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The main material for the production of flexible PCB in the circuit board factory
2021-08-26
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Author:Belle

As the rapidly growing field of electronics and telecommunications engineering requires technological innovation, engineers and scientists are constantly looking for novel ways to improve the quality, life cycle and reliability of the final product. For this reason, flexible PCB materials are the focus of current research. Flexible PCB circuit boards 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 reduce the production cost to the greatest extent and improve the quality and reliability. Final product. In this article, we will analyze the main material types used in the flexible PCB manufacturing process.
Properties of flexible PCB:
We know that flexible PCBs can be easily bent and can install micro electronic components. It is also very light and ultra-thin, so it can be installed in any small compartment or enclosure designed for themed electronic products or final products. Flexible printed circuit boards are most suitable for applications that need to address the space constraints of the housing.
Common substrate material types for flexible PCBs:
Matrix:
The most important material in a flexible PCB or a rigid PCB is its base substrate material. It is the material on which the entire PCB stands. 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. In addition, polymer films such as PEN (polyethylene phthalate) can also be used. Diester), PTFE and aramid etc.
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 operating temperature range of -200 O C to 300 O C, has chemical resistance, excellent electrical properties, high durability and excellent heat resistance. Unlike other thermosetting resins, it can maintain its elasticity even after thermal polymerization. However, the disadvantages of PI resin are 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. Another substrate, PEN, has a 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 PCB. This is because it overcomes the shortcomings of PI substrates while maintaining all the characteristics of PI. LCP has moisture resistance and moisture resistance in %, and the dielectric constant at 1 GHz is. This makes it famous in high-speed digital circuits and high-frequency RF circuit boards. 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.

The main material for the production of flexible PCB in the circuit board factory

Resin:
Another material is a resin that tightly bonds the copper foil and the base material. The resin may be PI resin, PET resin, modified epoxy resin and acrylic resin. The resin, copper foil (top and bottom) and substrate form a sandwich called "laminate". This laminate is called FCCL (Flexible Copper Clad Laminate) and is formed by applying high temperature and high pressure to the "stack" through automatic pressing in a controlled environment. Among the resin types mentioned, modified epoxy resins and acrylic resins have strong adhesion properties
These adhesive resins are detrimental to the electrical and thermal properties of Flex PCB and reduce dimensional stability. These adhesives may also contain halogens that are harmful to the environment and are restricted by European Union (EU) regulations. According to these environmental protection regulations, the use of 7 hazardous substances is restricted, including lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (Cr 6+), polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE), Bis(2-ethylhexyl) phthalate (DEHP) and butyl benzyl phthalate (BBP).
Therefore, the solution to this problem is to use a 2-layer FCCL without adhesive. 2L FCCL has good electrical properties, high heat resistance and good dimensional stability, but its manufacture is difficult and costly.
Copper foil:
Another top material in flexible PCBs is copper. PCB traces, traces, 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 the 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-Plating 2-Laminating. The electroplating method has fewer adhesives, while laminates contain adhesives.
plating:
In the case where an ultra-thin Flex PCB is required, the conventional method of laminating copper foil on a PI substrate through a 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 a wet or dry method by "electroless" electroplating. This electroless plating deposits a very thin copper layer (seed layer), and in the next step called "electroplating" another copper layer is deposited, where a thicker copper layer is deposited on the thin copper layer (seed layer) Layer). This method does not require the use of resin adhesives to form a strong bond between PI and copper.
laminated:
In this method, the PI substrate is laminated with the ultra-thin copper foil through a cover 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 a good electrical insulator, with bending, flame retardant and gap filling characteristics. The special type of cover layer is called "Photo Imageable Coverlay (PIC)", which has excellent adhesion, good flexibility and environmental friendliness. However, the disadvantages of PIC are poor heat resistance and low glass transition temperature (Tg)
Roll annealing (RA) and electrodeposition (ED) copper foil:
The main difference between the two 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. The RA copper with different thicknesses is made of high-purity copper (>%) through a press process.
Electrodeposited (ED) copper has better conductivity than roll annealed (RA) copper, and RA has much better ductility than ED. For Flex PCB boards, RA is a better choice in terms of flexibility, and ED is a better choice for conductivity.