Flexible circuit board (FPC) is a kind of flexible printed circuit board which is made of polyimide or polyester film with high reliability and excellent flexibility.FPC is also known as soft circuit board and flexible circuit board. It is favored for its light weight, thin thickness, free bending and folding.
FPC is a technology developed by the United States in 1970s for the development of space rocket technology. By embedding a circuit design on a flexible thin plastic sheet, a large number of precision components are embedded in a narrow and limited space, thus forming a flexible circuit. This kind of circuit can be bent and folded at will, with light weight, small volume, good heat dissipation and convenient installation, breaking through the traditional interconnection technology. In the structure of flexible circuit, the materials are insulating film, conductor and adhesive.
Composition material
1. insulating film
The insulating film forms the basic layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In the multi-layer design, it is bonded with the inner layer. They are also used as protective covers to isolate the circuit from dust and moisture, and to reduce the stress during flexing. Copper foil forms a conductive layer.
In some flexible circuits, rigid members made of aluminum or stainless steel are used, which can provide dimensional stability, physical support for the placement of components and wires, and stress relief. The adhesive bonds the rigid member and the flexible circuit together. Another kind of material is sometimes used in flexible circuits, which is adhesive ply, which is formed by coating adhesive on both sides of insulating film. Adhesive ply provides environmental protection and electronic insulation functions, and can eliminate one thin film, and has the ability to bond multiple layers with fewer layers.
2. Conductor
Copper foil is suitable for use in flexible circuits. It can be electrodeposited or plated. The electrodeposited copper foil has a glossy surface on one side, while the machined surface on the other side is dull. It is a flexible material and can be made into many thicknesses and widths. The dull side of ED copper foil is often specially treated to improve its bonding ability. Forged copper foil not only has flexibility and toughness, but also has the characteristics of hardness and smoothness. It is suitable for applications requiring dynamic deflection.
3. Adhesive
In addition to bonding the insulating film to the conductive material, the adhesive can also be used as a covering layer, as a protective coating, and as a covering coating. The main difference between them lies in the application mode used. The purpose of bonding the cover layer to cover the insulating film is to form a laminated circuit. Screen printing technology used for adhesive coating.
Not all laminated structures contain adhesives, and laminated structures without adhesives form thinner circuits and greater flexibility. Compared with the laminated structure based on adhesive, it has better thermal conductivity. Because of the thin structure characteristics of the adhesive-free flexible circuit, and because the thermal resistance of the adhesive is eliminated, thus improving the thermal conductivity, it can be used in the working environment where the adhesive-based laminated flexible circuit cannot be used.
Basic structure
How to weld
1. Operation steps
(1) Before welding, apply soldering flux on the pad, and treat it again with a soldering iron, so as to avoid bad tinning or oxidation of the pad, resulting in bad welding, and the chip generally does not need to be treated.
(2) Carefully put PQFP chip on PCB with tweezers, and be careful not to damage the pins. Align it with the bonding pad, and ensure that the chip is placed in the right direction. Adjust the temperature of the soldering iron to more than 300 degrees Celsius, dip a small amount of solder on the tip of the welding head, press down the aligned chip with a tool, add a small amount of flux to the pins in two diagonal positions, and still press down the chip to weld the pins in two diagonal positions, so that the chip can be fixed but not moved. Re-check whether the position of the chip is aligned after the diagonal welding. If necessary, it can be adjusted or removed and realigned on the PCB.
(3) When starting to weld all pins, solder should be added to the soldering iron tip, and all pins should be coated with flux to keep the pins moist. Touch the tip of the soldering iron to the end of each pin of the chip until the solder flows into the pin. When soldering, keep the soldering iron tip parallel to the soldered pin to prevent overlapping due to excessive soldering tin.
(4) After welding all the pins, soak all the pins with flux to clean the solder. Suck up excess solder where necessary to eliminate any short circuit and overlap. Finally, use tweezers to check whether there is virtual soldering. After the inspection is completed, remove the flux from the circuit board, dip the bristle brush in alcohol and wipe it carefully along the pin direction until the flux disappears.
(5) SMD resistance-capacitance components are relatively easy to weld. You can first put tin on a solder joint, then put on one end of the component, clamp the component with tweezers, and after welding one end, see if it is aligned; If it is aligned, weld the other end.
2. Matters needing attention
On the layout, when the size of the circuit board is too large, although the welding is easy to control, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; After hours, the heat dissipation decreases, the welding is not easy to control, and adjacent lines are prone to interfere with each other, such as the electromagnetic interference of circuit boards.
