Copper foil is the main material used in PCBs, primarily for transmitting current and signals. The copper foil on a PCB can also serve as a reference plane to control the impedance of transmission lines or as a shielding layer to suppress electromagnetic interference. During the PCB manufacturing process, characteristics such as the peel strength and etching performance of the copper foil also affect the quality and reliability of PCB manufacturing.
The copper foil industry began in 1937, produced by the Anaconda copper plant in New Jersey, USA, for the construction sector. Starting in the 1970s, Japanese copper foil companies rapidly developed, monopolizing the global copper foil market, making Japan the largest producer of copper foil in the world, followed by Taiwan, China. In 1955, the Yates company in the USA began specializing in the production of copper foil for PCBs; in the early 1960s, China began producing copper foil, and a relatively complete industrial chain has formed, although the technology for ultra-thin copper foil is still controlled by foreign countries, especially Japan.
The appearance of copper foil and the corresponding electron microscope scanning images:
1. Relevant Standards for Copper Foil:
The main standards include American IPC standards, European IEC standards, Japanese JIS standards, national standards, and military standards:
1. American Electronic Interconnection Association: IPC-CF-150 (1966), IPC-MF-150G (1999), IPC-4562A (2008) currently in use, IPC-4101D
2. International Electrotechnical Commission: IEC-249-3A (1976)
3. Japanese Industrial Standards: JIS-C-6511 (1992), JIS-C-6512 (1992), JIS-C-6513 (1996)
4. National standards and military standards: GB/T5230-2020, GB/T29847-2013, GJB 2142A-2011
2. Simple Flow Chart for Copper Foil Production:
3. Classification and Code Names of PCB Copper Foil:
1. Based on thickness, it can be divided into thick copper foil (≥70μm), conventional thickness copper foil (18μm – 70μm), thin copper foil (12μm – 18μm), and ultra-thin copper foil (<12μm);
2. Based on surface conditions, it can be divided into single-sided treated copper foil (single-sided rough), double-sided treated copper foil (double-sided rough), smooth treated copper foil (double-sided rough), double-sided smooth copper foil (double smooth), and very low profile copper foil (VLP copper foil);
3. Based on production methods, it can be divided into electrolytic copper foil and rolled copper foil.
4. Classification of surface roughness of electrolytic copper foil: high-temperature high-elongation copper foil HTE roughness Rz is approximately 7-8μm, reverse treated copper foil RTF roughness is approximately 4-6μm, low profile copper foil roughness is approximately 3-4μm, and ultra-low profile copper foil roughness is approximately 1-2μm.
5. IPC—4562A specifies the types and codes of metal foils based on different manufacturing processes: E- electrolytic foil W- rolled foil O- other foils copper
6. Common abbreviations for copper foil in the industry:
① STD：standard electrolytic copper foil
② HTE：high temperature high elongation copper foil
③ RTF：reverse treated copper foil
④ DST：double side treated copper foil
⑤ LP：low profile copper foil
⑥ VLP：very low profile copper foil
⑦ HVLP：high frequency very low profile copper foil
⑧ UTF：ultra thin copper foil
⑨ RCC：resin coated copper foil
4. Weight and Thickness of Copper Foil: Excerpt from IPC-4562A
The copper thickness of PCB copper clad laminates is commonly expressed in ounces (oz), where 1oz=28.3g, such as 1/2oz, 3/4oz, 1oz, 2oz. For example, the area mass of 1oz/ft2 is equivalent to 305 g/㎡ in metric units. By converting through the copper density (8.93 g/cm2), it corresponds to a thickness of 34.3um.
The definition of copper foil “1/1”: copper foil with an area of 1 square foot weighs 1 ounce; it means spreading 1 ounce of copper evenly over 1 square foot of substrate.
5. Introduction to Copper Foil Types:
1. Electrolytic copper foil, ED, Electrodeposited copper foil (ED copper foil), refers to copper foil made by electrodeposition. Its manufacturing process is an electrolytic process, where the electrolytic equipment generally uses titanium material for the surface roller as the cathode roller, with high-quality soluble lead-based alloys or insoluble titanium-based corrosion-resistant coatings as the anode. Copper sulfate electrolyte is added between the anode and cathode, and under the effect of direct current, metal copper ions are adsorbed on the cathode roller to form the electrolytic original foil. As the cathode roller continuously rotates, the generated original foil is continuously adsorbed and stripped from the roller. It is then washed, dried, and wound into rolls. Purity 99.8%.
