PCB quality technology: copper foil knowledge that PCB people should master
Copper foil is the main material used in PCBs, mainly used to transmit current and signals. The copper foil on PCBs can also be used as a reference plane to control the impedance of the transmission line, or as a shielding layer to suppress electromagnetic interference. During the PCB manufacturing process, the peel strength, etching performance and other characteristics of the copper foil will also affect the quality and reliability of PCB manufacturing.
The copper foil industry began in 1937 and was produced by the Anaconda Copper Refinery in New Jersey, USA, for use in the construction field. Since the 1970s, Japanese copper foil companies have developed rapidly and have completely monopolized the world's copper foil market. They are the world's largest copper foil producer, followed by Taiwan, China. In 1955, Yates Company in the United States began to specialize in the production of copper foil for PCBs;
The appearance of copper foil and the corresponding electron microscope scan:
1. Copper foil related standards:
mainly American IPC standards, European IEC standards, Japanese JIS standards, national standards and military standards:
1. American Electronics 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 of copper foil production:
3. PCB copper foil classification and code name:
1. According to thickness, it can be divided into thick copper foil (≥70μm), conventional thickness copper foil (18μm-70μm or less), thin copper foil (12μm-18μm), ultra-thin copper foil (<12μm);
2. According to surface condition, 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-light) and very low profile copper foil (VLP copper foil);
3. According to production method, it can be divided into electrolytic copper foil and rolled copper foil.
4. Classification of surface roughness of electrolytic copper foil: The roughness Rz of high temperature and high elongation copper foil HTE is about 7-8μm, the roughness of reverse copper foil RTF is about 4-6μm, the roughness of low profile copper foil is about 3-4μm, and the roughness of ultra-low profile copper foil is about 1-2μm.
5.IPC-4562A specifies the types and codes of metal foils according to their manufacturing processes: E-electrolytic foil, W-rolled foil, O-other foils.
6. Commonly used copper foil abbreviations in the industry:
① STD: Standard electrolytic copper foil
② HTE: High temperature high elongation copper foil
③ RTF: Reverse treated copper foil (reverse copper foil)
④ DST: Double-sided treated copper foil
⑤ LP: Low profile copper foil
⑥ VLP: Ultra low profile copper foil
⑦ HVLP: High frequency ultra low profile copper foil
⑧ UTF: Ultra thin copper foil
⑨ RCC: Resin coated copper foil
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4. Weight and thickness of copper foil:
Extracted from IPC-4562A The copper thickness of PCB copper-clad boards is usually expressed in imperial ounces (oz), 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, which is equivalent to a thickness of 34.3um through conversion of copper density (8.93 g/cm2).
The definition of "1/1" copper foil: copper foil with an area of 1 square foot weighs 1 ounce; 1 ounce of copper is evenly spread on a board with an area of 1 square foot.
5. Introduction to copper foil types:
1. ED, Electrodeposited copper foil (ED copper foil),
refers to copper foil made by electrodeposition. Its manufacturing process is an electrolytic process. The electrolytic equipment generally uses a surface roller made of titanium material as the cathode roller, and a high-quality soluble lead-based alloy or an insoluble titanium-based corrosion-resistant coating as the anode. Copper sulfate electrolyte is added between the cathode and the cathode. Under the action of direct current, the cathode roller is adsorbed with metal copper ions to form an electrolytic raw foil. As the cathode roller rotates continuously, the generated raw foil is continuously adsorbed and peeled off on the roller. After washing, drying, and winding into a rolled raw foil. The purity is 99.8%.
2. RA, Rolled annealed copper foil,
is made from copper ore to extract crude copper, which is smelted, processed, electrolytically purified, and made into a copper ingot about 2mm thick. The copper ingot is used as the base material, pickled, degreased, and repeatedly hot rolled and rolled (in the long direction) at a high temperature of more than 800°C. Purity 99.9%.
3. HTE, high temperature elongation electrodeposited copper foil,
is a copper foil that maintains excellent elongation at high temperature (180°C). Among them, the elongation of copper foils with a thickness of 35μm and 70μm at high temperature (180°C) should be maintained at more than 30% of the elongation at room temperature. It is also called HD copper foil (high ductility copper foil).
4. RTF, Reverse treated copper foil,
also called reverse copper foil, improves the bonding performance and reduces the roughness by adding a specific resin coating on the smooth surface of the electrolytic copper foil. The roughness is generally between 2-4 um. The side of the copper foil bonded to the resin layer has a very low roughness, while the rough side of the copper foil faces outward. The low copper foil roughness of the laminate is very helpful for making fine circuit patterns in the inner layer, and the rough surface ensures adhesion. When the low roughness surface is used in high-frequency signals, the electrical performance is greatly improved, but this will also reduce the peel strength.
5. DST, double side treatment copper foil,
both the glossy and rough surfaces are roughened. The main purpose at present is to reduce costs. Roughening the glossy surface can save the copper surface treatment and browning steps before lamination. It can be used as the copper foil of the inner layer of the multilayer board, and it is not necessary to brown (blacken) it before lamination of the multilayer board. The disadvantage is that the copper surface cannot be scratched, and it is difficult to remove it once it is contaminated. At present, the application of double-sided copper foil has gradually decreased.
6. LP, low profile copper foil,
this is a general term. VLP copper foil (Very low profile copper foil), HVLP copper foil (High Volume Low Pressure), HVLP2, etc. have further low profiles. The microcrystals of the original copper foil are generally very rough, with thick columnar crystals and large fluctuations. The transmission path of the current on the wire will become more circuitous, resulting in an increase in resistance. The crystals of low-profile copper foil are very fine (below 2?μm), equiaxed grains, without columnar crystals, and in the form of lamellar crystals with flat edges, which is conducive to signal transmission. LP copper foil is mainly used on high-frequency and high-speed boards.
7. RCC, resin coated copper foil,
also known as resin-attached copper foil in China. Taiwan calls it: adhesive-backed copper foil. Some foreign countries also call it: insulating resin sheet carried on copper foil, adhesive film with copper foil. It is a thin electrolytic copper foil (thickness is generally ≦18μm) coated with one or two layers of specially composed resin glue (the main component of the resin is usually epoxy resin) on the roughened surface, and the solvent is removed by drying in an oven, and the resin becomes a semi-cured B stage. The thickness of RCC is generally not more than 18μm, and 12μm is currently commonly used. The thickness of the resin layer is generally 40-100μm. It plays a role in replacing both the traditional prepreg and copper foil in the production process of multilayer boards by the build-up method.
8. UTF, ultra thin copper foil,
refers to copper foil with a thickness of less than 12μm. The most common is copper foil below 9μm, which is used in the manufacture of printed circuit boards with fine circuits. Since extremely thin copper foil is difficult to take, it is generally supported by a carrier, and the types of carriers include copper foil, aluminum foil, organic film, etc.
6. Conclusion:
PCB Layout engineers, PCB process engineers, CAM engineers, etc., all need to understand the characteristics of these copper foils and select them scientifically and rationally to ensure that the manufacture of PCBs can meet product quality requirements and exert high performance.
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