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The specification sheet provided by the magnetic separator supplier usually includes three sets of core data: initial magnetic permeability, saturation magnetic inductance, and room temperature loss. The engineer completes the selection based on these three sets of data, and the incoming inspection also revolves around them. This process may seem complete, but it cannot answer one question: how much magnetic permeability will remain on this magnetic separator after three years.
There is a systematic gap between the actual usage conditions of the wireless charging receiver and the testing environment specified in the specifications. Charging mobile phones under high temperatures in cars during summer, bending caused by repeated opening and closing of TWS headphone compartments, temperature shock and road vibrations experienced by in car wireless charging modules - these stresses alone may not be fatal, but when combined, the failure of magnetic separators often begins not from "insufficient magnetic permeability", but from delamination, corrosion and interface weakening.
Double 85 test exposed issue: qualified magnetic permeability does not mean complete structure
The constant temperature and humidity test at 85 ℃/85% RH (commonly known as "Double 85" in the industry) is a routine project for reliability verification of electronic components. For wireless charging magnetic separators, the Double 85 test not only examines the retention of magnetic performance, but also the integrity of multi-layer composite structures.
YS/T 1417-2021 specifies that the nickel zinc ferrite composite foil material must withstand constant temperature and humidity conditions of 85 ℃± 2 ℃, 85% ± 3% RH, and 120 hours. The rate of change in physical performance indicators after testing must meet the requirements. For nanocrystalline magnetic separators, the structure of alternating layers of magnetic layers and adhesive layers is more likely to expose weak links under high temperature and high humidity. After water vapor permeates through the gaps between fragments and interlayer interfaces, the adhesive strength of the adhesive layer decreases, and small delamination or bubbles may occur between the layers. Layering itself does not immediately change the magnetic permeability, but it changes the continuity of the magnetic circuit - the air gap replaces the adhesive layer, the effective magnetic resistance increases, and the magnetic permeability drifts silently.
The more hidden problem is corrosion. Nanocrystalline alloys are metallic materials that may undergo oxidation or electrochemical corrosion at the edges and cutting surfaces of fragments in humid and hot environments. The corrosion products change the insulation state of the fragment gaps, and the eddy current path may partially recover, leading to an increase in losses. An engineering observation on high temperature and high humidity testing indicates that after testing, it is necessary to disassemble and inspect the magnetic shielding sheet for corrosion or delamination to verify the long-term reliability of material selection and packaging design. The magnetic permeability data on the specification sheet cannot reflect the degradation at this structural level.
Cold and hot shock: the "breathing effect" between fragments
The temperature cycle experienced by the magnetic separator in the entire machine is much more rigorous than the room temperature test specified in the specifications. The cold and hot shock conditions specified in YS/T 1417-2021 are -40 ℃~80 ℃, 10 ℃/min, and 60 cycles. Within this temperature range, the materials of each layer of the magnetic separator expand and contract at different rates.
The thermal expansion coefficient of nanocrystalline alloys is much lower than that of polymer adhesive layers. During the temperature cycling process, the expansion and contraction amplitude of the adhesive layer is greater, resulting in periodic shear stress on the fragmented units. Under this' breathing effect ', the gaps between fragments undergo small opening and closing, and the length of the gaps is no longer stable. For the magnetic separator of the magnetic fragmentation process, the gap is the air gap in the magnetic circuit, and the drift of the gap length directly changes the effective magnetic permeability, thereby affecting the inductance and resonant frequency of the receiving coil.
The main risks of ferrite magnetic separators under thermal shock are different. The Curie temperature of ferrite is usually below 200 ℃, and at the high temperature end, the slope of magnetic permeability decreasing with increasing temperature is steeper; At low temperatures, hard and brittle ceramic structures may develop microcracks under thermal stress. For scenarios with large temperature fluctuations such as in car wireless charging, the temperature stability boundary of ferrite is narrower than that of nanocrystals.
Salt spray and vibration: overlooked mechanical and environmental coupling failure
Car mounted wireless charging modules in coastal areas or on salt sprinkled roads in winter may have their magnetic barriers exposed to salt spray environments. The corrosion risk of salt spray on metal based nanocrystalline materials is higher than that of ferrite. Corrosion starts from the edge of the fragments, gradually weakening the effective cross-sectional area of the fragments and decreasing the magnetic flux capacity. Corrosion products may also bridge adjacent fragments, locally restore eddy current paths, and increase losses. For magnetic separators without complete packaging protection or exposed edges, salt spray testing is a more direct corrosion assessment method than double 85.
The impact of mechanical vibration is reflected in another dimension. The continuous vibration during vehicle operation is transmitted to the magnetic separator through the module structure, and the relative micro motion between the fragment units accelerates the fatigue of the adhesive layer. Vibration itself rarely directly causes the rupture of the magnetic separator, but when combined with temperature cycling, the rate of interface debonding will significantly accelerate. The IP67 protection level can prevent liquid water from entering, but cannot eliminate the cumulative damage of temperature cycling and vibration to the internal interface.
The following table summarizes the main failure modes and verification gaps under four types of stress:
Selection judgment from "initial performance" to "retention rate"
The reliability verification of magnetic separators needs to shift the focus from "how good the performance is at the factory" to "how much performance is left after experiencing stress". The data from the following three dimensions can better predict long-term performance than the nominal magnetic permeability specified in the specifications.
Firstly, suppliers are required to provide the retention rate of magnetic properties after aging, rather than just the initial value
After 120 hours of double 85 and 60 cycles of thermal shock, the rate of change of the real part of the magnetic permeability μ 'of the magnetic separator should be controlled within a reasonable range. For applications in the Qi frequency band (110-205 kHz), a variation of more than ± 10% in μ 'means that the resonant matching at the receiving end may need to be readjusted. If the supplier can only provide initial data at room temperature, their ability to predict long-term reliability is limited.
Secondly, distinguish the corrosion risk of material systems from temperature boundaries
The Bs and Curie temperature advantages of nanocrystalline magnetic separators are obvious, but the corrosion risk of metal based materials in salt spray and humid environments is higher than that of ferrites. For vehicle exterior or coastal scenes, it should be confirmed whether the magnetic isolation sheet has complete edge encapsulation and anti-corrosion coating, and it is required to provide magnetic performance and structural integrity data after salt spray testing. Although ferrites have lower corrosion risk, their Curie temperature boundary (<200 ℃) triggers magnetic permeability decay earlier in high-temperature scenarios, and selection needs to be based on the actual upper limit of operating temperature.
Thirdly, evaluate the interface stability of multi-layer composite structures, rather than just looking at magnetic layer parameters
The thermal resistance, mechanical strength, and magnetic continuity of the magnetic separator all depend on the integrity of the interlayer interface. The delamination, bubbling, or edge warping after double 85 and thermal shock will deteriorate the overall performance without changing the intrinsic parameters of the magnetic layer. When selecting, the supplier's adhesive layer temperature resistance level and lamination process should be confirmed, and it is required to provide microscopic photos or ultrasonic scanning data of the cross-section after aging as direct evidence of interface integrity.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging magnetic isolation sheet covers two material routes, ferrite and nanocrystals, and can provide magnetic performance retention data under double 85, cold and hot shock, and salt spray conditions, as well as interface integrity evaluation after aging.
The effective magnetic permeability of the nanocrystalline series products can be customized from 150-2000, with a Curie temperature of ≥ 500 ℃, and supports edge packaging solutions with anti-corrosion coatings. The product can be customized with a reliability verification plan based on the temperature, humidity, and mechanical stress conditions of the application scenario. For more technical information, please visithttp://www.avanzado.cn/.