PCB Tech - FPC technology and material development and technology trends

Basic structure of FPC

The basic structure of a single-sided FPC. In the case of traditional FPC, the copper foil conductor is fixed on a base film such as polyimide interposed with an adhesive such as epoxy resin, and then the circuit formed by etching is covered with a protective film. This structure uses adhesives such as epoxy resin. Due to the high mechanical reliability of this layer composition, it is still one of the commonly used standard structures even now. However, the heat resistance of adhesives such as epoxy resin or acrylic resin is lower than that of polyimide resin matrix film, so it becomes a bottleneck that determines the upper limit of the use temperature of the entire FPC (Bottle Neck).

In this case, it is necessary to exclude the FPC structure of the binder with low heat resistance. This configuration not only minimizes the thickness of the entire FPC, greatly improves mechanical properties such as bending resistance, but also facilitates the formation of fine circuits or multilayer circuits. Adhesive-free copper clad laminate materials composed only of polyimide layer and conductor layer have been put into practical use, which expands the selection range of materials suitable for various purposes.

There are also FPCs with a double-sided through-hole structure or a multilayer structure in FPC. The basic structure of the double-sided circuit of FPC is roughly the same as that of rigid PCB. The adhesive is used for interlayer bonding. However, the recent high-performance FPC excludes the adhesive and only uses polyimide resin to form the copper clad board. There are many examples. The layer composition of FPC multilayer circuits is much more complicated than that of printed PCBs. They are called Multilayer Rigid Flex or Multilayer Flex. Increasing the number of layers will reduce the flexibility, and reducing the number of layers in the part for bending, or eliminating the adhesion between layers, can increase the degree of freedom of mechanical movement. In order to manufacture multilayer rigid-flex boards, many heating processes are required, so the materials used must have high heat resistance. The use of binder-free copper clad laminates is increasing.

FPC technology trends

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 through-hole diameter between conductor layers connecting double-sided circuits or multi-layer circuits is also getting smaller and smaller, and now holes with through-hole diameters below 100um have reached the mass production scale.

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 rate, 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.

The basic constituent 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.

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) 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.