Electronic Design - Flexible circuit board production process and PCB design software application

Flexible circuit board production process and PCB design software application

Flexible printed circuit (Flexible Printed Circuit) can be freely bent, wound, and folded. The flexible circuit board is processed by using polyimide film as the base material, and is also called soft board or FPC in the industry. Flexible circuit board is based on The number of layers is different, the process flow is divided into double-sided flexible circuit board process flow, multi-layer flexible circuit board process flow. The FPC soft board can withstand millions of dynamic bending without damaging the wires. It can be arranged arbitrarily according to the space layout requirements, and can be moved and stretched arbitrarily in three-dimensional space, so as to achieve the integration of component assembly and wire connection; flexible circuit boards can be The volume and weight of electronic products are greatly reduced, and it is suitable for the development of electronic products in the direction of high density, miniaturization and high reliability.

As more and more mobile phones adopt the flip structure, flexible circuit boards are also increasingly adopted.
According to the combination of base material and copper foil, flexible circuit boards can be divided into two types:
There are adhesive flexible boards and non-adhesive flexible boards.
Among them, the price of the glueless flexible board is much higher than that of the glued flexible board, but its flexibility, the bonding force of the copper foil and the substrate, and the flatness of the pad are also better than the glued flexible board. Therefore, it is generally only used for high-demand occasions, such as: COF (CHIP ON FLEX, bare chip mounted on flexible board, high requirements for flatness of the pad) and so on.
Due to its high price, most of the flexible boards currently used in the market are still flexible boards with glue.
Next we will introduce and discuss flexible boards with glue. Since the flexible board is mainly used in occasions that need to be bent, if the design or process is unreasonable, defects such as micro-cracks and open welding are likely to occur. The following is about the structure of the flexible circuit board and its special requirements in design and technology.

