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Briefly describe the selection criteria of five major materials for flexible circuits

Release time: 09/11/2018

 In recent years, the flexible circuit technology has been applied in various fields and has been applied in the fields of radio communication, computers and automotive electronics. In the past, flexible circuits were previously offered as an alternative to rigid cables, and they have matured as a replacement for rigid circuits and printed circuit boards where thin or three-dimensional circuits are required. In order to meet the application requirements of rigid and soft, rigid-flex technology combines flexible circuits on rigid circuit boards, and flexible circuit boards are the most widely used.

The functions of flexible circuit boards can be divided into four types, namely, lead lines, printed circuits, connectors, and multi-functional integrated systems, which cover applications such as computers, computer peripheral auxiliary systems, consumer livelihood appliances, and automobiles. In order to meet these functions of the flexible circuit board, the choice of materials is particularly important.

Material of flexible circuit 1: Insulation film

The insulating film has a flexible property, and serves as an insulating carrier of the circuit board to form a base layer of the circuit. When selecting a flexible dielectric film, it is necessary to investigate and test the heat resistance, shape properties, thickness, mechanical properties and electrical properties of the material. Insulation film is usually available on the market, and polyimide and polyester materials are the most commonly used. Nearly 80% of all flexible circuit manufacturers in the US use polyimide film as the material for flexible circuits, and about 20% of manufacturers use polyester film. Polyimide materials are non-flammable, geometrically stable, have high tear strength, and can withstand high temperatures during soldering.

Material of flexible circuit 2: adhesive sheet

The ability to bond the film is to bond the film to the metal foil, as well as the adhesion between the film and the film. The reinforcing rib is adhered to the flexible circuit with an adhesive, in order to provide mechanical support for the component and the connector to be inserted. And eliminate stress. It consists of an insulating film coated with adhesive on both sides. The adhesive sheet provides environmental protection and electrical insulation, and it can eliminate the film layer and bond in a multilayer circuit with a small number of layers. Different types of adhesive sheets can be used for different film substrates. For example, the adhesive sheets for polyester are different from the adhesive sheets for polyimide. The adhesive sheets for polyimide substrates are classified into epoxy and acrylic.


Material of flexible circuit 3: copper foil

The copper foil covers the conductor layer adhered to the insulating substrate, and is selectively etched to form a conductive line. Most of this copper foil is made of rolled copper foil or electrolytic copper foil. The ductility and bending resistance of the rolled copper foil are superior to those of the electrolytic copper foil, the elongation of the rolled copper foil is 20% to 45%, and the elongation of the electrolytic copper foil is 4% to 40%. Copper foil thickness is most commonly used for 35um (1oz), as well as thin 18um (0.5m) or 70um (2oz) or even 105um (30z). According to different applications, we have to choose different forms of copper foil. If it is only to replace the wires and connectors, thereby reducing manufacturing time and cost, then electrolytic copper foil which is well applied to the compatible circuit board is the best choice. Electrolytic copper foil is also used to increase the current carrying capacity by increasing the weight of copper, thereby obtaining a achievable copper skin width.

Material of flexible circuit 4: cover layer

The role of the cover layer is to protect the surface wires and increase the strength of the substrate. Covering the surface of the flexible printed circuit board with a cover layer, a functional layer for insulating protection, and a protective material for the outer layer are generally available in two types.

In the Novaclad brand product created by Sheldahl, a patented technology based on vacuum metallization is used, which applies a layer of pure copper on a surface of a polyimide film. This material is then overplated to a specific thickness to form Novalad's base material.

Novaclad base materials are used in the manufacture of Novaflex flexible circuits that do not use adhesives. After imaging the entire circuit, a layer of Novaflex insulation is applied. Novaflex flexible circuits are designed to withstand harsh environmental conditions. The Novaflex adhesive-free interconnect system provides better softness, chemical resistance, high temperature characteristics and maximum heat dissipation.

Flexible circuit material 5: reinforcement board

The reinforcing plate is adhered to the local position plate of the flexible board, and the flexible film substrate is super-supported and strengthened to facilitate the connection, fixing or other functions of the printed circuit board. The reinforcing plate material is selected according to the use, and polyester, polyimide flake, epoxy fiberglass cloth board, phenolic paper board or steel plate, aluminum plate, etc. are commonly used.