The main conductor material used in PCBs is copper foil, which is used for transmitting signals and currents. At the same time, the copper foil on the PCB can also serve as a reference plane to control the impedance of transmission lines or as a shielding layer to suppress electromagnetic interference (EMI). Additionally, during the PCB manufacturing process, characteristics such as the peel strength and etching performance of the copper foil can affect the quality and reliability of PCB manufacturing. PCB layout engineers need to understand these characteristics to ensure the successful manufacturing process of PCBs.
There are two types of copper foil used for printed circuit boards: electrodeposited copper foil (ED copper foil) and rolled annealed copper foil (RA copper foil). The former is produced through electroplating, while the latter is produced through rolling. In rigid PCBs, the primary choice is electrodeposited copper foil, while rolled annealed copper foil is mainly used for flexible circuit boards.
For applications in printed circuit boards, there is a significant difference between electrodeposited copper foil and rolled copper foil. The two surfaces of electrodeposited copper foil have different characteristics, meaning that the roughness of the two surfaces of the copper foil is not the same. As circuit frequencies and rates increase, specific features of the copper foil may affect the performance of millimeter-wave (mmWave) frequencies and high-speed digital (HSD) circuits. The surface roughness of the copper foil can influence the insertion loss, phase consistency, and propagation delay of the PCB. The surface roughness of the copper foil can cause performance variations within the same PCB, as well as variations in electrical performance between different PCBs. Understanding the role of copper foil in high-performance, high-speed circuits helps optimize and more accurately simulate the design process from models to actual circuits.
Copper Foil Surface Treatment
Electrodeposited Copper Foil (ED Copper Foil)
The manufacturing process of ED copper foil consists of two main stages: depositing base copper (electroplating process) and surface treatment of the copper foil (treatment process). This ensures optimal process flexibility for various copper foil products and copper thicknesses. For example, copper foil can be used for manufacturing automotive battery products and for the production of PCB copper-clad laminates, but the requirements for surface treatment processes vary depending on the application field. In each process stage, the copper foil has different characteristics, which are crucial for its subsequent use in printed circuit boards.
Image source: https://japan.denora.com/ja/applications/copper-foil-electrodeposition.html
Base Copper Deposition (electroplating process) is theoretically very simple. This process is completed in an electroplating tank consisting of an anode, a cathode, and a chemical plating bath.
Electrodeposited copper foil is made from electrolytic copper or copper wire with the same purity as electrolytic copper, dissolved in sulfuric acid to create a copper sulfate solution. After mixing the copper sulfate solution with additives to control the characteristics of the copper foil, it is pumped into the electroplating tank. A very large current is applied between the anode and the cathode roller in the electroplating tank, causing copper ions in the electroplating tank to deposit on the surface of the cathode. The cathode is a highly polished cylindrical roller, and the material of the roller surface can be made from titanium, chromium, or stainless steel. The roller is partially immersed in the copper sulfate solution and continuously rotated. When the roller surface contacts the copper sulfate solution as it rotates, copper ions begin to deposit on the surface of the roller until they leave the copper sulfate solution again with the rotation of the roller.
The corresponding anodes are arranged in a semicircular shape, ensuring that the surface of the cathode always maintains an equal distance from the anode. By changing the rotation speed of the roller, the copper deposition time can be controlled, thus controlling the thickness of the copper layer. By maintaining a constant current density, the optimal deposition rate can be maintained. In this way, it is possible to economically and efficiently produce copper foil with a thickness ranging from 9 μm to about 300 μm. If the thickness is less than 6 μm, operations during manufacturing or use become difficult, necessitating improvements to the characteristics of the copper foil, such as increasing strength.
Due to the smooth surface of the roller, the adhesion of the copper plating layer is poor, and the copper can easily peel off from the roller. When the copper foil peels off from the cathode, the cathode continues to rotate and continue depositing copper foil, thereby enabling continuous production of copper foil and simultaneous winding.
The electrodeposited copper foil produced by this process typically has one shiny side, known as the Shiny surface (S side) or Drum surface (the side in contact with the drum), while the other side has a matte finish, known as the Matte surface (M side) or rough surface, indicating that the roughness of the two sides of the copper foil is different. The shiny S side represents the beginning of copper deposition, that is, the side where copper ions begin to contact the surface of the roller. The flatness of the roller surface directly affects the surface flatness of this side of the copper foil. The matte M side represents the growth of copper grains (columnar grains or dendrites) during the copper deposition process, which helps form a microscopically rough structure, thus aiding in adhesion to the dielectric substrate.
The performance of the copper foil can be adjusted for various different environments by controlling the chemical composition of the electroplating solution, the surface state of the electroplating roller, and the electroplating parameters. For example, mechanical properties (such as tensile strength or elongation) or rough surface profiles can be adjusted by controlling these variables.