The structure of the flexible board
According to the number of layers of conductive copper foil, it is divided into single-layer boards, double-layer boards, multi-layer boards, and double-sided boards.
Single-layer board structure: The flexible board of this structure is the simplest flexible board. Usually the base material + transparent glue + copper foil is a set of purchased raw materials, and the protective film + transparent glue is another purchased raw material. First, the copper foil needs to be etched and other processes to obtain the required circuits, and the protective film needs to be drilled to expose the corresponding pads. After cleaning, use the rolling method to combine the two. Then the exposed pad part is electroplated with gold or tin for protection. In this way, the slab is ready. Generally, small circuit boards of corresponding shapes are also stamped.
There are also solder masks directly printed on the copper foil without a protective film, so the cost will be lower, but the mechanical strength of the circuit board will be worse. Unless the strength requirements are not high but the price needs to be as low as possible, it is best to apply a protective film method.
Double-layer board structure: When the circuit is too complicated, single-layer board cannot be wired, or copper foil is needed for grounding shielding, double-layer board or even multi-layer board is required.
The most typical difference between a multi-layer board and a single-layer board is the addition of a via structure to connect each layer of copper foil. The first processing technology of general substrate + transparent glue + copper foil is to make vias. First drill holes on the substrate and copper foil, and then plate a certain thickness of copper after cleaning, and the vias are completed. The subsequent production process is almost the same as the single-layer board.
Double-sided board structure: There are pads on both sides of the double-sided board, which are mainly used for connection with other circuit boards. Although it is similar to the single-layer board structure, the manufacturing process is very different. Its raw material is copper foil, protective film + transparent glue. First, drill holes on the protective film according to the requirements of the pad position, and then paste the copper foil to corrode the pads and leads, and then paste another drilled protective film.
Material performance and selection method
(1) Substrate:
The material is polyimide (POLYMIDE), which is a high-temperature resistant, high-strength polymer material. It is a polymer material invented by DuPont, and the polyimide produced by DuPont is called KAPTON. You can also buy some polyimides produced in Japan at a lower price than DuPont.
It can withstand a temperature of 400 degrees Celsius for 10 seconds, and its tensile strength is 15,000-30,000 PSI.
The 25μm thick substrate is the cheapest and the most common application. If the circuit board needs to be harder, a 50μm substrate should be used. Conversely, if the circuit board needs to be softer, a 13μm substrate is used.
(2) Transparent glue of base material:
It is divided into two types: epoxy resin and polyethylene, both of which are thermosetting adhesives. The strength of polyethylene is relatively low. If you want the circuit board to be softer, choose polyethylene.
The thicker the substrate and the transparent glue on it, the harder the circuit board. If the circuit board has a relatively large bending area, you should try to use a thinner substrate and transparent glue to reduce the stress on the surface of the copper foil, so that the chance of micro-cracks in the copper foil is relatively small. Of course, for such areas, single-layer boards should be used as much as possible.
(3) Copper foil:
There are two types: rolled copper and electrolytic copper. Rolled copper has high strength and bending resistance, but the price is more expensive. The price of electrolytic copper is much cheaper, but its strength is poor and it is easy to break. It is generally used in occasions where it is rarely bent.
The thickness of the copper foil should be selected according to the minimum lead width and minimum spacing. The thinner the copper foil, the smaller the minimum achievable width and spacing.
When choosing rolled copper, pay attention to the rolling direction of the copper foil. The rolling direction of the copper foil should be the same as the main bending direction of the circuit board.
(4) Protective film and its transparent glue:
Similarly, a 25μm protective film will make the circuit board harder, but the price is cheaper. For circuit boards with relatively large bends, it is best to choose a 13μm protective film.
Transparent glue is also divided into epoxy resin and polyethylene, and the circuit board using epoxy resin is relatively hard. After the hot pressing is completed, some transparent glue will be extruded from the edge of the protective film. If the size of the pad is larger than the opening size of the protective film, the extruded glue will reduce the size of the pad and cause its edges to be irregular. At this time, transparent glue with a thickness of 13μm should be used as much as possible.
(5), pad plating:
For circuit boards with relatively large bending and exposed pads, electroplated nickel + electroless gold layer should be used, and the nickel layer should be as thin as possible: 0.5-2μm, and the chemical gold layer 0.05-0.1μm.
Shape design of pads and leads
(1). SMT pad:
--Common pad:
Prevent the occurrence of microcracks.
--Reinforced pad:
If the pad strength is required to be very high or an enhanced design is required.
--LED pad:
Due to the high requirements for the position of the LED and often stress during the assembly process, the pads of the LED should be specially designed.
-QFP, SOP or BGA pads:
Due to the greater stress of the pads on the corners, an enhanced design is required.
(2). lead:
--In order to avoid stress concentration, the lead should avoid right-angle corners, but should use arc-shaped corners.
--The leads near the corners of the circuit board shape should be designed as follows to avoid stress concentration:
External interface design
(1) The circuit board design at the solder hole or plug:
Since the welding hole or the plug has a large stress during the insertion operation, a reinforced design should be made to avoid cracks.
Use a reinforcing plate to increase the hardness of the solder hole plug of the circuit board, the thickness is generally 0.2 to 0.3 mm, and the material is polyimide, PET or metal. For pad plating, it is best to choose electroplated nickel + hard gold for plugs, and electroplated nickel + chemical gold for solder holes.
(2), the design of hot-press welding:
Generally used for the connection of two flexible boards or a flexible board and a rigid circuit board.
If you need to bend the circuit board near the hot-pressing area, stick polyimide tape or dispense glue on this area to prevent the pad root from breaking.
(3) Design of ACF hot pressing area:
Design of punching holes and small circuit board corners (see Table 3)
Design for SMT:
(1) The direction of components:
The length direction of the element should avoid the bending direction of the flexible board.
(2) For large QFP or BGA, a reinforcement board should be attached to the reverse side of the flexible board or glue under the IC.
The material of the reinforcing plate is FR4, the thickness is greater than 0.2 mm, and the area is greater than 0.5 mm of the outer edge of the element.
(3) Two positioning holes should be made on the flexible board near the edge. It is also necessary to make two identification pads for the patch, the diameter of which is 1 mm, the distance from other pads is at least 3.5 mm, the surface is gold-plated or tin-plated, and the flatness is better.
(4) If the large flexible board is composed of many small boards, a ring-shaped (inner diameter 3.2 mm) bad circuit board identification pad should be made on each small board. If the small circuit board is damaged, use a black marker to blacken the identification pad to avoid future operations.
(5) The minimum distance between the components and the edge of the circuit board is 2.5 mm, and the minimum distance between the components is 0.5 mm.
(6) Do not have vias under the components, because the flux will flow through the vias and cause pollution.
Design for electrical performance
(1) The relationship between maximum current and line width and line height: (see table)

(2) Control of impedance and noise:
--Choose transparent glue with strong insulation, such as polyethylene. Avoid using epoxy resin.
--In high-impedance or high-frequency circuits, increase the distance between the wire and the ground plate.
--The above design methods can also be adopted:
Special design of SMT process
(1) Positioning method in solder paste printing and patch process:
Since the flexible board is thin and flexible, if the edge of the circuit board is used for positioning, the positioning accuracy is very poor. Positioning holes should be used for positioning. In order to avoid the missing plate when printing solder paste, a support plate with a spring pin is used.
(2), print solder paste, patch, over-heating furnace until the visual inspection is completed, use the support plate to fix the whole process. To avoid damage to the solder joints during operation.
Flexible circuit board manufacturers often use some PCB design software to produce flexible circuit boards. Generally, there are PCB design and wiring software such as POWERPCB, DXP, 99SE, AUTOCAD, etc. PCB design software can generally be downloaded in various versions on the Internet. CAM350 and genesis2000 are also commonly used engineering aided design software by manufacturers.