PCB Technical - Understand the technology and basic materials of FPC

The basic constituent materials of FPC are the base film or the heat-resistant resin that constitutes the base film, followed by the copper clad laminate and protective layer materials that constitute the conductor.

The base film material of FPC ranges from the initial polyimide film to the heat-resistant film that can withstand soldering. The first-generation polyimide film has problems such as high moisture absorption and large thermal expansion coefficient, so people use the second-generation polyimide material for high-density PCB circuits.

Copper clad laminate

Many FPC manufacturers often buy in the form of copper clad laminates, and then use the copper clad laminates as starting materials to process them into FPC products. The FPC copper-clad sheet or protective film (Cover Lay Film) using the first-generation polyimide film is made of an adhesive such as epoxy resin or acrylic resin. The heat resistance of the adhesive used here is lower than that of polyimide, so the heat resistance or other physical properties of FPC are limited.

In order to avoid the shortcomings of copper clad laminates using traditional adhesives, high-performance FPCs including high-density circuits use adhesive-free copper clad laminates. There have been many manufacturing methods so far, but the following three methods are now available for practical use:

1) PCB casting process

The casting process is based on copper foil as the starting material. Coating liquid polyimide resin directly on the surface-activated copper foil, and heat-treating it to form a film. The polyimide resin used here must have excellent adhesion to copper foil and excellent dimensional stability, but there is no polyimide resin that can meet these two requirements. First coat a thin layer of polyimide resin (adhesive layer) with good adhesion on the surface of the activated copper foil, and then coat a certain thickness of polyimide resin with good dimensional stability on the adhesion layer (Core layer). Due to the difference in thermal physical properties of these polyimide resins, if the copper foil is etched, large pits will appear in the base film. In order to prevent this phenomenon, the core layer is coated with an adhesive layer to obtain good symmetry of the base layer.

In order to manufacture a double-sided copper clad board, the adhesive layer uses a thermoplastic (Hot Melt) polyimide resin, and then a hot pressing method is used to laminate the copper foil on the adhesive layer.

2) Sputtering/plating process

The starting material of the sputtering/plating process is a heat-resistant film with good dimensional stability. The initial step is to use a sputtering process to form a seeding layer on the surface of the activated polyimide film. This seeding layer can ensure the bonding strength to the conductor base layer, and at the same time assumes the role of the conductor layer for electroplating. Usually nickel or nickel alloy is used. In order to ensure conductivity, a thin layer of copper is sputtered on the nickel or nickel alloy layer, and then copper is electroplated to a specified thickness.

3) Hot pressing method

The hot pressing method is to coat a thermoplastic resin (thermoplastic adhesive resin) on the surface of a heat-resistant polyimide film with good dimensional stability, and then laminate copper foil on the hot-melt resin at high temperature. Here A composite polyimide film is used.

This composite polyimide film is commercially available from a specialized manufacturer, and the manufacturing process is relatively simple. When manufacturing the copper clad laminate, the composite film and the copper foil are laminated together and hot pressed at a high temperature. The equipment investment is relatively small, which is suitable for the production of small quantities and multiple varieties. The manufacture of double-sided copper clad laminates is also easier.

Another important material element constituting the FPC is the protective layer (Cover Lay), and various protective materials have now been proposed. The first practical protective layer is to coat the same heat-resistant film as the substrate and use the same adhesive as the copper clad laminate. The characteristic of this structure is good symmetry, and it still occupies the main part of the market, usually called "Film Cover Lay". However, this kind of film protection layer is difficult to automate the processing process, which increases the overall manufacturing cost, and because it is difficult to perform fine window processing, it cannot meet the needs of high-density SMT that has become the mainstream in recent years.

In order to meet the requirements of high-density mounting, photosensitive protective layers have been adopted in recent years. Coat photosensitive resin on the copper foil circuit, and then use the PCB photolithography process to open windows in necessary parts. The photosensitive resin material has a liquid form and a dry film form. Now the protective layer materials based on epoxy resin or acrylic resin have been put into practical use, but their physical properties, especially mechanical properties, are far inferior to those of the protective layer based on polyimide. In order to improve this situation, it is necessary to use polyimide resin or to improve the physical properties of the PCB protective layer material based on epoxy resin or acrylic resin, or to improve the PCB process. The photosensitive polyimide resin used here is expected to be used as an interlayer insulating material in a multilayer circuit formation process.