The most important influencing factors in process control include:
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Material and surface flatness of the cathode roller
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Concentration of the copper sulfate solution
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Working temperature
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Flow conditions in the electroplating tank
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Duration of copper deposition (rotation speed of the roller)
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Current density
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Stability of the anode
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Composition and concentration of additives
Copper Foil Surface Treatment
The copper foil generated during the base copper deposition process also requires a treatment process, which involves electroplating copper nodules on the surface of the copper foil to achieve a rougher surface for better adhesion; this treatment process also uses other metals to create barrier layers and apply anti-oxidation coatings.
The Surface Roughness of Copper Foil is Very Important for PCB Manufacturing
A relatively rough surface profile helps enhance the adhesion between the copper foil and the resin system. However, a rough surface profile may require longer etching times, which can affect the production efficiency and pattern accuracy of the circuit board. Increasing etching time means more lateral etching of the conductors, leading to more severe side etching of the conductors. This poses significant challenges for fine line production and impedance control. Additionally, as the operating frequency of circuits increases, the effect of copper foil roughness on signal attenuation becomes apparent. At higher frequencies, more electrical signals are transmitted through the surface of the conductor, and a rough surface will increase the transmission distance of the signal, resulting in greater attenuation or loss. Therefore, high-performance substrates require copper foil with low roughness while ensuring sufficient adhesion to match high-performance resin systems.
The roughness of copper foil is defined in the IPC-4562 specification:
As can be seen from the above table, the current IPC-4562A standard provides a rather rough definition of the surface roughness parameters for copper foil, specifying the allowable maximum copper foil profile. These values cannot be directly applied to the simulation model parameters for copper foil roughness in simulation software.
Moreover, the standard specifications for copper foil roughness in IPC-4562A only describe the condition of copper foil delivered to PCB substrate manufacturers. PCB substrate manufacturers will further process these copper foils, laminating them with resin and reinforcement materials into copper-clad laminates for PCB manufacturers. PCB manufacturers will then perform multiple surface treatments on the copper-clad laminates during the manufacturing processes, which can also cause changes in the surface roughness of the copper foil. Therefore, the IPC-4562A standard’s definition of the surface roughness profile of copper foil does not equate to the actual roughness profile of copper foil after manufacturing.
The roughness classification standards in IPC-4562A are also significantly outdated. Many new copper foils have been developed, some of which even have lower roughness. The IPC-4562A specification has not yet covered these new ranges of lower roughness.
Glossy Treated Copper Foil or Reverse Treated Copper Foil
Glossy copper foil (DSTFoil) or reverse treated copper foil (RTF) are also types of electrodeposited copper foil (ED copper), but they are treated on the glossy side (S side), unlike conventional electrodeposited copper foil which is treated on the matte side (M side), as shown in the diagram provided by Isola:
Therefore, the side of the copper foil that adheres to the resin layer has a very low roughness, while the rough side faces outward. The low roughness of the copper foil helps in the fine production of circuit patterns for inner layers, while the rough side ensures adhesion. The low roughness surface shows significant improvements in electrical performance when applied to high-frequency signals, but this may slightly weaken the peel strength.
Double-Sided Treated Copper Foil
As mentioned above, the side of the copper foil in contact with the substrate undergoes special treatment, primarily to enhance the adhesion between the copper foil and the resin and ensure reliability. In double-sided treated copper foil, the surface of the copper foil facing outward from the substrate will also be treated. In the case of reverse treated double-sided copper foil, the glossy side contacts the substrate while the matte side faces outward, and both sides are treated.
The advantage of double-sided treated copper foil is that it can eliminate the need for browning or other surface processes of the inner core board before multilayer board lamination. However, there is a problem: double-sided treated copper foil cannot have any wear or scratches; moreover, it becomes more challenging to remove surface contamination from the copper foil. Therefore, during the circuit board manufacturing process, if double-sided treated copper foil is used, production operations must be very careful. Currently, the use of double-sided treated copper foil is gradually decreasing.
In addition to obtaining a rougher surface through the electroplating of copper nodules on the copper foil surface for better adhesion, the copper foil surface treatment process also uses other metals to create thermal protection layers and apply anti-oxidation coatings.
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Bonding Treatment or Spherical Treatment
Bonding or spherical treatment refers to the deposition of plated copper or oxidized copper nodules onto the surface of the copper foil to increase the surface area of the copper foil. This treatment layer is relatively thin but significantly enhances the adhesion between the copper foil and some high-performance resins (such as polyimide and BT resin).
