Introduction to Basic Materials of FPC

Material of Flexible Printed Circuit Board I. Insulation Substrate
The insulating substrate is a flexible insulating film. As the insulation carrier of PCB, flexible dielectric film is chosen, which requires comprehensive investigation of the heat resistance, cladding performance, thickness, mechanical and electrical properties of the material. At present, engineering commonly used are polyimide (PI: Polyimide, trade name Kapton) film, polyester (PET: Polyester, trade name Mylar) film and polytetrafluoroethylene (PTFE: Ployterafluoroethylene) film. Generally, the thickness of films is in the range of O.0127-O.127 mm (O.5-5 ml). Properties of polyimide film, polyester film and polytetrafluoroethylene film

Material of Flexible Printed Circuit Board II. Bonding Sheet
The function of bonding sheet is to bond film to metal foil, or to bond film to film (covering film). Different types of bonding sheets can be used for different film substrates, such as polyester bonding sheets and polyimide bonding sheets. The bonding sheets of polyimide substrates can be divided into epoxy and acrylic. The fluidity and thermal expansion coefficient of the material are mainly investigated when the bonding sheet is selected. There are also polyimide copper clad laminates without bonding sheets, which have better chemical resistance and electrical properties.

Due to the low glass transition temperature of acrylic bonding sheet, the large amount of contamination produced during drilling is not easy to remove, which affects the quality of metallized holes and other bonding materials. Therefore, polyimide materials are commonly used in the interlayer bonding sheet of multilayer flexible circuits, because the CTE (thermal expansion coefficient) of the interlayer bonding sheet is consistent with the polyimide substrate, thus overcoming the multi-layer flexible circuits. Medium size instability and other performances are satisfactory.

Material of Flexible Printed Circuit Board III. Copper Foil
Copper foil is a conductive layer that covers and adheres to insulating substrates, and then selectively etches to form conductive circuits. Most of these copper foils are rolled Copper Foil or Electrodeposited Copper Foil. The ductility and bending resistance of rolled copper foil are better than those of electrolytic copper foil. The elongation of rolled copper foil is 20%-45%, and that of electrolytic copper foil is 4%-40%. Copper foil thickness is the most commonly used 35um (1oz), but also thin 18um (O.5oz) or thick 70UM (2oz), or even 105um (30z). Electrolytic copper foil is formed by electroplating. The crystalline state of copper particles is vertical needle-like, which is easy to form vertical line edges during etching, which is conducive to the fabrication of precision circuits. However, when the bending radius is less than 5 mm or dynamic flexure, the needle-like structure is easy to break. Therefore, calendered copper foil is often used as flexible circuit substrates, and its copper particles are horizontal axis-like structure, which can adapt to multiple windings. 。

Material of Flexible Printed Circuit Board IV. Overlay
Overlay is an insulating layer covering the surface of flexible printed circuit board, which can protect the surface wire and increase the strength of the substrate. There are generally two kinds of protective materials for outer graphics.
The first type is dry film type (covering film). Polyimide material is chosen, which is laminated directly with the circuit board to be protected after etching without any adhesive. This kind of coating requires pre-forming before pressing to expose welded parts, so it can not meet the requirements of finer assembly.
The second type is photosensitive development. The first type of photosensitive development is to expose the welded part by photosensitive development after covering the dry film with the laminator, which solves the problem of high density assembly. The second type is liquid screen printing covering material, commonly used are thermosetting polyimide material, and special solder resistance ink for photosensitive development flexible circuit board.
This kind of material can better meet the requirements of flexible plates with fine spacing and high density.

Material of Flexible Printed Circuit Board V. Reinforcement Board
The reinforcing plate adheres to the local position of the flexible plate, which strengthens the super-support of the flexible thin film substrate and facilitates the connection, fixing or other functions of the printed circuit board. Reinforcement board materials are selected according to their different uses, such as polyester, polyimide sheet, epoxy glass fibre sheet, phenolic paper sheet or steel plate, aluminium sheet, etc.