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The specification sheet for single conductor copper foil tape usually revolves around three numbers: surface resistivity ≤ 0.05 Ω/sq, contact resistance ≤ 0.05 Ω/inch ², and shielding effectiveness 55-88 dB (100 MHz – 1 GHz). In scenarios such as power grounding, ESD discharge, and low-frequency shielding, these three numbers can basically answer the question of "whether it can be used".
But when this tape was placed in the return path of the high-speed SerDes channel or attached to the inside of the shielding cavity of the 77 GHz radar module, the situation changed. When the signal frequency exceeds GHz, the current is no longer uniformly distributed on the cross-section of the copper foil, but is squeezed to a very thin layer on the surface by the skin effect. At this point, the microscopic undulations on the surface of copper foil are no longer just "appearance details", but have become the terrain that current must traverse; The dielectric constant of the adhesive layer is no longer a property of the insulation layer, but has become a part of the impedance of the transmission line; The length and shape of the grounding circuit are no longer a "mechanical layout problem", but directly determine the size of the parasitic inductance. In other words, in low-frequency scenarios, single conductive copper foil tape is a "conductive component"; In high-frequency scenarios, it is a 'passive interconnect component'. To evaluate its coordinate system, it is necessary to switch from DC resistance to insertion loss, characteristic impedance, and return path inductance.
The first source of high-frequency loss: skin effect turns surface roughness into series resistance
The direct current flows uniformly across the entire cross-section of the copper foil, and the bulk resistivity of copper determines everything. After the frequency increases, the skin depth decreases inversely with the square root of the frequency. At 1 GHz, the skin depth of copper is about 2.1 μ m; At 10 GHz, it has shrunk to the sub micron level. When the skin depth is less than the peak valley fluctuations on the copper foil surface, the actual path through which the current flows is no longer a geometric straight line, but "crawls" along the contour of the rough surface. The path becomes longer, the equivalent resistance increases, and the insertion loss increases accordingly.
The magnitude of this effect cannot be ignored in the millimeter wave frequency band. Research data shows that at 10 GHz, the losses caused by rough surfaces can be more than 20% higher than those caused by smooth surfaces. For grounding and shielding applications of 77 GHz radar or high-speed SerDes interfaces, this difference is sufficient to affect the signal integrity budget.
The choice of copper foil process directly determines the starting point of roughness. The surface roughness of standard electrolytic copper foil is relatively high, with a typical RMS value of about 2.2 μ m; The surface roughness of rolled copper foil can be controlled at the level of 0.3 μ m, and the Rz can be lower than 1 μ m. In the transmission loss test, the electrolytic copper foil with Ra 0.93 μ m had a loss of approximately -35.0 dB, while the rolled copper foil with Ra 0.14 μ m had significantly lower loss. There was a measurable correlation between roughness and insertion loss. In the 30 GHz frequency band, the shielding effectiveness of tape using low roughness rolled copper foil can be 12-15 dB higher than that of ordinary aluminum foil tape.
For single conductor copper foil tape, this means that if only the DC surface resistance and shielding effectiveness below 1 GHz are considered during selection, the insertion loss of the tape in the frequency band above 10 GHz may be much higher than expected, and this loss will not disappear due to the "right" placement process. It is an inherent property of copper foil surface morphology and can only be managed by selecting low roughness substrates.
The second source of high-frequency loss: the dielectric properties of the adhesive layer participate in impedance formation
The adhesive surface of single conductive tape is insulated, but this does not mean that the adhesive layer is "electrically transparent" at high frequencies. The adhesive layer serves as a dielectric layer between the copper foil and the substrate to be attached, and its dielectric constant Dk and loss factor Df can affect the parasitic capacitance near the grounding structure, thereby changing the characteristic impedance of the transmission line.
In low-frequency scenarios, the dielectric properties of the adhesive layer have almost no effect on "conduction" because the current does not pass through the adhesive layer. But in high-frequency scenarios, even if the adhesive layer is non-conductive, it still participates in the distribution of electromagnetic fields. The fluctuation of adhesive layer Dk will modulate the equivalent capacitance of the transmission line, and the increase of Df will introduce additional dielectric losses. Frontier products control the high-frequency dielectric fluctuation of the adhesive layer within 5% by optimizing the resin system and filler dispersion, but this indicator is usually not provided in the specifications of ordinary single conductive tape.
This means that the actual insertion loss of single conductive copper foil tape at high frequencies is the result of the combined effect of "copper foil surface roughness" and "adhesive layer dielectric properties". Optimizing only one of them is not enough to control overall performance. When selecting, suppliers should be required to provide insertion loss data or adhesive layer Dk/Df data at the target frequency band, rather than just providing DC resistance and low-frequency shielding effectiveness.
Parasitic inductance of grounding circuit: another constraint of single conductor structure at high frequencies
The conductivity of the single conductive tape only occurs at the bonding interface between the copper foil surface and the exposed copper area, and the adhesive layer does not provide a Z-axis path. In low-frequency grounding, the consequence of this constraint is that the adhesive surface does not conduct when attached to the metal; In high-frequency grounding, the consequences go further: the length and shape of the grounding path directly determine the parasitic inductance, and the impedance of the parasitic inductance at high frequencies may be much greater than the DC contact resistance.