2. Rolled annealed copper foil, RA, Rolled annealed copper foil, is made from rough copper extracted from copper ore, processed through smelting, electrolytic purification, and made into copper ingots of about 2mm thickness. The copper ingot is used as the base material, and after acid washing and de-oiling, it undergoes repeated hot rolling and calendering (longitudinal processing) at high temperatures above 800°C. Purity 99.9%.
3. High temperature high elongation copper foil, HTE, high temperature elongation electrodeposited copper foil, maintains excellent elongation at high temperatures (180°C). The elongation rate of copper foil with thicknesses of 35μm and 70μm at high temperatures (180°C) should maintain more than 30% of the elongation rate at room temperature. It is also known as HD copper foil (high ductility copper foil).
4. Reverse treated copper foil, RTF, Reverse treated copper foil, also known as reverse copper foil, improves adhesion and reduces roughness by adding a specific resin coating on the smooth surface of electrolytic copper foil, with roughness generally between 2-4 um. The side of the copper foil bonded with the resin layer has very low roughness, while the rough side of the copper foil faces outward. The low roughness of the copper foil helps in making fine circuit patterns for inner layers, while the rough side ensures adhesion. The low roughness surface shows significant improvements in electrical performance when used for high-frequency signals, but this may also lead to a decrease in peel strength.
5. Double side treated copper foil, DST, double side treatment copper foil, undergoes roughening treatment on both the smooth and rough surfaces. The main purpose currently is to reduce costs; roughening the smooth surface can eliminate the need for copper surface treatment and browning steps before lamination. It can be used as the copper foil for inner layers of multilayer boards without additional browning (blackening) treatment before lamination. The downside is that the copper surface should not have scratches, and once contaminated, it is difficult to remove. The application of double side treated copper foil has gradually decreased.
6. Low profile copper foil, LP, low profile copper foil, is a general term. There are even lower profile types such as VLP copper foil (Very low profile copper foil), HVLP copper foil (High Volume Low Pressure), HVLP2, etc. Generally, the original foil of copper foil has very rough microcrystallization, showing coarse columnar crystals with significant fluctuations, which causes the current transmission path on the wire to become more convoluted, leading to increased resistance. In contrast, low profile copper foil has fine crystallization (below 2 μm), with equiaxed grains, free of columnar crystals, presenting a layered crystallization structure with flat edges, which is beneficial for signal transmission. LP copper foil is mainly used in high-frequency and high-speed boards.
7. Resin coated copper foil, RCC, resin coated copper foil, also known domestically as attached resin copper foil. In Taiwan, it is referred to as adhesive copper foil. It is also known abroad as an insulating resin sheet coated on copper foil or a bonding film with copper foil. It is produced by coating one or two layers of a special resin adhesive solution (the main component of the resin is usually epoxy resin) on the rough surface of thin electrolytic copper foil (generally ≤18μm thick), and dried in an oven to remove the solvent, resulting in a semi-cured B stage form of resin. The thickness used in RCC is generally not more than 18μm, with 12μm being common, and the thickness of the resin layer is generally between 40—100μm . It serves as a substitute for traditional semi-cured sheets and copper foil in the multilayer board manufacturing process.
8. Ultra thin copper foil, UTF, ultra thin copper foil, refers to copper foil with a thickness of less than 12μm. Commonly, it is copper foil below 9μm, used in manufacturing fine line printed circuit boards. Due to the difficulty of handling extremely thin copper foil, it generally has a carrier for support, with the carrier types including copper foil, aluminum foil, and organic films.
6. Conclusion:
PCB layout engineers, PCB process engineers, CAM engineers, etc., all need to understand these copper foil characteristics and select them scientifically and reasonably to ensure that the manufacturing of PCBs meets product quality requirements and achieves high performance.
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