Flexible printed circuit board (FPC Flexible Circuit) is a flexible circuit in the end of the surface to produce a circuit in the form of Printed, there can be no cover layer (usually used to protect the FPC circuit). Because FPC can be bent, folded or repetitive motion in a variety of ways, compared with the ordinary hard board (PCB), has the advantages of light, thin, flexible, so its application is more and more widely.
FPC Film (Base) material is generally used polyimide (Polyimide, referred to as PI), but also useful polyester
(Polyester, referred to as PET), the material thickness of 12.5/25/50/75/125um, commonly used 12.5 and 25um. If the FPC needs to be welded at high temperature, the material is often selected by PI, and the PCB substrate is usually FR4.
Covering FPC (Cover Layer) material for the pressure fit dielectric film and adhesive coating, or flexible medium, which can avoid contamination, damp, scratches and protective effect of main materials and basic materials, namely polyimide
(Polyimide) and polyester (Polyester), commonly used material thickness of 12.5um.
FPC design need to be bonded together, this time the need to use FPC glue (Adhesive). Flexible board is commonly used in acrylic acid (Acrylic), modified epoxy resin (Modified Epoxy), FPC (Phenolic Butyrals), adhesive, pressure sensitive adhesive, etc., and a single layer of adhesive glue will not be used.
In many applications, such as device welding, flexible plates need to use Stiffener (plate) to obtain external support. The main materials are PI or Polyester film, glass fiber, polymer materials, steel, aluminum, etc.. PI or Polyester thin film is commonly used in flexible plate reinforcement, the thickness is generally 125um. Glass fiber (FR4) reinforced plate hardness is higher than PI or Polyester, used for some of the more demanding areas, relative processing difficulties.
Compared with the PCB pad processing, FPC pad processing also has a variety of methods, the following are common:
1. chemical nickel gold also known as chemical leaching gold or gold. Generally used in PCB copper metal surface electroless nickel layer thickness is 2.5um-5.0um, leaching of gold (99.9% gold) thickness of 0.05um-0.1um (before the news that workers in a PCB factory by replacement method to replace the PCB pool of gold). The technology has the advantages of smooth surface, long storage time and easy welding, and is suitable for fine pitch components and thinner PCB. For FPC, because the thickness is thin so it is suitable for use. Disadvantages: not environmentally friendly.
2. (Tin-Lead Plating) Pb Sn plating can be directly to the pad with flat tin, good weldability, good uniformity. For some processing technology such as HOTBAR, FPC must use this method. Disadvantages: lead easy oxidation, preservation time is short; need to pull the plating wire; not environmental protection.
3. selective gold plating (SEG) selective plating gold refers to the local area of PCB gold plating, other regions with another surface treatment. Gold plating is used on the surface of PCB copper coated with nickel layer, after plating gold layer. The thickness of the nickel layer is 2.5um-5.0um, and the thickness of the gold layer is generally 0.05um-0.1um. Advantages: thick gold plating, oxidation and wear resistance. "Golden finger" generally use this treatment. Disadvantages: environmental protection, cyanide pollution.
4. organic solderability protection layer (OSP) this process is the surface of the exposed PCB copper surface with specific organic matter. Advantages: can provide very smooth PCB surface, in line with environmental requirements. PCB suitable for fine pitch components.
Disadvantages: the need for conventional wave soldering and selective wave soldering process PCBA, not allowed to use OSP surface treatment process.
5. hot air leveling (HASL) refers to the process of Pb Sn alloy exposed in the PCB final metal surface coverage of 63/37. The coating thickness of hot air leveling Pewter requirements for 1um-25um. Hot air leveling control of the coating thickness and the pad pattern is difficult, not recommended to fine pitch components of PCB, the reason is the fine pitch components pad flatness requirement is high; the influence of FPC hot air leveling for thin large, does not recommend the use of this kind of surface treatment.
In the design, FPC often need to be used in conjunction with PCB, in both the connection board to board connectors, connectors, HOTBAR, gold finger flex board, manual welding mode is usually used to connect, for different application environments, the designer can use the corresponding connection mode.
In practical applications, according to the application needs to determine whether the need for ESD shielding, when the FPC flexibility requirements are not high can be solid copper skin, thick media. The copper wire mesh and conductive silver paste can be used when the flexibility is high.