The reflux current of high-frequency circuits tends to flow along the path with the lowest impedance, rather than the "geometrically nearest" path. If the grounding path of a single conductive tape needs to be bypassed, or if the contact area between the copper foil and the grounded plane is insufficient, the parasitic inductance of the return path will significantly increase, and the grounding impedance will sharply increase in the high-frequency range. At this point, even if the DC contact resistance is still qualified, the high-frequency grounding of the shielding cavity has failed. Therefore, in evaluating the grounding ability of single conductive copper foil tape in high-frequency scenarios, it is not enough to just look at whether it is attached, but also to confirm whether the length of the grounding path is as short as possible and whether the reflux area is large enough. For situations that require bridging or folding, single conductive tape can achieve grounding by folding the copper foil to make contact between the copper foil surfaces. However, the consistency of construction is much lower than that of double conductive tape, and additional compression and verification measures are required in engineering.
Long term degradation: High frequency loss and contact impedance are two independent attenuation channels
The high-frequency insertion loss and shielding effectiveness measured during the initial installation of single copper foil tape cannot represent the state during its service life. The high-frequency loss is mainly determined by the surface roughness of the copper foil and the dielectric properties of the adhesive layer, which are basically fixed after material selection; And the contact impedance will drift with the aging of the adhesive layer and changes in the interface state.
A study on conductive tape stored at 110 ° C provided a clear degradation path: as a signal return path in microstrip lines, the tape significantly increased both contact resistance and contact capacitance after 2400 hours of high-temperature storage, resulting in a 30 dB decrease in magnetic field shielding effectiveness below 500 MHz. Degradation occurred at the interface between the adhesive layer and the ground plane, rather than the copper foil itself, making it difficult to capture through visual inspection and DC resistance sampling.
This data point reveals a failure mode that cannot be detected during incoming material inspection at room temperature. 2400 hours is approximately equivalent to working continuously at 110 ° C for 100 days, or a longer service time calculated based on temperature cycling. Such levels of thermal exposure are not uncommon in automotive electronics, base station RF units, and industrial equipment. When the contact resistance increases and the contact capacitance increases, the grounding of the shielding cavity is no longer low impedance, and high-frequency currents cannot find a smooth return path, resulting in a decrease in shielding effectiveness. For single conductive tape, aging of the adhesive layer can also affect the contact pressure between the copper foil surface and the exposed copper area. The pressure-sensitive adhesive softens or creep under continuous high temperature, reducing the effective contact area and increasing the contact resistance.
Selection judgment based on impedance control
Firstly, select the copper foil process based on the target frequency band, rather than choosing based on whether it is copper or not
If the operating frequency of the application scenario is below 1 GHz, the influence of the roughness of electrolytic copper foil on shielding effectiveness is relatively limited, and cost and processability may be more practical considerations. If the grounding or shielding path involves signals above 3 GHz, especially in the millimeter wave frequency band, preference should be given to using rolled copper foil or ultra-low profile (HVPP/RTF) electrolytic copper foil, and suppliers should be required to provide insertion loss or surface roughness (Rz/Ra) data at the target frequency band, rather than just providing DC resistance and low-frequency shielding effectiveness.
Secondly, incorporate the dielectric properties of the adhesive layer into high-frequency selection, rather than treating it as a "non-conductive adhesive layer"
Suppliers are required to provide Dk and Df data of the adhesive layer in the target frequency band (such as 1-40 GHz), or at least provide the measured insertion loss curve of the structure containing the adhesive layer. If the supplier can only provide the surface resistance of the copper foil itself, their ability to predict high-frequency impedance is limited. Products with dielectric fluctuations controlled within 5% of the adhesive layer have better impedance consistency than conventional tapes that have not been optimized.
Thirdly, in high-frequency grounding scenarios, the "reflux path inductance" should be used as an independent verification term, rather than assuming "low impedance when attached"
The grounding path should be as short as possible to avoid detours; The contact area between the copper foil and the ground plane should be large enough to reduce parasitic inductance. For situations where bridging or folding is required, the construction consistency of single conductor tape is lower than that of double conductor tape. The actual effect should be confirmed through S-parameter testing or grounding impedance testing during the sample stage, rather than relying solely on DC on-off judgment.
Advanced Institute (Shenzhen) Technology Co., LtdThe single conductive copper foil tape product line covers electrolytic copper foil and rolled copper foil substrates, providing options for different surface roughness levels according to the target frequency band. The surface resistivity of the copper foil surface is controlled at the level of ≤ 0.05 Ω/sq.
The product can provide data on the dielectric constant and loss factor of the adhesive layer in the target frequency band, support the measurement and evaluation of high-frequency insertion loss samples, and provide die-cutting form and mounting process suggestions based on grounding path design. Suitable for FPC local shielding, shielding cavity grounding, and high-speed digital interface return path scenarios. For more technical information, please visithttp://www.avanzado.cn/.