PCB Tech - FPC circuit board technology trends

The technological development trend of FPC and the technological trend of FPC materials.

1. The latest technological trends of FPC

With the diversification of uses and compactness, FPCs used in electronic devices require high-density circuits as well as high performance in a qualitative sense. Recent changes in FPc circuit density. The subtractive method (etching method) can be used to form a single-sided circuit with a conductor pitch of 30um or less, and a double-sided circuit with a conductor pitch of 50um or less has also been put into practical use. The via holes between conductor layers connecting double-sided circuits or multi-layer circuits are getting smaller and smaller, and now holes with via hole diameters below 100um have reached the scale of mass production.

Based on the standpoint of manufacturing technology, the possible manufacturing range of high-density circuits. According to the circuit pitch and via hole diameter, high-density circuits are roughly divided into three types: (1) traditional FPC; (2) high-density FPC; (3) ultra-high-density FPC.

In the traditional subtractive method, FPC with a pitch of 150um and a via hole diameter of 15um has been mass-produced. Due to the improvement of materials or processing equipment, a circuit pitch of 30um can be processed even in the subtractive method. In addition, due to the introduction of processes such as CO2 laser or chemical etching, mass production and processing of via holes with a diameter of 50um can be achieved, and most of the high-density FPCs currently mass-produced are processed by these technologies.

However, if the pitch is less than 25um and the via hole diameter is less than 50um, even if the traditional technology is improved, it is difficult to increase the yield, and new processes or new materials must be introduced. There are various processing methods for the proposed process, but the semi-additive method using electroforming (sputtering) technology is the most suitable method. Not only the basic process is different, but the materials and auxiliary materials used are also different.

On the other hand, the advancement of FPC joining technology requires FPC to have higher reliability performance. With the high density of circuits, the performance of FPCs has put forward diversified and high-performance requirements. These performance requirements depend to a large extent on the circuit processing technology or the materials used.

2. FPC manufacturing process

Almost all the FPC manufacturing processes so far have been processed by the subtractive method (etching method). Usually, a copper clad board is used as a starting material, a resist layer is formed by photolithography, and the unnecessary part of the copper surface is etched and removed to form a circuit conductor. Due to problems such as undercutting, the etching method has limitations in the processing of fine circuits.

Because it is difficult to process or maintain high-yield micro circuits based on the subtractive method, the semi-additive method is considered to be an effective method, and various semi-additive methods have been proposed. An example of micro-circuit processing using the semi-additive method. The semi-additive process starts with a polyimide film, and first casts (coats) a liquid polyimide resin on a suitable carrier to form a polyimide film. Next, a sputtering method is used to form a seeding layer on the polyimide base film, and then a resist pattern of the reverse pattern of the circuit is formed on the seeding layer by photolithography, which is called a plating resist layer. The blank part is electroplated to form a conductor circuit. Then, the resist layer and unnecessary seeding layer are removed to form the first layer circuit. Coating photosensitive polyimide resin on the first layer of circuit, using photolithography to form holes, protective layer or insulating layer for the second layer of circuit layer, and then sputtering on it to form a seeding layer, as the second The base conductive layer of a two-layer circuit. By repeating the above process, a multilayer circuit can be formed.

Using this semi-additive method, it is possible to process ultra-fine circuits with a pitch of 5um and a via hole of 10um. The key to the production of ultra-fine circuits using the semi-additive method is the performance of the photosensitive polyimide resin used as the insulating layer.

Three, the basic constituent materials of FPC

The basic constituent material of FPC is the base film or the heat-resistant resin that constitutes the base film, followed by the copper clad laminate and protective layer material 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 circuits.

So far, people have developed several heat-resistant films for FPC that can replace the first-generation polyimide film. However, in the next 10 years, it is believed that the position of polyimide resin, which is the main material of FPC, will not change. In addition, with the high performance of FPC, the material form of polyimide resin will change, and it is necessary to develop polyimide resin with new functions.

Four, copper clad laminate

Many FPC manufacturers often buy in the form of copper clad laminates, and then process them into FPC products from the copper clad laminates. The copper-clad sheet for FPC or the 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) Casting process

The casting process is based on copper foil. 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 double-sided copper clad laminates, the adhesive layer uses hot melt polyimide resin, and then the copper foil is laminated on the adhesive layer by hot pressing.

(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.

Six, concluding remarks

The demand for FPC is increasing rapidly, circuit density continues to increase, and manufacturing technology is also improved and advanced year by year. The rapid growth of FPC base materials, protective layers and interlayer insulating materials will still be centered on polyimide resins in the future.

With the high performance and high density of FPC, not only the development of higher performance polyamide resin film, but also the development of more diversified product forms are required.