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Thermal Protection Layer
Coatings containing zinc, nickel, or brass are applied to the surface of the copper foil’s honeycomb structure. During laminate manufacturing, PCB processing, and circuit board assembly, this coating can effectively prevent the effects of thermal or chemical degradation on the adhesion between the copper foil and the resin. The thickness of this coating is generally a few hundred Angstroms (A), and the color is related to the specific type of metal alloy, mostly brown, gray, or mustard yellow. Low-arsenic content has been used in copper foil coatings for a long time, but the use of arsenic-free copper foil is increasing recently.
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Passivation and Anti-Oxidation Layer
Unlike other coatings, this treatment is generally applied to both sides of the copper foil. Passivation and anti-oxidation layers typically use chromium compounds and sometimes organic coatings, primarily to prevent oxidation of the copper foil during storage and lamination. The coating thickness is usually less than 100 Angstroms (A) and is removed during PCB manufacturing processes such as cleaning, etching, or brushing.
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Coupling Agents
Coupling agents are primarily silanes, used to enhance the adhesion between glass fibers and resin, and can also be used on copper foil. These coupling agents can enhance the chemical bonding between the copper foil and the resin and prevent the copper foil from oxidizing or being contaminated.
Inspection and Packaging for Shipment
During the electroplating and surface treatment process, copper foil is wound into rolls, which will be cut into sheets or rolled for shipment according to subsequent usage. At the same time, important quality checks are performed on the copper foil. As the cleanliness of the laminate surface becomes increasingly important, the copper rolls can be cut in cleanroom conditions, and methods such as CCD optical inspection can be used to check the surface of the copper foil. For more details on copper foil inspection, please refer to the document titled “Key Raw Materials for Copper-Clad Laminates – Copper Foil.pdf” in the attachment page of the “One Board Success” column.
Characteristics of Copper Foil
The IPC standard specifies the basic specifications for copper foil substrates. The most important specifications for copper foil are IPC-4562A and the base material specification IPC-4101D, with corresponding test specifications detailed in IPC-TM-650.
Copper Foil Thickness and Unit Area Quality
IPC-4562A Table 1-1 provides specifications for copper foil weight and thickness.
It should also be noted that the IPC-4562A specification allows for a tolerance range of ±10 %. Due to cost considerations, laminate manufacturers typically use the lower limit of the tolerance range in the manufacturing of copper-clad laminates, such as 1oz copper thickness being approximately 32 μm. At the copper foil manufacturers, the unit area quality of the copper foil is used as a inspection standard, such as 1oz, while the thickness of the copper foil on the manufactured circuit board can only be determined through layer thickness measurement, such as metallographic slicing. The IPC-A-600 standard provides the minimum acceptable copper foil thickness requirements after processing:
For the inner layer copper foil of a PCB, its thickness corresponds to the copper foil thickness of the core board used. However, during the PCB manufacturing process, the substrate undergoes cleaning and degreasing treatment, resulting in some loss of copper thickness. As shown in the diagram of a 1oz copper thickness core board, the nominal thickness in the IPC-4562A specification is 34.3μm, while the actual thickness measured after manufacturing is 30.17μm.
For the outer layer copper thickness, since the PCB manufacturing process requires electroplating through holes, the outer layer copper foil will also be electroplated, resulting in the final copper thickness being the original copper foil thickness minus the copper thickness lost during cleaning plus the electroplated copper thickness.
Due to the roughness of the copper foil, accurately determining the thickness becomes challenging. Therefore, when determining its thickness, one can refer to the scoring method provided in the IPC-A-600 standard, excluding the protrusions used to enhance the bonding strength of the metal foil, which are the rough parts of the copper foil surface.
Physical Performance Requirements
Section 3.5 of the IPC-4562A specification provides the minimum requirements for the physical performance of copper foil and the corresponding testing methods. Since the attachment page of the column course will include the IPC-4562A specification PDF document, it will not be repeated here. Interested students can download the IPC-4562A specification PDF document from the attachment page for reference.
The most important requirements for copper foil will be discussed in detail below.
IPC-4562 Copper Foil Grades
The most commonly used copper foils in PCBs are Grade 1 and Grade 3 copper foils. Unlike Grade 1 copper, Grade 3 copper is required to meet specific ductility requirements at higher temperatures (180℃). Grade 3 copper foil is often referred to as “high-temperature ductile copper foil,” or simply “HTE copper foil,” and is the main type of copper foil for multilayer PCB substrates.
Under high-temperature conditions, the excellent ductility of copper foil will effectively reduce the likelihood of cracks in copper when multilayer PCBs experience thermal stress and Z-axis expansion. Changing electroplating parameters can alter the grain structure of HTE copper foil, but this may lead to changes in the mechanical properties of the copper foil. The following table provides the tensile strength and ductility requirements for standard Grade 1 copper foil, as well as the requirements for high-temperature ductility of Grade 3 copper foil. These regulations are all sourced from IPC-4562, and the requirements for other grades of metallic copper foil are also specified in this regulation.
