PCB Technical - FPC technology and material development and technology trends

FPC technology refers to circuit boards made from flexible materials by printing,plating and other processes.Compared with traditional rigid circuit boards, FPC can be freely bent and folded within a certain range, thus making it ideal for use in electronic devices with space constraints. Due to the advantages of lightweight, small size, foldable, etc.,FPC is widely used in all kinds of electronic products, such as smart phones, tablet PCs, wearable devices and so on.

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,FPC 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 FPC 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

Flexible Printed Circuit Board (FPC) Basic Composition

1.Substrate

The substrate of a flexible circuit board is mainly made of polyimide (PI) or polyester film (PET), which provides the structural foundation of the board. Polyimide has excellent high temperature resistance and bending characteristics, making it the material of choice for many applications. Polyester film, on the other hand, is relatively economical but not as good as PI in terms of flexibility and high temperature resistance.

2.Conductive layer

The conductive layer is mainly made of copper foil and is responsible for transmitting electronic signals. The surface treatment of copper foil can affect its bonding properties, such as through the formation of electroplating with a glossy or non-glossy surface, these characteristics directly affect the reliability and performance of the line.

3.Insulating Layer

The insulating layer is usually made of polyester or polyimide film, and its main function is to isolate and protect the conductive layer. The insulating layer not only ensures electrical isolation between different circuits, but also prevents moisture and dust from affecting the circuits, and has an important protective role in multi-layer design.

4.Covering film

Covering film is used to protect the surface of the flexible circuit board is an important component of the common thickness of 1 mil and 1/2 mil two. This film enhances the durability of the circuit, while providing additional insulation protection to reduce the impact of external environmental factors.

5.Bonding agent

The role of the adhesive is to provide bonding between the insulating layer and the conductive layer. It is used not only to bond insulating films to conductive materials, but also as a cover layer to provide protection and insulation. Adhesives are usually overcoated by screen printing techniques.

6.Other elements

In addition, flexible circuit boards can also contain reinforcement boards to increase their mechanical strength and facilitate their use in practice. Reinforcement plates are usually located between other materials to provide additional support.

Laminates

Many FPC manufacturers often purchase laminates in the form of a laminate and then use the laminates as a starting material to process them into FPC products.FPC laminates or protective films (overlay films) using first generation polyimide films are made from adhesives such as epoxy or acrylic resins. The heat resistance of the adhesives used here is lower than that of polyimide,and the heat resistance or other physical properties of the 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.