As the rapidly growing field of electronic and telecommunication engineering requires technological innovation, engineers and scientists are constantly looking for novel ways to improve the quality, life cycle and reliability of final products. Therefore, flexible PCB materials are the focus of current research. Flexible PCB can be found in almost all electronic devices around us (such as printers, scanners, high-definition cameras, mobile phones, calculators, etc.). Therefore, the research of flexible PCB materials and the improvement of manufacturing process can minimize the production cost and improve the quality and reliability. The final product. In this article, we will analyze the main material types used in the manufacturing process of flexible PCB.
Attributes of flexible PCB:
As we know, flexible PCB can be easily bent and micro-electronic components can be installed. Its weight is also very light and ultra-thin, so it can be installed in any cubicle or shell designed for the theme electronic products or final products. The flexible printed circuit board is most suitable for applications that need to solve the housing space limitation.
Common substrate material types of flexible PCB:
Matrix:
The most important material of flexible PCB or rigid PCB is its basic substrate material. It is the material that the whole PCB stands on. In rigid PCB, the substrate material is usually FR-4. However, in Flex PCB, the commonly used substrate materials are polyimide (PI) film and PET (polyester) film. Besides, polymer films such as PEN (polyethylene phthalate), PTFE and aramid fiber can also be used.
Polyimide (PI) “thermosetting resin” is still the most commonly used material for Flex PCB. It has excellent tensile strength, is very stable in a wide working temperature range from-200 C to 300 C, and has chemical corrosion resistance, excellent electrical properties, high durability and excellent heat resistance. Unlike other thermosetting resins, they can keep their elasticity even after thermal polymerization. However, PI resin has the disadvantages of poor tear strength and high moisture absorption rate. On the other hand, PET (polyester) resin has poor heat resistance, “making it unsuitable for direct welding”, but it has good electrical and mechanical properties. PEN, another substrate, has better medium-level performance than PET, but not better than PI.
Liquid crystal (LCP) substrate:
LCP is a rapidly popular substrate material in Flex PCB. This is because it overcomes the shortcomings of PI substrate while maintaining all the characteristics of PI. LCP has% moisture resistance and moisture resistance, and its dielectric constant at 1GHz is. This makes it famous in high-speed digital circuits and high-frequency RF circuits. The molten form of LCP is called TLCP, which can be injection molded and pressed into flexible PCB substrate, and can be easily recycled.
Resin:
The other material is the resin that tightly bonds the copper foil and the base material together. The resin can be PI resin, PET resin, modified epoxy resin and acrylic resin. Resin, copper foil (top and bottom) and substrate form a sandwich called “laminate”. This kind of laminate, called FCCL (Flexible Copper Cladding Laminate), is formed by applying high temperature and high pressure to the “stack” through automatic pressing in a controlled environment. Among these mentioned resin types, modified epoxy resin and acrylic resin have strong adhesive properties.
These adhesive resins are not conducive to the electrical and thermal performance of Flex PCB, and reduce the dimensional stability. These adhesives may also contain halogen harmful to the environment and are restricted by European Union (EU) regulations. According to these environmental protection laws and regulations, seven harmful substances are restricted, including lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (Cr 6+), polybrominated biphenyl (PBB), polybrominated diphenyl ether (PBDE)), di (2- ethylhexyl) phthalate (DEHP) and butyl benzyl phthalate (DEHP)
Therefore, the solution to this problem is to use 2 layers of FCCL without adhesive. 2LCL has good electrical properties, high heat resistance and good dimensional stability, but its manufacturing is difficult and its cost is high.
Copper foil:
Another top material in flexible PCB is copper. PCB traces, traces, pads, vias and holes are filled with copper as conductive material. We all know the conductive characteristics of copper, but how to print these copper traces on PCB is still the topic of discussion. There are two copper deposition methods on 2L-FCCL(2-layer flexible copper clad laminate) substrate. 1- Electroplating 2- Laminating. The electroplating method has less adhesive, while the laminate contains adhesive.
Electroplating:
In the case of ultra-thin Flex PCB, the conventional method of laminating copper foil on PI substrate by resin adhesive is not suitable. This is because the lamination process has a 3-layer structure, that is, (Cu-Adhesive-PI) makes the stacked layer thicker, so it is not recommended for double-sided FCCL. Therefore, another method called “sputtering” is used, in which copper is sputtered on the PI layer by wet or dry method through “electroless” plating. This electroless plating deposited a very thin copper layer (seed layer), while another copper layer was deposited in the next step called “electroplating”, in which a thicker copper layer was deposited on a thin layer (seed layer) of copper. This method can form strong adhesive force between PI and copper without using resin adhesive.
Lamination:
In this method, the PI substrate is laminated with the ultra-thin copper foil through a covering layer. Coverlay is a composite film in which a thermosetting epoxy adhesive is coated on a polyimide film. This covering adhesive has excellent heat resistance and good electrical insulation, and has the characteristics of bending, flame retardant and gap filling. A special type of cover layer is called “Photo Imageable Coverlay(PIC)”, which has excellent adhesion, good flexibility and environmental friendliness. However, PIC has the disadvantages of poor heat resistance and low glass transition temperature (Tg).
Rolling annealed (RA) and electrodeposited (ED) copper foil;
The main difference between them lies in their manufacturing process. ED copper foil is made from CuSO4 solution by electrolysis, in which Cu2+is immersed in a rotating cathode roll and peeled off, and then ED copper is made. RA copper with different thickness is made of high purity copper (>%) by pressing process.
Electrodeposited (ED) copper has better conductivity than rolled annealed (RA) copper, while the ductility of RA is much better than that of ED. For Flex PCB, RA is a better choice in terms of flexibility, while ED is a better choice for conductivity.