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The most commonly cited numbers in the selection review of high-temperature resistant copper foil tape are "long-term temperature resistance" and "short-term temperature resistance". The labeling of 200 ℃ long-term and 260 ℃ short-term seems sufficient to cover most high-temperature scenarios, and engineers judge based on this that 'the tape can work at this temperature'.
But the source of this number needs to be dissected. The melting point of copper foil is 1083 ℃, and copper will not melt or significantly soften at 200 ℃. The long-term temperature resistance of PI substrate can reach 250 ℃, and it can withstand temperatures above 260 ℃ in the short term. If we only look at the substrate, it seems that there is no problem with the tape working above 200 ℃. The problem is not with the substrate, but with the adhesive layer and the interface between the adhesive layer and the substrate.
The adhesive layer is the weakest link on the heat-resistant chain, but its failure is not due to "melting"
The upper limit of temperature resistance of pressure-sensitive adhesives is not determined by the "decomposition temperature", but by the "rate at which the modulus decreases with temperature". Pressure sensitive adhesives are in a high elastic state above the glass transition temperature Tg, with a low modulus, and rely on viscoelastic flow to wet the surface and form adhesion. When the temperature further increases to near the softening point, the modulus further decreases, and the adhesive layer undergoes creep under sustained stress. The contact pressure at the interface relaxes, and the adhesive strength decreases.
Thermal expansion mismatch: The more heat-resistant the substrate, the greater the interfacial stress may be
The long-term temperature resistance of PI substrate is high, but the coefficient of thermal expansion (CTE) of PI is usually in the range of 20-60 ppm/℃ (depending on the grade and orientation). The CTE of copper is about 16-17 ppm/℃, while the CTE of pressure-sensitive adhesive is in the range of 100-300 ppm/℃. The expansion amplitude of three-layer materials varies with temperature changes, resulting in shear stress at the interface.
Differences in thermal expansion of three-layer materials
In the temperature cycle from room temperature to 150 ℃, the expansion amplitude of the adhesive layer is the largest, the expansion amplitude of the copper foil is the smallest, and the PI substrate is between the two. The interface between the adhesive layer and the copper foil, as well as the interface between the adhesive layer and the substrate, are subjected to periodic shear. The higher the temperature, the greater the temperature difference, and the greater the amplitude of shear stress. When the temperature approaches the softening range of the adhesive layer, the modulus of the adhesive layer decreases. It should have absorbed stress through viscoelastic deformation, but the decrease in modulus also means that the interface contact pressure relaxes. The two effects compete, and the net result is delamination starting from the edge where stress is most concentrated.
This explains a counterintuitive phenomenon: a piece of tape that can work for a long time at 120 ° C, when switched to a 180 ° C environment, fails not from "layer carbonization", but from "edge warping and delamination". The adhesive layer has not decomposed, the substrate has not melted, but the interface is no longer continuous.
The second failure path under high temperature: synergistic oxidation of copper foil and overflow of adhesive layer
High temperature not only changes the mechanical state of the interface, but also accelerates the chemical evolution of the copper foil surface. The oxidation rate of copper significantly increases in high temperature and high humidity environments, and the oxide layer increases the contact resistance and deteriorates the conductivity. At the same time, the adhesive layer softens at high temperatures. If the thickness of the adhesive layer is large or the cross-linking density is insufficient, the colloid will overflow from the edge of the copper foil, and the overflowing adhesive layer may contaminate adjacent pads or change the dielectric environment of the edge.
Oxidation and overflow are two independent degradation pathways, but they accelerate simultaneously at high temperatures and overlap with each other. The oxidation of copper foil increases the overlap impedance, and the overflow of adhesive changes the physical boundary of the edge. The net effect of the two is that the shielding effectiveness and grounding impedance of the tape after high-temperature aging cannot be predicted by adding the temperature coefficient to the initial value at room temperature.
What is the difference between "substrate temperature resistance" and "tape temperature resistance"
Using the temperature resistance number of the substrate as the temperature resistance number of the tape can cause systematic overestimation in selection. The PI substrate is labeled with 250 ℃, but when paired with modified acrylic adhesive for PI copper foil tape, the long-term usable temperature usually falls within the range of 150 ℃ -180 ℃, rather than 250 ℃. PI substrate tape with silicone adhesive can be used for a long time at temperatures close to 200 ℃ -220 ℃, but the initial adhesion of silicone adhesive is low, requiring additional compression protection in production line scenarios that require quick fixation.
The temperature resistance standard for industrial grade copper foil tape has a more conservative statement: the long-term working temperature covers -40 ℃ to 150 ℃, and the short-term tolerance is 180 ℃. Within this range, the conductivity performance does not decrease by more than 15%, and the adhesive layer does not crack or peel off. The boundary of "150 ℃ long-term" is not determined by the substrate, but by the comprehensive thermal stability of the "substrate adhesive layer interface" as a system.
Selection judgment based on "service temperature x duration x interface stress"
Firstly, break down the temperature resistance index into two dimensions: "adhesive layer temperature resistance" and "interface temperature resistance", rather than just looking at the substrate labeling
Suppliers are required to provide both the Tg or softening point data of the adhesive layer and the peel strength retention rate after high-temperature aging. A reference engineering boundary is a peel strength retention rate of ≥ 75% after thermal aging at 120 ℃/168 hours; If the target temperature is higher, data on the order of 150 ℃/1000 hours should be required.
Secondly, evaluate the long-term usability of the adhesive layer based on the product of temperature and time, rather than just looking at short-term peaks
Arrhenius' law suggests that for every 10 ℃ increase in temperature, the rate of material aging approximately doubles. The aging process that runs for 1000 hours at 150 ℃ may take several times longer to be equivalent at 120 ℃. When selecting, it is necessary to confirm at what temperature and duration the supplier's temperature resistance data was measured, rather than accepting an isolated "long-term temperature resistance" label.
Thirdly, in high-temperature scenarios, priority should be given to selecting silicone adhesive systems, but at the cost of initial adhesion and cost
The continuous temperature resistance of silicone adhesive can reach 180 ℃ -220 ℃, and its adhesion retention rate and lifespan at high temperatures are usually better than modified acrylic acid. However, the initial adhesion of silicone adhesive is relatively low, and more thorough pressing or rolling is required during installation; The cost is usually 2-3 times that of acrylic acid systems. If the target temperature is in the range of 120 ℃ -150 ℃, modified acrylic acid may be the starting point for a more balanced cost and performance.
Fourthly, consider the edge interface as an independent verification item for high-temperature reliability, rather than only measuring the peel strength in the central area
High temperature delamination starts from the edges, and the peel strength test in the central area cannot expose the stress concentration at the edges. For high-temperature service scenarios, during the sample stage, the edge of the tape should be checked for warping, glue overflow, or delamination after high-temperature aging, and local changes in overlap impedance or shielding effectiveness should be retested in the edge area.
Advanced Institute (Shenzhen) Technology Co., LtdThe high-temperature resistant copper foil tape product line covers modified acrylic and silicone adhesive systems, PI and modified PET substrates, providing adhesive layer Tg/softening point data, high-temperature aging peel strength retention data at 120 ℃/168 hours and 150 ℃/1000 hours, as well as edge overflow and interface integrity assessment after high temperature.
The product supports matching the adhesive layer system with the substrate based on the target service temperature and duration. For more technical information, please visithttp://www.avanzado.cn/.