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The selection of tinned copper foil tape is usually based on the core reason of "oxidation resistance": the tin layer covers the copper surface, preventing copper from coming into contact with air and maintaining stable contact resistance over long-term use.
This logic holds true in scenarios of room temperature, short-term, and dryness. But when the working temperature increases and the service time lengthens, the tin layer itself will participate in a slow interface reaction. Tin is not an inert "protective film", it will continuously react with the copper substrate and be converted into intermetallic compounds; The Kirkendall voids formed during the reaction process will ultimately alter the electrical behavior of the contact interface. At this point, the tin layer not only provides protection, but also accumulates interface resistance over time.
The tin layer is not a "passivation layer", but a consumed reactant
Tin and copper begin to diffuse into each other at room temperature, forming Cu ₆ Sn ₅ at the interface, and further forming Cu ₆ Sn at high temperatures. Cu ₆ Sn ₅ and Cu ₅ Sn are both intermetallic compounds with lower conductivity than pure tin and pure copper, and higher brittleness. As the temperature increases and time prolongs, the IMC layer continues to thicken, and the tin layer and copper substrate are consumed synchronously.
The electrical consequence of this process is not that the tin layer oxidizes, but that the tin layer becomes thinner and the interface becomes more complex. At DC and low frequencies, the contact resistance is composed of a residual tin layer, an IMC layer, and an unreacted copper substrate connected in series. The resistivity of the IMC layer is higher than that of pure metal, and its thickening directly increases the series resistance. More importantly, there are Kirkendall voids: the atomic diffusion rates of Cu and Sn are different, and vacancies aggregate and merge on one side of the Cu ∝ Sn/Cu interface, forming sub micron sized voids. Hollow reduces the effective conductive cross-sectional area, increases contact resistance, and in severe cases, forms a "semi broken" state.
The patent data indicates the magnitude of this process: during heating at 175 ° C for 1000 hours, the contact resistance of structurally defective samples increases to over 10 m Ω; The optimized sample with a complete Cu ∝ Sn coating only showed a slight increase in contact resistance. This indicates that the degradation of contact resistance under long-term high temperature is not a problem of "insufficient thickness" of the tin layer, but rather a problem of whether the interface IMC structure is controlled.
Degradation of shielding effectiveness after high-temperature storage: direct evidence at the tape level
The electrical function of tinned copper foil tape in shielding cavity grounding and FPC return path is not only "copper foil surface conductivity", but also includes the impedance stability of the adhesive layer interface and copper foil substrate interface at high frequencies.
Research on High Temperature Storage at 110 ° C/2400 Hours
After storing at 110 ° C for 2400 hours, both contact resistance and contact capacitance significantly increased, resulting in a 30 dB decrease in magnetic field shielding effectiveness below 500 MHz. Degradation occurs at the contact impedance level, rather than the bulk resistance of the copper foil itself. For tin plated copper foil tape, the IMC growth between the tin layer and the copper substrate, as well as the aging of the interface between the adhesive layer and the substrate, will jointly contribute to this impedance increase.
The time scale of this data is worth noting: 2400 hours is approximately equivalent to working continuously at 110 ° C for 100 days. Such levels of thermal exposure are not uncommon in automotive electronics, base station RF units, and industrial equipment. Room temperature incoming inspection and short-term EMC testing cannot expose this failure mode, as it occurs in a slow evolution after being "qualified".
High frequency dimension: Tin layer thickness becomes a double-edged sword under skin effect
In low-frequency and DC scenarios, the thickness of the tin layer mainly affects how much tin is left to participate in conductivity; In the frequency band above GHz, the role of tin layer thickness has changed.
The resistivity of tin is more than 6 times that of copper. At 10 GHz, the skin depth of copper is about 0.66 μ m, while the typical thickness of tin plating is 0.8-1.2 μ m, which is close to or exceeds the skin depth. This means that the conduction path of high-frequency current is no longer dominated by copper, but is "intercepted" by the tin layer. The thicker the tin layer, the higher the proportion of high-frequency current propagating in the tin, and the greater the insertion loss. In the 28 GHz frequency band, the additional insertion loss introduced by the tin deposition process can reach over 0.2 dB/cm. For tape with grounding and shielding as its main functions, this high-frequency loss may not directly damage the signal, but it changes the impedance of the grounding circuit. In the grounding path near the millimeter wave module or the return path of high-speed SerDes, the high-frequency resistance component of the tin layer overlaps with the interface resistance of the IMC layer, making "grounding" no longer a low impedance node in the high-frequency range. IMC growth accelerates at high temperatures, which means that the high-frequency grounding impedance depends not only on the initial tin layer thickness, but also on the service temperature and time.
