Materials for Flexible Printed Boards

 i. Insulating Substrates

Insulating substrates are flexible insulating films. As the insulating carrier of the circuit board, when selecting the flexible dielectric film, the heat resistance performance, shape forming performance, thickness, mechanical performance, and electrical performance of the material are required to be comprehensively examined. Now the material commonly used in engineering are polyimide (Kapton) film, polyester (PET, Mylar) film, and polytetrafluoroethylene (PTFE) film. The general film thickness is selected within the range of 0.0127 to 0.127 mm (0.5 to 5 mil).

ii. Adhesive Sheet

   The function of the adhesive sheet is to bond the film with the metal foil, or to bond the film with the film (cover film). Different types of adhesive sheets can be used for different film substrates. For example, polyester adhesive sheets and polyimide adhesive sheets are different. The adhesive sheets of polyimide substrates are classified into epoxy and acrylic. Choosing a cohesive sheet mainly examines the fluidity of the material and its coefficient of thermal expansion. There are also non-adhesive polyimide copper clad laminates with better chemical and electrical properties.

iii. Copper Foil

   The copper foil is a conductor layer that covers the insulating substrate and is selectively etched to form a conductive line. The vast majority of such copper foils use Rolled Copper Foil or Electrodeposited Copper Foil. The ductility and bending resistance of the rolled copper foil is better than that of the electrolytic copper foil. The elongation rate of the rolled copper foil is 20% to 45%, and the elongation of the electrolytic copper foil is 4% to 40%. The thickness of copper foil is most commonly 35 um (1 oz), thin 18 um (O.5 oz) or thick 70 um (2 oz), or even 105 um (3oz). The electrolytic copper foil is formed by electroplating, and the crystal state of the copper microparticles is vertical needle-like, easy to form a vertical line edge during etching, which is favorable for the production of a precision circuit; but when the bending radius is less than 5mm or dynamic deflection, the needle shape structure is prone to breakage; therefore, the flexible circuit substrate is mostly made of rolled copper foil, and the copper particles have a horizontal axis structure and can adapt to multiple windings.

iv. Cover Layer

   The cover layer is an insulating protective layer covering the surface of the flexible printed circuit board, which plays a role of protecting the surface wires and increasing the strength of the substrate. There are generally two types of protective materials for the outer graphics.

v. Enhanced Board

   The reinforcing plate is adhered to the local plate of the flexible plate, and the super-strength of the flexible film substrate is strengthened to facilitate the connection, fixing, or other functions of the printed plate. Reinforcement plate materials are selected according to different applications. The commonly used are polyester, polyimide sheets, epoxy glass cloth sheets, phenolic paper plates or steel plates, and aluminum plates.

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