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In the selection review of EMI shielding copper foil tape, engineers usually confirm two things: whether the shielding effectiveness of the copper foil is high enough, and whether the conductivity of the tape meets the standard. The number of "shielding effectiveness 80 dB" on the specification sheet seems sufficient to meet most EMC requirements. The material passes and the plan enters assembly.
The EMC test results of the whole machine are out, and the radiation emission exceeds the standard. The engineer retested the shielding effectiveness of the copper foil tape and found no issues with the material itself. The problem lies in whether the shielding layer is continuous after the copper foil is attached.
The ceiling of shielding effectiveness is not on the copper foil, but on the seams
The shielding effectiveness of metal shielding bodies is theoretically determined by the conductivity and thickness of the material. For copper foil, the skin depth at 1 GHz is about 2.1 μ m, and a copper foil thickness of 0.03 mm far exceeds the skin depth. The shielding effectiveness of the material itself can reach 80-100 dB. This number describes the shielding ability of a "continuous seamless copper foil".
Discontinuities are the starting point of leakage
But in actual assembly, copper foil tape is not a continuous and seamless copper foil. It is made up of multiple tapes spliced together, with seams between each tape; There is an overlapping interface between the tape and the shielding cover, chassis, and PCB ground plane; The edge of the tape has a cut, and the coating at the cut is exposed but not connected to any conductor. These 'discontinuous points' constitute the channels for electromagnetic leakage.
Gap antenna effect: leakage of 0.1 mm gap at 1 GHz
The degree to which electromagnetic waves leak through gaps depends on the ratio of the size of the gap to its wavelength. When the gap length approaches λ/2, the gap itself becomes an efficient "gap antenna", and leakage significantly increases. The free space wavelength of 1 GHz is about 300 mm, with a λ/2 of 150 mm; the wavelength of 10 GHz is about 30 mm, with a λ/2 of 15 mm.
This relationship may seem to imply that 'small gaps will not leak', but in practical engineering, the leakage of gaps depends not only on their length, but also on the voltage difference between the conductors on both sides of the gap and the depth of the gap. A gap 0.1 mm wide and several millimeters long may not form an efficient slot antenna at 1 GHz, but it will create a local impedance discontinuity, disrupting the continuity of the surface current of the shielding layer. The surface current is forced to bypass at the gap, and the inductance of the bypass path raises the local impedance, causing leakage from these high impedance points. When multiple tapes are spliced, the number and total length of seams directly determine the total amount of leakage. A 1-meter-long seam, even if the gap is only 0.05 mm wide, the accumulated leakage in the GHz frequency band may lower the system shielding effectiveness from 80 dB to below 40 dB.
Overlapping impedance: the "last centimeter" between the tape and the shielding cover
The overlapping interface between copper foil tape and shielding cover, chassis or PCB ground plane is another easily overlooked source of leakage. The adhesive tape is bonded to the metal surface through a backing adhesive, and the conductivity of the adhesive layer (single or double) determines the bonding impedance. Even with double conductive tape, Z-axis conductivity depends on the physical contact of particles under pressure. When the pressure is insufficient or the surface is contaminated, the overlap impedance may be much higher than expected.
More importantly, the area and shape of the overlapping interface. If the tape only overlaps with the shielding cover at the edge, the overlap area is small, and the current density of high-frequency current at the overlap is high, causing local impedance to rise. If the tape covers the entire inner wall of the shielding cover, the overlapping area is large, the current distribution is uniform, and the influence of overlapping impedance is dispersed. The design of the overlapping method determines the shielding effectiveness of the system more than the conductivity of the tape itself.
Edge effect: "open port" at the edge of the tape
The edge of copper foil tape is the physical boundary of the shielding layer. The copper foil cut at the edge is exposed to the environment and not connected to any conductor. From the perspective of electromagnetic fields, this edge is an "open port" - where surface currents terminate and the electromagnetic field radiates outward.
The severity of edge effects depends on the distance between the edge and the shielding wall. If the edge of the tape is tightly attached to the wall of the shielding cover and the edge is "sealed", the leakage is limited; If the edge of the tape is suspended or there is a gap between it and the wall, the edge becomes the concentrated location of leakage. The common practice in engineering is to extend the edge of the tape beyond the wall of the shielding cover, and then use conductive pads or a second layer of tape to press the edge and "seal" it.
Adhesive wrinkles and bubbles: leakage channels invisible to the naked eye
Copper foil tape is prone to wrinkles and bubbles during manual installation. The copper foil at the crease is raised, forming an air gap between it and the substrate, which becomes a leakage channel. The adhesive layer at the bubble is not in contact with the substrate, and Z-axis conductivity is not established, resulting in an increase in local overlap impedance.
These defects may not be obvious during visual inspection, but at high frequencies, each wrinkle and bubble is a local impedance transition point. For scenarios that require high shielding effectiveness, the consistency of mounting quality is more difficult to control than the material parameters of the tape itself, and it is also more difficult to detect through incoming inspection.
Selection and construction judgment of EMI shielding copper foil tape based on "system continuity"
Firstly, convert the target value of shielding effectiveness from "material level" to "system level"
The 80 dB on the specification sheet represents the shielding effectiveness of the material under ideal conditions. The shielding effectiveness at the system level needs to deduct the leakage contributions of seams, overlaps, and edges. If the system target requires 40 dB, material grade 80 dB may be sufficient; If the system goal requires a level of 60 dB or higher, the design of joints and overlaps needs to be given equal attention as material selection.
Secondly, reduce the number of seams and prioritize using whole piece die-cutting instead of multiple splices
Seams are the main source of leakage. If the size of the shielding area allows, priority should be given to using whole die cut copper foil tape instead of splicing multiple standard width tapes together. The cost of whole piece die-cutting is higher than splicing, but it eliminates seam leakage and improves the consistency of system shielding effectiveness.
Thirdly, the overlapping interface should have "surface contact" instead of "line contact"
The overlap between the tape and the shielding cover or ground plane should be maximized in contact area. The edge of the tape extends beyond the wall of the shielding cover, and the edge is compressed with a conductive pad or edge pressing structure to form a surface contact rather than a wire contact. The cleanliness of the overlap surface is equally crucial, as oil stains and oxide layers can block particle contact and increase the overlap impedance.
Fourthly, incorporate the quality of mounting into the process specifications, rather than relying on the operator's hand feel
Wrinkles and bubbles are hidden sources of high-frequency leakage. When mounting, use a rolling tool to remove air from the center to the edges, rather than smoothing with fingers. For scenarios requiring high shielding effectiveness, sampling inspection should be conducted after installation to confirm that the tape surface is flat, free of visible bubbles and curled edges.
Advanced Institute (Shenzhen) Technology Co., LtdThe EMI shielding copper foil tape product line covers bare copper, tin plated, and nickel plated substrates, with single and double conductive structures available. It supports whole piece die-cutting and irregular die-cutting to reduce the number of seams.
The product can provide material level shielding effectiveness data and provide die-cutting scheme suggestions based on the customer's shielding area size and overlap method, helping engineers convert material level shielding effectiveness into system level continuity design. For more technical information, please visithttp://www.avanzado.cn/.