Engineering boundaries for suppressing pathways: Ni barrier layer, annealing, and alloying
Since the consumption of tin layer and Kirkendall voids are the root causes of long-term degradation, the engineering response mainly revolves around "slowing down the Cu Sn reaction".
Ni barrier layer
Insert a layer of nickel between copper and tin to block or slow down the diffusion of Cu into Sn. When Ni is used as the substrate material, the tendency for tin whisker growth is also relatively small. However, there is a window in the thickness of the Ni layer: if it is too thin, the diffusion barrier is insufficient, and if it is too thick, it is prone to cracking during bending processing. In the patent, the Ni based substrate layer is formed with a thickness of 0.05-0.3 μ m, and the coverage rate of the Ni substrate surface is ≥ 60% when combined with a Cu ∝ Sn layer, in order to effectively suppress the degradation of high-temperature contact resistance and Kirkendall voids.
Annealing process
Annealing at 150 ° C for 1 hour after electroplating can reduce residual stress in the coating, promote uniformity of the IMC layer, and suppress premature formation of tin whiskers and interface voids. The effect of annealing depends on temperature and time window: insufficient stress release may occur, while excessive stress release may accelerate the overgrowth of IMC and consume the tin layer.
Alloying (such as Sn Bi, SnAgCu)
Inhibit the growth of tin whiskers and IMC by changing the grain structure and diffusion behavior of the tin layer. The cost of alloying may affect weldability and coating cost, and the long-term behavior of different alloy systems at high temperatures needs to be separately verified. The common boundary of the three paths is that they delay degradation but do not eliminate it. At sufficiently high temperatures and long periods of time, the tin layer will eventually be consumed by IMC reactions, and Kirkendall voids will eventually accumulate. The task of selection is to ensure that the degraded contact impedance remains within an acceptable range within the target service temperature and lifespan.
Selection judgment based on "temperature x frequency"
Firstly, confirm the target service temperature and time, and then discuss the thickness of the tin layer
If the working temperature remains above 105 ° C for a long time, the consumption rate of the tin layer and the accumulation of Kirkendall voids will significantly accelerate. Suppliers should be required to provide contact resistance changes after high-temperature storage data (such as 110 ° C/1000-2400 hours) at the target temperature, rather than just providing initial values at room temperature. The data of contact resistance rising to 10 m Ω or above at 175 ° C for 1000 hours can serve as a warning boundary for structural defects; The optimization plan should maintain a smoother upward trend under this condition.
Secondly, in high-temperature scenarios, inquire about the ability to control interface structure, rather than just looking at "tin plating thickness"
Whether the IMC layer is continuous, whether the coverage rate of Cu ∝ Sn on the bottom layer is sufficient, and whether a Ni barrier layer is set, can better predict long-term contact resistance than the nominal thickness of the tin layer. When selecting, it should be confirmed whether the supplier provides IMC structure or barrier layer process instructions, as well as interface void data after high-temperature storage.
Thirdly, in high-frequency applications, the thickness of the tin layer should be managed as a "high-frequency loss variable" rather than just a "anti-oxidation layer"
Above 10 GHz, a tin layer of 0.8-1.2 μ m has already covered the skin depth, and the resistivity of tin directly enters the insertion loss budget. For millimeter wave grounding or high-speed reflow paths, the matching relationship between the thickness of the tin layer and the target frequency band should be confirmed. If necessary, a thinner tin layer or single-sided tin plating scheme should be selected, and the interface resistance contribution of the IMC layer at high frequencies should be evaluated at the same time.
Fourth, use "shielding effectiveness/contact impedance after high-temperature storage" as an independent verification term, rather than assuming that "tin plating will not degrade"
The data showing a 30 dB decrease in shielding effectiveness below 500 MHz after 110 ° C/2400 hours indicates that degradation of the tape layer can occur with qualified appearance and initial resistance. For long-life or high-temperature scenarios, the contact impedance or shielding effectiveness should be retested after high-temperature storage during the sample stage, rather than relying solely on the room temperature specification sheet for judgment.
Advanced Institute (Shenzhen) Technology Co., LtdThe tin plated copper foil tape product line covers rolled copper foil and electrolytic copper foil substrates. The tin layer thickness can be adjusted in the range of 0.7-3 μ m, and supports Ni barrier layer, annealing treatment, and alloying coating options to control the IMC growth and Kirkendall voids at the Cu Sn interface.
The product can provide contact resistance change data and interface structure control instructions after high-temperature storage, suitable for vehicle grounding, base station shielding, and FPC grounding scenarios that require long-term high-temperature service. For more technical information, please visithttp://www.avanzado.cn/.