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In the selection review of wireless charging magnetic separators for mobile phones, engineers usually confirm three things: material system, nominal thickness, and saturation margin at target power. If these three items are passed, the plan will enter sample verification. The data in the sample stage is often very beautiful - efficiency meets standards, temperature rise is controllable, and NFC reading distance is normal.
The problem occurred during the ramp up phase of mass production. There is a measurable difference in charging efficiency between the first and second batches using the same set of matching parameters; Some batches intermittently disconnect from charging at specific placement locations; The production line needs to adjust the testing threshold for different batches separately. When investigating the cause, magnetic permeability tolerance is usually the first to suspect, but the contribution of thickness tolerance is often underestimated.
Thickness is not a 'geometric parameter', it is a part of inductance
The inductance L of the receiving coil depends on the geometric structure of the coil and the effective magnetic permeability of the surrounding magnetic materials. The magnetic separator is attached to the back of the coil, and its thickness participates in the formation of effective magnetic permeability - the change in thickness changes the effective cross-sectional area and demagnetization effect in the magnetic circuit, thereby changing the equivalent inductance of the coil.
Conductive chain from thickness deviation to resonance shift
Resonant frequency f=1/(2 π√ LC). As L changes, the resonant frequency shifts accordingly. In the 100-205 kHz frequency band of Qi, the receiving IC usually has a built-in resonant tracking mechanism that can compensate for L drift within a certain range. But the tracking range is limited. If the variation in L caused by thickness tolerance exceeds the tracking margin, the resonance point cannot be pulled back, and the system enters a detuning state, resulting in a decrease in efficiency and an increase in temperature rise.
The nominal thickness of 0.11 mm varies by approximately ± 9% if it is actually distributed between 0.10-0.12 mm (± 0.01 mm). The thickness fluctuations of this magnitude typically correspond to changes in effective magnetic permeability and inductance within a range of a few percentage points. For systems with sufficient resonance tracking margin, a few percentage points of L variation can be absorbed; For systems with tight tracking margins or operating points close to the boundary, these few percentage points are the difference between "stable charging" and "intermittent charging".
Tolerance bands can better predict mass production consistency than nominal values
The specification sheet for magnetic separators usually states "thickness 0.11 mm", with occasional annotations of "± 0.01 mm" or "± 0.02 mm". The width of this tolerance zone directly determines the inductance dispersion within and between batches.
Narrow tolerance band: One set of matching parameters covers multiple batches
Suppliers with narrow tolerance zones have concentrated thickness distribution within batches, low dispersion of coil inductance, and the production line can cover multiple batches with a set of matching parameters.
Tolerance bandwidth: Discrete stacking within batches and drift between batches
Suppliers with tolerance bandwidth may have scattered thickness distribution within the batch, and some units' inductors may fall at the edge of the matching window, resulting in "lower efficiency of some units in the same batch of incoming materials". If there is still a drift in the center value between batches (such as a batch with an overall thickness deviation of 0.005 mm), the problem will escalate from "dispersion within batches" to "drift between batches", and the production line will need to adjust batch by batch, resulting in an increase in production consistency costs.
This is also why the weight of thickness tolerance should not be lower than the nominal thickness itself in the selection of magnetic separators at the receiving end. The nominal value answer is' What is the design goal ', and the tolerance band answer is' Can mass production consistently fall near the design goal'. The former affects sample performance, while the latter affects mass production yield.
Thickness tolerance and magnetic permeability tolerance: two independent drift channels
The performance drift of magnetic separators comes from two independent sources: magnetic permeability tolerance and thickness tolerance. The magnetic permeability tolerance affects the distribution of μ 'in the material itself, while the thickness tolerance affects the geometric dimensions of the magnetic circuit. Both will conduct to the coil inductance, but the conduction path is different: the magnetic permeability deviation changes the "magnetic flux guiding ability per unit thickness", while the thickness deviation changes the "total thickness of the magnetic layer involved in guiding". Within a batch, both factors may coexist, and the combined inductance dispersion is greater than that of a single factor.
If only the magnetic permeability tolerance (such as μ'150 ± 10) is confirmed during selection, and the thickness tolerance is only written as "typical value 0.11 mm" without providing a tolerance zone, the risk of mass production consistency is actually only quantified in half. A complete judgment requires confirmation of both tolerance zones and whether the supplier provides thickness distribution data within the batch.
The fragmented magnetic structure makes the transmission of thickness tolerances more complex
The thickness tolerance of the nanocrystalline magnetic separator is also influenced by the fragmentation process. The magnetic isolation sheet after magnetic fragmentation is composed of multiple layers of magnetic layers stacked through adhesive film, and the total thickness is determined by the thickness of the single-layer strip, the number of layers, and the thickness of the adhesive layer. The thickness tolerance of single-layer strip, the uniformity of coating of adhesive layer, and the pressure consistency of laminating process all contribute to the distribution of finished product thickness.
More importantly, the effective μ 'after demagnetization is sensitive to thickness. The equivalent air gap formed by fragments has a higher relative proportion in the thin layer; When the thickness changes, the relative influence of fragment gaps on the magnetic circuit also changes accordingly. This means that the influence of the thickness tolerance of the fragmented magnetic nanocrystal separator on inductance may be more complex than that of continuous materials of the same thickness - thickness deviation not only changes the cross-sectional area, but also changes the weight of the fragment gap in the magnetic circuit.
Selection and judgment of magnetic separators at the receiving end based on the "tolerance zone"
Firstly, treat the thickness tolerance zone as a selection parameter parallel to the nominal thickness, rather than a "reference information"
Require suppliers to clearly indicate the thickness tolerance range (such as 0.11 mm ± 0.01 mm) and provide thickness distribution data within the batch. If the supplier can only provide nominal values and cannot provide tolerance bands or distribution data, the risk of mass production consistency cannot be quantified.
Secondly, distinguish between "intra batch distribution" and "inter batch drift" and evaluate their respective impacts
The distribution within a batch determines the inductance dispersion of the same batch of incoming materials, while the drift between batches determines whether the production line needs to adjust the matching parameters batch by batch. When selecting, suppliers should be required to provide at least 3 batches of thickness measurement data, observing the distribution width within each batch and the center value drift between batches.
Thirdly, in the fragmented magnetic nanocrystal scheme, investigate the joint distribution of thickness tolerance and effective μ 'tolerance
The thickness deviation and μ 'deviation of the fragmented magnetic structure are not independent, but are interrelated through fragment gaps and layer design. When selecting, it should be confirmed whether the supplier can provide joint data of "thickness effective μ '" instead of providing two isolated tolerance zones separately.
Fourth, use "tolerance zone boundary samples" for verification during the sample stage, rather than just using "typical samples"
Sample validation typically uses samples with a thickness close to the nominal value, which masks the performance at the tolerance zone boundary. If the matching parameters of the production line are set according to typical samples, the boundary samples (thickness deviation upper or lower limit) may experience resonance offset after actual assembly. When selecting, the supplier should be required to provide samples at the boundary of the tolerance zone to verify the robustness of the matching parameters.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of wireless charging magnetic separators for mobile phones covers ferrite composite sheets and nanocrystalline composite sheets. The nanocrystalline series has a typical thickness of 0.11 mm under the same efficiency, and the thickness tolerance can be controlled in the order of ± 0.01 mm. It supports thickness specifications such as 0.08/0.11/0.15 mm and ± 0.05 mm die-cutting accuracy.
The product can provide thickness distribution data within batches and center value drift data between batches, helping engineers verify the resonance matching robustness at the tolerance zone boundary during the sample stage. For more technical information, please visithttp://www.avanzado.cn/.