What are the differences between flexible board copper foil and rigid board copper foil
Jul 20, 2024
Introduction to FPC
FPC is a technology developed by the United States in the 1970s for the development of space rocket technology. It is a printed circuit board with high reliability and excellent flexibility made ²of flexible copper-clad laminate as the substrate.
Sorting out the flexible circuit board industry chain
The upstream of the direct raw materials of the flexible circuit board (FPC) industry chain is flexible copper clad laminate (FCCL), and the downstream is terminal consumer electronic products, such as: smart phones, automotive electronics, etc. Among them, the upstream of the direct raw materials of flexible copper clad laminate (FCCL) are: copper foil, polyimide (PI) film or polyester (PET) film substrate and adhesive.
Types and differences of copper foil for flexible boards
The copper foil materials for flexible boards are mainly divided into rolled copper foil and electrolytic copper foil. In terms of performance, the ductility and bending resistance of rolled copper foil are better than those of electrolytic copper foil. The elongation of rolled copper foil can reach 20%-45%. It is used in early flexible board process copper foil due to its excellent ductility and bending resistance. The elongation of electrolytic copper foil is only 4%-40%. The advantage of electrolytic copper foil is that its copper foil material is formed by electro-deposition with copper sulfate electrolyte under the action of external direct current. The crystal structure of its copper particles is arranged uniformly, and the formed coating and the surface formed after the final surface treatment are relatively flat. In addition, the copper particle structure is easy to form vertical line edges during the etching process, which is conducive to the production of fine wires. Electrolytic copper foil is used for circuit boards with low flexibility requirements because of its low manufacturing cost and brighter soft board surface.
| Rolled copper foil | Electrolytic copper foil | |
| Before treatment |
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| After treatment |
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Types of electrolytic copper foil for flexible boards
Electrolytic copper foil for flexible boards can be divided into high temperature high elongation copper foil (HTE copper foil), reverse copper foil (RTF copper foil), low profile copper foil (VLP copper foil) and ultra low profile copper foil (HVLP copper foil) according to the roughness. Among them, the roughness of HTE copper foil is about 4μm-6.5μm, which is suitable for occasions with low requirements on folding resistance, such as connecting wires and light strips, and is mostly used in the three-layer method; the special feature of RTF copper foil is that the treated surface is processed with a low-roughness glossy surface, with a roughness between 2μm-4μm, and is mainly used for medium and fine lines and high-end high-frequency and high-speed electronic circuits, such as mobile phones, VR equipment, etc., in conjunction with the 2-layer and 3-layer methods; the roughness of VLP copper foil and HVLP copper foil is below 2μm. This extremely low roughness can reduce signal loss during signal transmission and is mainly used in high-frequency and high-speed electronic circuits, such as 5G, cloud servers, etc. It is mostly used in the 2-layer method and is suitable for low dielectric constant substrates, such as PTFE and LCP (liquid polymer).
Performance indicators of copper foil for flexible boards
In order to meet the requirements of electronic products to be light, thin, short and small, flexible boards have certain requirements for the copper foil substrate in copper clad boards, such as: copper foil thickness, unit area weight, roughness, ductility, peel strength, pinholes/penetration points, etc., as shown in the following table:
| Item | Unit | HTE copper foil | RTF copper foil | |||
| Standard thickness | um | 12 | 18 | 35 | 18 | |
| Weight per unit area | g/m² | 105±10 | 148±10 | 275±15 | 156±5 | |
| Roughness | Untreated surface Ra | um | ≤0.43 | 0.2-0.4 | ||
| Treated surface Rz | ≤4 | ≤6 | 1.6-2.6 | |||
| Tensile strength | R.T(25℃) | N/mm² | ≥320 | ≥40 | ||
| H. T (180℃, 1h) | ≥180 | 18±3 | ||||
| Elongation | R. T(25℃) | % | ≥5 | ≥7 | ≥12 | 7±4 |
| H. T (180℃, 1h) | ≥10 | ≥12 | ≥16 | ≥7 | ||
| Peel strength | Kg/cm² | ≥0.8 | ≥0.95 | ≥1.25 | ≥0.62 | |
| Resistivity | Ω.g/cm² | ≤0.17 | ≤0.17 | ≤0.162 | / | |
| Pinhole/Penetration Point | Number | No | ||||
| High temperature anti-oxidation | / | 180℃, 60min, no discoloration | ||||
As wearable devices, flexible displays and smart devices tend to be lighter, thinner, shorter and smaller, and as flexible printed circuit boards (FPC) are expected to replace traditional electronic components in the future, they will have great application value in the field of new energy vehicle power battery modules. Therefore, the demand for flexible printed circuit boards will increase significantly. In order to cater to market changes, the copper foil industry is also facing greater challenges. In the future, copper foil for flexible printed circuit boards will develop in the direction of thinner thickness, lighter weight and lower roughness.