With the rapidly growing field of electronics and telecommunications engineering requiring technological innovation, engineers and scientists are constantly searching for innovative methods to improve the quality, lifecycle, and reliability of the final product. 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 on flexible PCB materials and the improvement of manufacturing processes can minimize production costs and improve quality and reliability to the greatest extent. Final product. In this article, we will analyze the main types of materials used in the manufacturing process of flexible PCBs.
Properties of flexible PCB:
We know that flexible PCBs can be easily bent and can be used to install micro electronic components. Its weight is also very light and ultra-thin, so it can be installed in any small compartment or shell designed for themed electronic products or final products. Flexible printed circuit boards are most suitable for applications that require addressing shell space limitations.
Common substrate material types for flexible PCBs:
Matrix:
The most important material in flexible or rigid PCBs is their basic substrate material. It is the material from which the entire PCB stands. In rigid PCBs, the substrate material is usually FR-4. However, in Flex PCB, the commonly used substrate materials are polyimide (PI) film and PET (polyester) film. In addition, polymer films such as PEN (polyethylene Phthalic acid), PTFE and aramid can also be used.
Polyimide (PI) “thermosetting resin” is still the most commonly used material for Flex PCB. It has excellent tensile strength and is very stable within a wide operating temperature range of -200 ° C to 300 ° C. It has chemical corrosion resistance, excellent electrical performance, high durability, and excellent heat resistance. Unlike other thermosetting resins, they can maintain their elasticity even after thermal polymerization. However, the drawbacks of PI resin are poor tear strength and high moisture absorption. On the other hand, PET (polyester) resin has poor heat resistance, making it unsuitable for direct welding, but has good electrical and mechanical properties. Another substrate, PEN, has better mid level performance than PET, but not better than PI.
Liquid Crystal Polymer (LCP) Substrate:
LCP is a rapidly popular substrate material in Flex PCBs. This is because it overcomes the drawbacks of PI substrates while maintaining all the characteristics of PI. LCP has 0.04% moisture resistance and moisture resistance, with a dielectric constant of 2.85 at 1GHz. 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 a flexible PCB substrate, and can be easily recycled.
Resin:
Another material is a resin that tightly bonds copper foil and substrate material together. Resins can be PI resin, PET resin, modified epoxy resin, and acrylic resin. Resin, copper foil (top and bottom), and substrate form a sandwich called a “laminate”. This type of laminated plate, called FCCL (Flexible Copper Clad Laminate), is formed by applying high temperature and pressure to the “stack” through automatic pressing in a controlled environment. Among the 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 PCBs and reduce dimensional stability. These adhesives may also contain harmful halogens to the environment and are subject to EU regulations. According to these environmental protection regulations, the use of seven harmful substances is restricted, including lead (Pb), mercury (Hg), cadmium (Cd), Hexavalent chromium (Cr 6+), polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE)), di (2-ethylhexyl) phthalate (DEHP) and Benzyl butyl phthalate (BBP).
Therefore, the solution to this problem is to use 2-layer FCCL without adhesive. 2L FCCL has good electrical performance, high heat resistance, and good dimensional stability, but its manufacturing is difficult and costly.
Copper foil:
Another top-level material in flexible PCBs is copper. PCB wiring, wiring, solder pads, vias, and holes are filled with copper as a conductive material. We all know the conductive properties of copper, but how to print these copper traces on a PCB is still a topic of discussion. There are two copper deposition methods on the 2L-FCCL (2-layer flexible copper clad laminate) substrate. 1- Electroplating 2- Lamination. The electroplating method has fewer adhesives, while laminates contain adhesives.
Electroplating:
The conventional method of pressing copper foil on the upper layer of PI substrate using resin adhesive is not suitable when ultra-thin Flex PCBs are required. This is because the lamination process has a three-layer structure, namely (Cu Adhisive PI), which makes the stacking layer thicker, and therefore is not recommended for double-sided FCCL. Therefore, another method called “sputtering” was used, where copper was sputtered onto the PI layer by wet or dry methods through “non electric” electroplating. This chemical plating deposited a very thin copper layer (seed layer), while in the next step called “electroplating”, another copper layer was deposited, with the thicker copper layer deposited on the thin layer (seed layer) of copper. This method does not require the use of resin adhesive to form a strong adhesion between PI and copper.
Lamination:
In this method, the PI substrate is laminated together with ultra-thin copper foil through a covering layer. Coverlay is a composite film in which a thermosetting epoxy adhesive is applied to a polyimide film. This covering adhesive has excellent heat resistance and good electrical insulation properties, with bending, flame retardancy, and gap filling properties. A special type of overlay called “Photo Imageable Overlay (PIC)” has excellent adhesion, good flexibility resistance, and environmental friendliness. However, the drawbacks of PIC are poor heat resistance and low glass transition temperature (Tg)
Rolling Annealing (RA) and Electrodeposition (ED) Copper Foil:
The main difference between the two lies in their manufacturing process. ED copper foil is made by electrolysis from a CuSO4 solution, in which Cu2+is immersed in a rotating cathode roller and peeled off to produce ED copper. The copper with different thicknesses of RA is made from high-purity copper (>99.98%) through a pressure process.
The conductivity of electrodeposited (ED) copper is better than that of rolled annealed (RA) copper, and the ductility of RA is much better than that of ED. For Flex PCBs, in terms of flexibility, RA is a better choice, while ED is a better choice for conductivity.
