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The specifications of a piece of anti magnetic tape are written as μ'150, tan δ ≤ 0.05, and thickness 0.10 mm. All incoming materials have passed the inspection. But after being attached to the phone, NFC card swiping still works intermittently, and wireless charging occasionally disconnects.
The first reaction of engineers is often 'the material is not good', so they switch suppliers, change material systems, and add thickness. After several rounds of replacement, it was found that the problem still exists. The real reason at this point is most likely not in the material itself, but in the interface between the material and the phone.
Location: The wrong place to apply the anti magnetic tape is more common than selecting the wrong material
The position of the NFC antenna inside the phone varies depending on the model, usually near the lens module or on the upper part of the body. If the anti magnetic patch completely covers the NFC antenna area and the antenna is directly loaded with high magnetic permeability material, the inductance offset exceeds the matching range, and the reading distance decreases instead. Actual test data shows that induction failure only occurs in extreme cases where the patch is intentionally completely covered over the antenna area; And those cases of 'completely no signal' are mostly due to the wrong placement, directly 'covering' the antenna.
The correct mounting position is between the antenna and the metal interference source, rather than directly pressing on the antenna. The function of anti magnetic tape is to provide a bypass for magnetic field lines to bypass the metal, which needs to be located in front of the metal surface on the magnetic flux path, rather than covering the receiving end of the magnetic flux. The design of patch size should also follow the same logic: covering the metal interference area but avoiding the main sensing area of the NFC antenna. Industry experience shows that SMT usually only covers the magnetic ring area and avoids the NFC antenna position. As long as the bonding position is accurate, NFC card swiping can be performed normally.
A finer engineering criterion is area coverage. The relative position between the antenna unit and the magnetic sheet is limited, and when the area coverage (Sb/Sa) is above 90%, the deviation of the resonant frequency can be suppressed, and the decrease in communication distance can be controlled. This means that the size of the anti magnetic patch is not "bigger is better", but needs to be accurately matched with the relative position of the antenna and metal.
Thickness: 0.3 mm is a structural boundary, not an electromagnetic boundary
The issue of thickness is often understood as "too thin to absorb enough", but in practical engineering, the more common failure direction is "too thick to fit in".
The internal space of a mobile phone is measured in micrometers. The anti magnetic tape is attached to the inside of the back cover or above the battery, and for every 0.1 mm increase in thickness, it will be directly transmitted to the assembly gap of the phone case. The boundary given by actual testing experience is that the probability of the phone case buckle failing or the side bulging significantly increases when the thickness exceeds 0.3 mm; Even TPU shells with good elasticity may have visible bulges near the lens module. For transparent or original shells that pursue sealing, the thickness usually needs to be controlled at the level of 0.1 mm.
From an electromagnetic perspective, increasing beyond a certain thickness does not bring proportional benefits. At 13.56 MHz, the 0.15 mm ferrite layer is close to the skin depth of the material in this frequency band, and the attenuation of the interference magnetic field can reach 22 ± 3 dB. Continuing to increase to 0.5 mm, the optimization amplitude of the loss tangent is limited, and the marginal gain decreases. In NFC scenarios, controlling the thickness between 0.06-0.12 mm is usually sufficient to restore the reading distance at the correct position without compensating for positional deviations with thickness.
Bubbles and adhesive layer: interface failure obscured by appearance
The anti magnetic tape is attached to the structural components of the mobile phone through adhesive backing. If bubbles are trapped during the bonding process, there will be an air gap between the magnetic layer in the bubble area and the substrate, which is equivalent to introducing a high magnetic resistance in the magnetic circuit. Bubbles do not change the material's μ 'and tan δ, but they alter the coupling conditions for magnetic flux entering the surface of the anti magnetic sticker. In the shielding film with a layered structure, trapped bubbles can cause a decrease in inductance in that area, resulting in performance degradation when modularized.
Surface cleaning before bonding is equally crucial. The grease or fingerprints on the back cover may weaken the actual bonding strength of the adhesive layer. After temperature cycling or vibration, the anti magnetic tape may experience local lifting, forming air gaps more severe than bubbles. It is recommended to clean the surface before installation and gradually press the air from one side to the other, rather than pressing the entire piece at once.
Long term interface reliability: adhesive aging is more concealed than copper foil oxidation
The short-term performance of anti magnetic tape can be confirmed through incoming inspection, but its long-term reliability depends on the stability of the adhesive layer and interface under temperature, humidity, and time.
A high-temperature and high humidity study on magnetic tape showed that after exposure to 60 ° C/90% RH for four weeks, the decrease in magnetization strength was about 7%. For nanocrystalline magnetic isolation materials, the magnetic properties of modified acrylic double-sided tape fluctuate less after high-temperature storage, and the magnetic permeability remains almost unchanged at 128 kHz. Another set of data shows that the efficiency decay of the nanocrystalline scheme can be controlled within 1% after 500 hours at 85 ° C/85% RH, and the magnetic performance decay is less than 2% after 1000 bending cycles (R=5 mm).
The common point of these data is that the degradation of the material itself under accelerated aging conditions is limited, but changes in the adhesive layer and interface state may become weaker links that appear earlier. If the adhesive layer softens or creep under long-term high temperature, the contact pressure between the anti magnetic sticker and the surface to be pasted decreases, the equivalent air gap increases, and the magnetic flux guiding ability decreases accordingly. This process will not be reflected in the initial μ 'of the specification sheet, nor will it be exposed in the incoming material inspection, but it will manifest as a shortened NFC reading distance or decreased charging efficiency after the entire machine has been in service for a period of time.
The following table summarizes the four variables of the mounting interface and their engineering boundaries:
Selection judgment based on interface management
Firstly, confirm the mounting position and then discuss the material parameters
Before selecting, confirm the actual location of the NFC antenna and metal interference zone on the phone. The size and shape of the anti magnetic patch should match the magnetic flux path between the antenna and the metal, rather than simply covering the antenna or the entire backplate. If the supplier can only provide standard sized sheets, it should be confirmed whether their die-cutting scheme supports local coverage and avoidance based on the antenna position.
Secondly, manage the thickness boundary as a structural constraint rather than adjusting it as an electromagnetic parameter
The starting thickness for NFC scenarios is 0.06-0.12 mm, and the upper limit of the structure is 0.3 mm. If the performance does not meet the standard within this window, priority should be given to checking the position and interface rather than thickening. Thickening often introduces new structural issues such as phone case fit and wireless charging coupling distance while solving electromagnetic problems.
Thirdly, in reliability assessment, separate the verification of "interface aging" and "material aging"
The aging data of materials (such as μ 'and loss retention after 85 ° C/85% RH) is necessary, but it is not sufficient to cover the risks of adhesive layer creep and contact pressure relaxation. For scenarios that require long-term reliability, suppliers should be required to provide data on the bonding strength or contact resistance changes after high-temperature storage, or at least confirm whether the long-term temperature resistance level and glass transition temperature of the adhesive layer are higher than the upper limit of the service temperature.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of anti magnetic stickers for mobile phones covers ferrite composite sheets and nanocrystalline composite sheets, with a thickness ranging from 0.03-0.3mm. It supports precision die-cutting and local avoidance according to the position of NFC antennas.
The typical control of μ 'for nanocrystalline composite sheets at 13.56 MHz is 150 ± 10, which can provide magnetic performance retention data after aging at 85 ° C/85% RH, and supports thermal conductivity or low modulus adhesive layer options to meet the long-term reliability requirements of different mounting interfaces. For more technical information, please visithttp://www.avanzado.cn/.