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In the design review of shielding cover lining or connector compression, the selection of conductive tape is usually based on surface resistance and shielding effectiveness. The engineer confirms that the surface resistance is ≤ 0.05 Ω/sq and the shielding effectiveness is>60 dB. Then, the thickness is selected according to the structural gap, and the plan enters assembly verification.
The problem often occurs in the assembly pressure setting stage. In order to ensure reliable contact, the production line tends to increase the pressing pressure; The structural design tends to leave a large amount of compression to avoid looseness. The combination of two forces pressed the conductive cloth tightly. The initial contact resistance test passed, but after temperature cycling or vibration, the grounding impedance of some units increased and the shielding effectiveness decreased. Conductive cloth is not a material that becomes better with increasing pressure. There is a non monotonic relationship between its contact resistance and compression rate, which first decreases and then increases.
Initial compression stage: fiber overlap area increases, contact resistance decreases
The substrate of conductive cloth tape is a network formed by interweaving metalized fibers. In the uncompressed state, the contact between fibers is "line contact" or "point contact", and the overlapping area is limited. When the conductive fabric is compressed, the relative position between fibers is adjusted, the contact area at the interweaving point increases, the conductive path increases, and the contact resistance decreases accordingly. This is the first layer of benefit brought by compression.
Interface filling effect caused by micro compressibility
Conductive fabric itself has a certain degree of micro compressibility. At the compression interface, the fiber network can fill the micro unevenness of the substrate surface, turning "point contact" into "surface contact" and reducing the interface contact resistance. For metal shells or inner walls of shielding covers with high surface roughness, this filling effect is more effective than rigid materials. At this stage, as the compression ratio increases from 0 to 20% -30%, the contact resistance usually shows a decreasing trend. This is also the source of the belief that 'the tighter the pressure, the better'.
Exceeding the critical compression rate: Fiber crushing and coating damage begin to dominate
When the compression ratio continues to increase and exceeds a certain critical value, the downward trend of contact resistance stops and even begins to rise. The reason is not 'insufficient contact area', but 'damage to the conductive network'.
Fiber crushing and microcracks in coating
The fiber of the conductive cloth is polyester fiber, and the surface is coated with a metal layer. Fibers themselves have a certain degree of elasticity, but beyond their elastic deformation range, fibers will be crushed and bent, and the interwoven structure will undergo irreversible deformation. After the fiber is crushed, micro cracks appear in the metal coating at the crease, and the continuity of the conductive path is disrupted. The adhesion between the coating and the fibers is also tested under large deformation, and local peeling can directly cut off the current path.
Decline in rebound rate and permanent compression deformation
More importantly, rebound. Conductive cloth tape usually serves the function of "compression rebound" in laminated structures: it is compressed during assembly and maintains contact pressure with both sides of the structure during service by relying on the rebound force. If the compression rate is too high, the fiber network undergoes plastic deformation, the rebound rate decreases, and the compression permanent deformation rate increases. After temperature cycling or vibration, the conductive cloth cannot recover to a sufficient thickness, the contact pressure relaxes, and the grounding impedance increases.
In industry engineering practice, it is usually recommended to control the compression rate of conductive fabric liners within the range of 20% to 40%. Less than 20%, insufficient contact area, high contact resistance; Above 40%, the rebound rate decreases and the long-term reliability risk increases. The specific window depends on the fabric structure, coating system, and adhesive layer modulus, but the trend of "decreasing first and then increasing" is common.
Plain weave and mesh: differences in compression characteristics
The compression behavior of plain weave fabric and mesh fabric is not the same. The warp and weft lines of plain weave fabric are tightly interwoven, with high fiber density. The contact area increases faster during the initial compression stage, and the decrease in contact resistance is more significant. However, the compressible space of plain weave fabric is relatively limited, and beyond a certain compression rate, the risk of fiber crushing is higher. The weaving of mesh fabric is loose, with a high porosity and a larger compressible space, which can still maintain a certain degree of rebound at higher compression rates; But the initial contact area is small, requiring a larger compression amount to achieve a contact resistance level comparable to plain weave fabric.
The engineering significance of this difference is that plain weave fabric is suitable for scenarios with low compression and high initial contact resistance requirements; Mesh fabric is suitable for scenarios with high compression, high requirements for rebound and flexibility. When selecting, one should not only consider the "surface resistance" and "thickness", but also confirm the recommended compression rate window corresponding to the base fabric structure.
Adhesive modulus: How compressive stress is transmitted to fiber networks
The adhesive layer of conductive tape plays two roles during compression: bonding and stress transmission. The modulus of the adhesive layer determines how compressive stress is distributed on the fiber network. The stress transmitted by the high modulus adhesive layer during compression is more concentrated, and the local pressure borne by the fiber interweaving points is higher, resulting in an increased risk of crushing; Low modulus adhesive layers can cushion some stress, but may experience creep under long-term compression, leading to contact pressure relaxation.
For applications that require long-term maintenance of compression rebound, the glass transition temperature and compression permanent deformation rate of the adhesive layer are more critical parameters than the initial viscosity. If the adhesive layer softens or creep at the service temperature, even if the fiber network itself is not crushed, the contact pressure will decrease over time and the grounding impedance will increase.
Selection judgment based on "compression rate x rebound rate"
Firstly, use the compression ratio window as a selection parameter instead of 'the tighter the compression, the better'
Confirm with the supplier the recommended compression range for the target fabric structure (typically 20% -40%), and set the gap and compression amount according to this window in the structural design. When the compression ratio is lower than the lower limit of the window, the contact resistance is higher, and when it is higher than the upper limit, the rebound rate decreases. Both deviations will manifest as ground impedance drift in long-term use.
Secondly, consider the rebound rate and compression permanent deformation rate as independent reliability indicators, rather than just looking at the initial contact resistance
The initial contact resistance is qualified, but it does not mean that it is still qualified after temperature cycling and vibration. Request the supplier to provide compression permanent deformation rate data (optimized system can be controlled below 15%), as well as contact resistance change data after compression cycling. Conductive cloth with insufficient rebound rate will gradually "lose pressure" in the compression structure, and the grounding impedance will increase accordingly.
Thirdly, distinguish the compression characteristics of plain weave and mesh, and match the fabric structure according to the application scenario
For scenarios with low compression and high initial contact resistance requirements, plain weave fabric is preferred; Priority should be given to mesh fabric in scenarios where compression is high and flexibility and rebound are required. When selecting, confirm whether the supplier provides the compression ratio contact resistance curve or recommended compression window for the corresponding base fabric structure, rather than just providing a single surface resistance value.
Fourth, control the consistency of pressing pressure in the assembly process
The contact resistance of conductive fabric is sensitive to compression, and uneven distribution of compression pressure can lead to different compression rates in different areas of the same conductive fabric, resulting in discrete contact resistance. The production line should confirm the pressure uniformity of the compression equipment and measure the contact resistance at different positions during the sample stage, rather than just randomly checking the center point.
Advanced Institute (Shenzhen) Technology Co., LtdThe conductive cloth tape product line covers two types of base fabric structures, plain weave and grid, with surface resistivity controlled at the level of ≤ 0.05 Ω/sq. It can provide recommended compression rate windows, compression permanent deformation rates, and contact resistance change data after compression cycles for the target base fabric structure.
The product is suitable for scenarios that require compression rebound, such as shielding cover lining, connector crimping, and shell grounding. For more technical information, please visithttp://www.avanzado.cn/.