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In the incoming inspection of the wireless charging receiver, a 0.08 mm thick nanocrystalline magnetic separator may be fully qualified: the actual magnetic permeability meets the specifications, the thickness tolerance is within ± 3 μ m, and there is no visible damage to the appearance. But when it is mounted on the back of the coil, after several bending steps of module assembly and whole machine assembly, the resonant frequency of the receiving end may have deviated from the design value, the charging efficiency may decrease, and some units may even fail to vibrate normally.
The problem does not lie in the incoming materials, but in the "invisible cracks" during the assembly process.
How bending stress changes magnetic permeability: crack state is a variable
The flexibility of nanocrystalline magnetic separators comes from the demagnetization process. The continuous nanocrystalline strip is divided into small, mutually insulating fragments, and the gaps between the fragments absorb bending strain. Macroscopically, the sheet can adhere to curved surfaces without breaking. But the fragments themselves are still nanocrystalline alloys, which are mechanically very brittle. When bending stress is applied to the magnetic separator, the crack state inside the fragments will change - existing microcracks may propagate, and the gap length between fragments may also change due to deformation of the adhesive layer.
The direct consequence of this change is the drift of the real part of magnetic permeability μ '. The gap between fragments is the "air gap" of the magnetic circuit, and any change in the length of the gap will alter the effective magnetic resistance, thereby changing μ '. There is a clear statement in the patent literature regarding this: external stress causes changes in the length of the gap between fragments, the brittleness of nanocrystalline alloys causes the crack state to change with bending, and μ 'also changes accordingly. The drift of μ 'does not decrease unidirectionally - depending on the stress direction and fragment orientation, μ' may increase or decrease, and this uncertainty is precisely the difficulty of yield control.
In other words, the yield loss of ultra-thin magnetic separators is often not caused by "bad pieces", but by "good pieces deteriorating after assembly".
Drifting from μ 'to resonance detuning: the location of yield loss
The wireless charging receiver operates on an LC resonant circuit with a resonant frequency of f=1/(2 π√ LC). The μ 'of the magnetic separator directly participates in the inductance L of the receiving coil, and the drift of μ' is equivalent to the drift of L, causing the resonant frequency to shift accordingly.
After the resonant frequency deviates from the IC set value, the system transmission efficiency decreases. Slight offset may only manifest as slower charging speed or slightly higher temperature rise, and engineers may not immediately attribute it to the magnetic separator during functional testing; When the offset is large, the receiving end may not be able to vibrate normally under specific coil alignment or load conditions. The trouble with this type of failure is that it is not visible during incoming inspection and may not be obvious after static mounting. It is only exposed after undergoing assembly bending and overall stacking stress, and the magnetic piece has already been encapsulated inside the module, making the repair cost extremely high.
Improvement space for material end: fracture strain and annealing process
From a material perspective, there is room for improvement in the bending brittleness of nanocrystalline magnetic separators. The fracture relative strain of Co based nanocrystalline/amorphous composite tapes can reach more than twice that of traditional Fe based Nanoperm tapes. A higher fracture strain means that under the same bending radius, the probability of new cracks forming inside the fragments is lower, and the stability of μ 'under assembly stress is better.
The annealing process also affects the bending toughness. Research on Fe based nanocrystalline alloys shows that rapid annealing (520 ° C, 3-30 seconds) can achieve soft magnetic properties of Bs 1.82 T and Hc 6.2 A/m while maintaining fracture strain at approximately 1.04%. However, prolonged annealing time can exacerbate the element gradient at the amorphous/crystalline interface, suppress the formation of shear bands, and increase macroscopic brittleness. This means that the annealing process window control accuracy of the material supplier directly determines the yield performance of ultra-thin magnetic separators under assembly stress.
Response at the process end: no broken magnetic path and control of adhesive layer stress
In addition to material composition and heat treatment, the structural process of the magnetic separator is also changing the boundary of bending stability. The non fragmented magnetic double-layer structure is a path towards ultra-thin development in recent years: it does not rely on the absorption of strain by the gaps between fragments, but reduces the overall thickness by reducing the number of nanocrystalline layers, and structurally reduces the gap changes caused by relative sliding between fragments during bending. The bending stability of such schemes depends on the fracture strain of the nanocrystalline strip itself and the constraint method of the adhesive layer on the strip, rather than the random variation of fragment gaps.
The filling method of fragment gaps is another key variable for the scheme that preserves the fragmented magnetic structure. If the gaps between the fragments are completely filled with a glue layer, the viscosity and curing extrusion force of the glue layer will generate stress on the fragments, causing deformation, orientation change, gap expansion, and a decrease in magnetic permeability. Filling at least part of the gap with air, or setting up an isolation coating between the adhesive layer and the magnetic layer, can reduce the stress of the adhesive layer on the fragments, making the drift of μ 'more controllable after bending and thermal cycling.
Framework for evaluating suppliers based on assembly stress and batch consistency
For the selection of ultra-thin magnetic separators, relying solely on the incoming magnetic permeability and thickness tolerance is no longer sufficient to predict the assembly yield. The following three dimensions of data better reflect the availability of materials under real assembly stress.
Firstly, the stability data of the bent μ ', rather than just the initial magnetic permeability
Suppliers should be required to provide data on the changes in μ 'before and after bending of the magnetic separator at a specific curvature radius. A reference criterion is that before and after bending with a curvature radius of 5 mm, the μ '(5)/μ' (0) at 145 kHz should fall within the range of 0.9-1.1. This indicator directly corresponds to the allowable drift window of the LC resonant frequency. If the supplier can only provide magnetic permeability curves in a planar state, their ability to predict assembly yield is limited.
Secondly, the process description of fragment size and gap filling method
The size of the fragment unit is usually in the range of 0.3-1.5 mm. Smaller fragments are beneficial for dispersing bending strain, but they increase the total gap area and reduce the effective magnetic permeability. When selecting, it is necessary to confirm the matching relationship between the supplier's fragment size design and the target bending radius, as well as whether the gap is filled with air or adhesive. Adhesive layer filling may introduce greater stress drift after long-term thermal cycling and bending.
Thirdly, consistency of magnetic performance sampling after multiple batches of bending
The permeability retention ability of ultra-thin magnetic separators in the thickness range below 0.1 mm after bending is the "touchstone" of process capability. Suppliers should be required to provide at least 3 batches of size reports and magnetic performance sampling data after bending, and observe the dispersion of μ 'drift between batches. If the deviation of μ 'after bending between batches exceeds ± 5%, the resonance matching of the whole machine may need to be adjusted separately for different batches, and the risk of mass production consistency will significantly increase.
Advanced Institute (Shenzhen) Technology Co., LtdThe ultra-thin magnetic isolation sheet covers a thickness range of 0.03-0.5 mm, and the single-layer thickness of the magnetic layer can be as low as 16 μ m, supporting nanocrystalline, ferrite, and composite structure schemes.
The product can provide stability data of μ 'before and after bending, fragment size and gap process description, as well as multiple batches of magnetic performance sampling reports, with typical thickness tolerances controlled within ± 3 μ m. In flagship mobile phone and wearable device scenarios, parameter customization can be performed based on the target bending radius and assembly stress conditions. For more technical information, please visithttp://www.avanzado.cn/.