Surface Roughness
In the IPC-4562A specification, the maximum foil profile based on the roughness of the copper foil treatment surface is specified in section 3.4.5. The maximum foil profile for the two sides of the metallic foil should comply with the regulations in the table below and be indicated with parameters Rz (DIN) or RTM.
The surface roughness of the smooth side of the copper foil is defined by the roughness value Rz (IPC-4562A, section 3.5.6) and should not exceed 0.43μm. The testing method in IPC-TM-650 2.2.17A is applicable for determining the roughness values of both sides of the copper foil.
It is important to note that there are various methods for calculating Rz. The most common Rz calculation methods come from the German Institute for Standardization (DIN), Japanese Industrial Standards (JIS), and the International Organization for Standardization (ISO), with differences as follows:
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Rz (DIN) utilizes the absolute average of the five highest peaks and five lowest valleys over a sample length.
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Rz (JIS) utilizes the absolute average of the five highest peaks and five lowest valleys over a five sample length.
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Rz (ISO) is the maximum peak-to-valley distance over the sample length.
Rz(ISO) is not recommended as it has fewer data points. Generally, Rz(DIN) and Rz(JIS) are comparable, but Rz(JIS) is always less than Rz(DIN) because Rz(JIS) includes more data points. If numerical conversion is needed, Rz(JIS) = 0.85∙Rz(DIN) is a good approximation.
The existing roughness profile type classifications in IPC-4562A can no longer meet market demands, especially in high-frequency/high-speed applications, where lower surface roughness copper foils are increasingly needed to reduce conductor losses, leading to the emergence of more roughness profile specifications for copper foil.
The Rz (DIN) or RTM values specified in IPC4562A are obtained using a contact-type profilometer, with a probe tip diameter of 5μm or 2μm, while the Rz of ultra-low roughness copper foil is already less than 1μm. As shown in the schematic diagram of the cross-section of copper foil with Rz of 1μm and the contact probe tip diameter of 2μm:
It is clear that this is not a method capable of accurately tracing the surface texture of high-frequency copper foil. Therefore, for ultra-low roughness profiles of copper foil, it is necessary to adjust the measurement methods used, transitioning from contact-based (IPC-TM-650, test method 2.2.17A) to non-contact roughness measurement methods defined by ISO 25178-2 and the newly added test method 2.22.22 in IPC-TM-650.
The transition from contact-based measurement methods to non-contact measurement methods has led to different definitions of characteristic values for copper foil roughness. In the newly added test method 2.22.22 in IPC-TM-650, three main measurement indicators are defined:
• Sa is defined as the average absolute height relative to the mean plane.
• Sq is defined as the root mean square (RMS) height relative to the mean plane.
• Sz is defined as the absolute vertical distance between the highest peak and the lowest valley.
Conductivity/Purity
High-purity copper foil (≥99.8%) is a prerequisite for meeting the high conductivity requirements of circuits on subsequent PCBs. In the manufacturing environment of the base copper deposition process for copper foil, continuous control of the process parameters of the copper sulfate electrolyte ensures the purity of the deposited copper.
IPC-4562 standards describe the purity and resistivity of electrodeposited copper foil and rolled copper foil. The minimum purity of untreated electrodeposited copper foil is 99.8%, with silver content counted as copper content. The purity value of rolled copper foil is 99.9%.
Trends in Copper Foil for PCBs
Although most applications of copper thickness on PCBs currently are 1/2oz (approximately 18μm), 1oz (approximately 35μm), and 2oz (approximately 70μm), mobile devices are driving the trend of PCB copper thickness towards thinner than 1μm. On the other hand, due to new application fields (such as automotive electronics, LED lighting, etc.), copper thickness above 100μm will again become important.
Additionally, with the development of 5G millimeter waves and high-speed serial links, the demand for copper foil with lower roughness profiles is clearly increasing.
References
[1] https://www.polarinstruments.com/support/si/AP8206.html
[2] https://www.olympus-ims.com/en/applications/copper-foil-surface-roughness-for-5g-printed-circuit-boards/
[3] Printed Circuit Handbook, 6th Edition Chinese Revised Version, Science Press
[4] IPC-4562A
[5] IPC-TM-650
[6] https://www.isola-group.com/wp-content/uploads/Understanding-Laminate-Prepreg-Manufacturing.pdf
Copyright Statement
This article is a chapter from Wu Chuanbin’s blog website “PCB Design One Board Success Column”. If there are any errors, please feel free to point them out 
