Rolled copper and electrolytic copper are the two main types of copper foil used in high frequency PCBs for lamination with PTFE boards. Due to differences in their manufacturing processes, there are significant variations between the two in terms of lamination strength, high-frequency performance, process compatibility and environmental resistance.
Rolled copper is produced through a physical rolling process: high-purity copper ingots are heated and subjected to repeated rolling and annealing, ultimately forming thin copper foil. Its grains are arranged in a fibrous pattern, with a dense and uniform structure, high surface finish (Ra ≤ 2.0 μm), and excellent ductility (elongation of 15%–40%), enabling it to withstand significant tensile and bending deformation.
However, this process is complex and requires sophisticated equipment, resulting in costs 2–3 times higher than those of electrolytic copper. Mass production is challenging, and currently only a few companies possess the capacity for large-scale production. Prior to lamination with PTFE sheets, rolled copper must undergo surface roughening and activation processes such as plasma treatment and blackening to enhance adhesion to the PTFE board.
Electrolytic copper, on the other hand, is produced using an electrochemical deposition method: copper feedstock is dissolved in sulphuric acid to form a copper sulphate electrolyte; under the influence of direct current, copper ions deposit onto the surface of the cathode roller to form the base foil, which is then subjected to roughening and anti-oxidation treatments. Its grains grow vertically in a columnar structure; whilst the structure is regular, it is relatively brittle and has poor ductility (with an elongation rate of only 5%–15%) and a relatively rough surface (Ra ≤ 3.0 μm).
Although this rough surface helps to improve mechanical bonding strength with the substrate, it increases signal transmission loss at high frequencies. Electrolytic copper offers a simple process, low cost and ease of mass production, making it the most widely used type of copper foil in the PCB industry. When laminated onto PTFE boards, bonding stability can be further enhanced by optimising the lamination process.
The process differences between the two types of copper foil can be vividly summarised as the distinction between ‘hand-polishing’ and ‘mass casting’: rolled copper pursues ultimate performance and consistency, making it suitable for high-end applications; electrolytic copper balances performance and cost, making it suitable for standard high-frequency applications. This fundamental difference is directly reflected in their respective performance characteristics after lamination with PTFE boards.
Comparison of Properties After Laminating with PTFE Sheets
1.Laminating Strength
The surface of the PTFE substrate is highly inert, making it difficult to bond with copper foil. Electrolytic copper has a rough surface; the uneven texture formed by columnar grains allows for mechanical interlocking with the PTFE substrate.
Combined with the high temperature and pressure of the lamination process, the lamination peel strength can reach 1.2–1.8 N/mm, meeting the requirements of most standard high-frequency applications. However, this method is sensitive to lamination parameters; improper control of temperature and pressure can easily result in poor bonding or localised delamination.
Rolled copper has a smooth surface; even after roughening treatment, its surface roughness remains lower than that of electrolytic copper, resulting in weaker mechanical interlocking. Its lamination strength relies primarily on the chemical bonding formed by surface activation treatment, with a typical peel strength of 1.5–2.2 N/mm, which is slightly higher than that of electrolytic copper. More importantly, the fibrous grain structure of rolled copper allows for uniform stress distribution, preventing delamination caused by localised stress concentration.
Consequently, its laminate stability under extreme conditions (such as thermal cycling and vibration) is far superior to that of electrolytic copper. For example, in temperature cycling tests ranging from -55°C to 125°C, the peel strength of rolled copper decreases by no more than 5%, whereas that of electrolytic copper decreases by 10%–15%, and local delamination may even occur.
2.High-Frequency Performance
Rolled copper has a smooth surface with low roughness, resulting in minimal skin effect losses during signal transmission. Its fibrous grain structure reduces signal reflection, with insertion loss controlled below 0.1 dB/in (at 10 GHz). It is particularly suitable for millimetre-wave applications such as 77 GHz automotive radar and satellite communications, and works synergistically with the low-dielectric properties of PTFE boards to ensure signal integrity to the greatest extent possible.
Electrolytic copper has a rough surface, causing increased signal reflection and loss at surface irregularities, resulting in higher insertion loss (0.15–0.2 dB/in @ 10 GHz). Whilst it meets the requirements of 5G base stations and standard high-frequency modules, signal attenuation is significant at extreme high frequencies such as millimetre wave, making it unsuitable for high-end applications. Furthermore, rolled copper has a conductivity of approximately 58 mS/m, which is slightly superior to the 56 mS/m of electrolytic copper; in high-power, high-frequency scenarios, this further amplifies the difference in transmission efficiency.
3.Processability
PTFE boards are inherently flexible and difficult to process; the ductility and etchability of the copper foil directly impact processes such as drilling, etching and soldering. Rolled copper offers excellent ductility and, once laminated, can withstand complex processing such as laser drilling and precision etching without fracturing or warping. However, due to its smooth surface and dense grain structure, it is difficult to form fine circuits during etching. It is recommended that the copper thickness be kept below 18 μm, with line widths and spacings designed at 4/4 mils or greater; otherwise, uneven circuit edges or incomplete etching may occur.
Electrolytic copper has relatively poor ductility; if subjected to excessive stretching or bending after lamination, it is prone to cracking or even breaking, with a particularly high risk during micro-hole drilling (hole diameter ≤ 50 μm). However, its relatively loose grain structure offers superior etching performance, allowing for finer circuit patterns with a minimum line width and spacing of 2/2 mil, making it suitable for standard high frequency PCBs with stringent requirements for circuit precision. Furthermore, the overall rigidity of electrolytic copper laminated with PTFE is slightly higher than that of rolled copper. It is less prone to warping during SMT soldering, resulting in higher soldering yield, and is therefore more suitable for mass production.
4.Environmental Resistance
Rolled copper has a dense, fibrous grain structure, offering superior oxidation and corrosion resistance. When laminated with PTFE, it can operate stably over the long term in extreme environments such as high temperatures, high humidity and radiation. For example, after 1,000 hours of storage at 150°C, the reduction in laminate strength and electrical conductivity does not exceed 3%, making it suitable for high-reliability applications such as aerospace and automotive electronics.
Electrolytic copper, with its columnar grain structure, has grain boundary defects, resulting in relatively weaker oxidation and corrosion resistance. When exposed to high-temperature and high-humidity environments for extended periods, the surface is prone to oxidation, leading to a decline in electrical conductivity, reduced laminate strength, and even copper foil delamination. Consequently, products laminated from electrolytic copper and PTFE sheets are better suited to conventional high-frequency applications in indoor, ambient-temperature environments; if used in extreme environments, additional anti-oxidation treatment is required, thereby increasing production costs.
Rolled copper offers superior performance in terms of lamination strength, high frequency pcb performance and environmental resistance, making it suitable for high-end and extreme-environment applications; electrolytic copper, on the other hand, offers advantages in terms of cost, machining precision and mass production, and is suitable for conventional high frequency scenarios.
