (single-sided substrate). membrane. The structure of other multi-layer boards can be organized according to specific needs and designs, and can be sublimated from the structure of three or four-layer boards.
In addition, Flex Pcb often has structures such as pads and gold fingers, which generally need to be strengthened, that is, the FR4 or PI of an appropriate size is pressed on the back of these structures. The thickness is selected according to the specific use environment of the product.
Sometimes, Steel sheets are also used. Reinforcement can only be done on the surface of the Flex Pcb.
It is made of single-sided copper-clad laminate material. After the circuit is completed, it is covered with a protective film or coating to form a flexible circuit board with only a single-layer conductor.
Schematic diagram of single-sided Flex Pcb stack structure
After the double-sided circuit is completed using the double-sided copper-clad board material, a layer of protective film is added on both sides to become a circuit board with double-layer conductors.
Schematic diagram of ordinary double-sided flex pcb laminated structure
Using pure copper foil material in the circuit manufacturing process, a layer of protective film is added on both sides successively to form a circuit board with only a single layer of conductors but conductors exposed on both sides of the circuit board.
Two layers of single-sided copper-clad board materials are used in the middle to be laminated with an adhesive glue that opens a window at a specific position, and it becomes a double-sided conductor circuit board that is laminated in a local area and has a two-layer separation structure in the local area to achieve a layered area. High flex performance circuit board.
Using single-sided copper-clad board material and adhesive glue as the basic material, it adopts a process similar to that of the substrate to form a double-sided board. After multiple pressings, it becomes a circuit board with a multi-layer conductor structure. It can be designed as a local layered structure to achieve high performance. purpose of flexibility.
Rigid-flex printed boards are composed of rigid and flexible substrates selectively laminated together, with a tight structure and metallized holes to form conductive connections. Each rigid-flex printed board has one or more rigid areas and one or more flexible areas.
In the structure of flexible circuits, the constituent materials are insulating films, adhesives and conductors.
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 then bonded to the inner layers. They are also used as a protective cover to insulate the circuit from dust and moisture and to reduce stress during flexing, the copper foil forms the conductive layer. In some flexible circuits, rigid members formed of aluminum or stainless steel are used to provide dimensional stability, physical support for component and wire placement, and stress relief.
The adhesive bonds the rigid member and the flexible circuit together. Another material that is sometimes used in flexible circuits is the adhesive layer, which is formed by coating adhesive on both sides of an insulating film. Adhesive plies provide environmental protection and electrical insulation, as well as the ability to eliminate one film and have the ability to bond multiple layers with fewer layers.
There are many types of insulating film materials, but the most commonly used are polyimide 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 material is non-flammable, geometrically stable, has high tear strength, and has the ability to withstand welding temperature.
Polyester, also known as polyethylene terephthalate (Polyethylene terephthalate for short: PET), whose physical properties are similar to polyimide, has a low 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 to applications that require extensive end soldering. In low temperature applications, they appear rigid. 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 materials with the same properties is the ability to have dimensional stability after dry welding, or after multiple lamination cycles. Other important properties in adhesives are low dielectric constant, high insulation resistance, high glass transition temperature (Tg) and low moisture absorption.
In addition to being used to bond insulating films to conductive materials, the adhesive can also be used as a cover layer, as a protective coating, and as a cover coating. The main difference between the two is the method of application used, the cover layer is bonded to cover the insulating film in order to form the circuit of the laminated structure.
Screen printing technique for overlay application of adhesive. Not all laminates include adhesives, and laminates without adhesives result in thinner circuits and greater flexibility. It has better thermal conductivity than adhesive-based laminate construction. Due to the thin structural characteristics of adhesive-free flexible circuits and the improved thermal conductivity due to the elimination of thermal resistance of adhesives, it can be used in working environments where flexible circuits based on adhesive laminate structures cannot be used .
Copper foil is suitable for use in flexible circuits, it can be electrodeposited (Electrodeposited: ED), or plated. The surface of the electrodeposited copper foil is glossy on one side, while the processed surface on the other side is dull and dull. It is a compliant material and can be made in many thicknesses and widths. The matte side of ED copper foil is often specially treated to improve its bonding ability. In addition to its flexibility, wrought copper foil also has the characteristics of hard and smooth, which is suitable for applications that require dynamic deflection.
