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The specification sheet for high-temperature resistant copper foil tape usually provides two temperature numbers: long-term resistance and short-term resistance. Engineers use these two numbers to determine whether the tape can work at the target temperature. This judgment implies an assumption: high temperature is the main source of risk, and as long as the maximum temperature does not exceed the limit, the tape is usable.
The results of temperature cycling tests sometimes refute this hypothesis. The tape remains stable at the target high temperature for hundreds of hours, maintaining good peel strength and shielding effectiveness; After dozens of cycles from high temperature to low temperature, the performance of the same tape at room temperature has significantly decreased. The failure is not in the 'hot' itself, but in the 'cold' half of the cycle.
The mechanical state of the adhesive layer is completely different between heating and cooling
Pressure sensitive adhesive is a viscoelastic material whose modulus and mechanical response vary with temperature and in different directions.
Heating stage: The adhesive layer becomes "soft" and the stress can flow
During the heating stage, the adhesive layer transitions from room temperature to high temperature. The increase in temperature causes a decrease in the modulus of the adhesive layer, making the colloid softer and more prone to deformation. At this point, the relative displacement between the copper foil, adhesive layer, and substrate due to differences in thermal expansion coefficients is partially absorbed by the viscoelastic deformation of the adhesive layer. The adhesive layer acts as a cushion, dissipating the shear stress generated by thermal mismatch through creep and stress relaxation. The slower the heating rate, the more sufficient the time for the adhesive layer to absorb stress, and the lower the accumulated stress at the interface.
Cooling stage: The adhesive layer becomes "hard" and stress can only accumulate
During the cooling phase, the situation reverses. The temperature drops from high temperature to room temperature, and the modulus of the adhesive layer gradually recovers, changing from "soft" to "hard". But at the same time, the copper foil and substrate have started to shrink, and the shrinkage amplitude of the adhesive layer is much greater than both - the thermal expansion coefficient of the adhesive layer is usually in the range of 100-300 ppm/℃, the copper foil is about 16-17 ppm/℃, and the PI substrate is in the range of 20-60 ppm/℃. The adhesive layer "wants" to shrink more, but is "pulled" by the copper foil and substrate. This shrinkage mismatch generates tensile and shear stresses at the interface, and the adhesive layer has already hardened, making it impossible to release stress through creep as it does during heating.
Cooling stage: window where stress cannot be released
When heated, the adhesive layer is "soft" and stress can flow; When cooling down, the adhesive layer is "hard" and stress can only accumulate. This is the root cause of asymmetric temperature cycling damage.
Two consequences of the cooling process
During the cooling process, the stress at the interface increases monotonically as the temperature decreases. When the temperature returns to around room temperature, the modulus of the adhesive layer has already recovered to a relatively high level, and the interfacial stress reaches its peak during cycling. If this peak exceeds the adhesion strength between the adhesive layer and the copper foil or substrate, local debonding begins at the location where stress is most concentrated - usually at the edges, corners, or adjacent areas of the tape to the rigid structure.
More importantly, this damage is cumulative. The stress of the first cycle may not be sufficient to trigger debonding, but each cooling is "loaded" at the interface, and micro damage gradually expands. After dozens to hundreds of cycles, the debonding area extends inward from the edge, the continuity of the interface is disrupted, the contact resistance increases, and the shielding effectiveness decreases. IEEE research observed a significant increase in contact resistance and contact capacitance after 2400 hours of storage at 110 ℃, resulting in a 30 dB decrease in magnetic field shielding effectiveness below 500 MHz - even at steady-state high temperatures, the degradation rate of the interface will be faster when combined with cyclic stress during the cooling stage.
Why is cooling failure "invisible" during steady-state high-temperature testing
The steady-state high-temperature test only considers the "hot" half. The tape is placed in a constant temperature chamber without temperature cycling, and the adhesive layer does not undergo repeated switching from "softening to hardening". The interface stress is basically stable after the initial thermal equilibrium, and there will be no monotonic accumulation of stress peaks during the cooling stage.
Therefore, a tape that has passed 1000 hours of testing at a constant temperature of 150 ℃ may experience edge debonding after dozens of cycles between -40 ℃ and 150 ℃. This is not because the material is "not heat-resistant", but rather because its adhesive layer system lacks sufficient stress relaxation ability or interfacial adhesion strength during the cooling stage. Steady state data cannot predict cyclic behavior because they examine different mechanical paths.
Selection judgment based on "interface stress during cooling stage"
Firstly, suppliers are required to provide performance data after temperature cycling, rather than just providing steady-state high-temperature data
The temperature cycle should cover the low and high temperature endpoints of actual service, with a minimum of ten cycles. After testing, the edges should be checked for warping, delamination, or glue overflow, and the contact resistance or shielding effectiveness should be retested. Steady state high-temperature data can only prove that the tape can be used when it is hot, but cannot prove that it can still be used after alternating between hot and cold.
Secondly, in the selection of adhesive layer system, attention should be paid to the stress relaxation ability during the cooling stage, not just the softening point
Whether the adhesive layer can maintain a certain degree of viscoelasticity during the cooling process and partially release interfacial stress is more important than simply having a high softening point. The stress relaxation behavior of modified acrylic and silicone adhesives at high temperatures is different. When selecting, it should be confirmed whether the supplier provides data on the retention rate of peel strength after cycling, not just the initial peel strength.
Thirdly, reduce stress concentration during the cooling phase in structural design
The edges and corners of the tape are the concentrated locations of cooling stress. If space permits, using rounded transitions, avoiding right angle corners, or extending the edge of the tape beyond the rigid structure can reduce stress peaks at the edges. For large-area shielding layers, use whole piece die-cutting instead of splicing to reduce the number of interfaces at the seams.
Fourth, use "room temperature performance after cycling" as the acceptance indicator during the sample stage, rather than "high temperature performance during cycling"
The consequences of cooling damage usually only become fully apparent after returning to room temperature. After the temperature cycling test is completed, the lap resistance, peel strength, and shielding effectiveness should be retested at room temperature and compared with the room temperature data before cycling. If the performance at room temperature decreases beyond an acceptable range after cycling, it indicates that interface damage has occurred during the cooling stage and the adhesive layer system or structural design needs to be adjusted.
Advanced Institute (Shenzhen) Technology Co., LtdThe high-temperature resistant copper foil tape product line covers modified acrylic and silicone adhesive systems, providing data on peel strength retention, contact resistance changes, and edge interface integrity after temperature cycling from -40 ℃ to 150 ℃, helping engineers extend "temperature resistance" from the highest temperature indicator to "usability after cycling".
For more technical information, please visithttp://www.avanzado.cn/.