What other variables can engineers adjust when the saturation margin narrows after the thickness limit of ultra-thin magnetic separators

In the design of wireless charging receivers for flagship phones, the thickness budget of the magnetic separator is usually limited to within 0.15 mm, and some ultra-thin models even require a thickness lower than 0.1 mm. Engineers' first reaction to this constraint is often to find the material with the highest magnetic permeability and then thin it.

But after thinning, the real problem is usually not magnetic permeability, but saturation.

The magnetic flux carrying capacity of a magnetic separator is directly proportional to its cross-sectional area. The thickness is compressed from 0.27 mm to 0.11 mm, and the cross-sectional area decreases by about 60%. The magnetic flux that can be carried by the same material system decreases synchronously. When the working magnetic flux density approaches the saturation magnetization of the material, the magnetic permeability drops sharply, and the magnetic field lines penetrate the magnetic separator into the metal backplate, causing eddy current losses and temperature rise to increase accordingly. At this point, even if the nominal magnetic permeability is still 'high enough', the actual magnetic flux guidance capability has already failed.

Why is saturation margin more critical than magnetic permeability after thickness compression

The magnetic separator at the wireless charging receiver operates in an alternating magnetic field, and its core function is to establish a low magnetic resistance path between the coil and the metal component, while preventing magnetic field lines from entering the metal layer. The 'capacity' of this pathway is determined by the saturation magnetic inductance Bs of the material and the effective cross-sectional area of the magnetic separator.

The Bs of ferrites are usually between 0.4-0.5 T, while nanocrystalline materials can reach 1.2-1.25 T. This difference means that at the same working magnetic flux density, the working point of nanocrystalline magnetic separators is further away from the saturation zone, and the thickness can be made thinner without triggering saturation. When the magnetic permeability is the same as 600, nanocrystals only need 0.11 mm to achieve the same magnetic conductivity as 0.27 mm ferrite. Under the same testing conditions, the charging efficiency of the nanocrystalline scheme is about 2.5 percentage points higher than that of ferrite, and the surface temperature rise of the device is 7-8 ℃ lower.

Path 1: Non fragmented magnetic double-layer structure, using layer subtraction to replace thickness

One engineering path is to abandon magnetic fragmentation and instead reduce the number of nanocrystalline layers to achieve thinning. The solution disclosed in patent CN109817442A adopts this idea: a single-layer nanocrystalline strip of 16 μ m or 25 μ m is selected, and after two heat treatments, it is directly coated and bonded. The laminated structure only contains two layers of nanocrystals, and the total thickness is controlled within 80 μ m.

The core logic of this path is that although fragmentation reduces eddy current losses, fragmentation itself introduces leakage flux, and in order to compensate for leakage flux, it is necessary to increase the number of layers, but the thickness cannot be reduced. After canceling the demagnetization process, the leakage path is reduced, and two layers of nanocrystals can meet the charging efficiency requirements, and the thickness can be compressed. On the 5W testing platform, the charging efficiency of the 80 μ m magnetic separator can reach up to 77.95% at a current of 0.6A, and still maintain 75.88% at a current of 1A.

The cost of the non-magnetic solution is higher precision requirements for the heat treatment process. Due to the lack of insulation gaps between broken magnetic fragments to suppress eddy currents, the material's own losses must be suppressed through heat treatment. The first heat treatment in the patent was held at 430-450 ℃ for 30 minutes to increase nucleation points and refine grains, while the second heat treatment was held at 550-560 ℃ for 120 minutes to reduce losses and suppress the precipitation of Fe-B hard magnetic phase. The time temperature window for heat treatment parameters is narrow, and the difficulty of batch consistency control is higher than that of conventional demagnetization processes.

Path 2: Magnetic fragmentation and multi-layer stacking, exchanging magnetic flux capacity with process complexity

The other path preserves the fragmented magnetic field and controls both eddy current losses and magnetic flux capacity by stacking multiple layers of thin films. Magnetic fragmentation divides a continuous strip into small, mutually insulating fragments, with air gaps typically ranging from 0.01 to 20 μ m between the fragments, cutting off the eddy current path. The thickness of single-layer fragmented magnetic pieces can reach 30 μ m or even lower, and the target magnetic flux capacity can be achieved through multi-layer composite.

The advantage of this path is that the control of eddy current losses is more thorough, and the fragmentation of each layer independently suppresses the eddy current closure path of that layer. After multi-layer stacking, although the total thickness is higher than that of the non-magnetic bilayer structure, the low loss characteristics of each layer make the overall temperature rise performance still controllable. The ultra-thin nanocrystal solution of Advanced Institute Technology belongs to this route, with a thickness coverage of 0.03-0.15 mm. In the application of smartphone receiving end, it can lower the temperature rise and fall of the metal side by 5-8 ℃, and improve the receiving efficiency by 3-5 percentage points.

The choice between the two paths depends on the priority of the application scenario for "extreme thickness" and "process robustness". For scenarios where the thickness budget is extremely tight and the charging power does not exceed 5-10W, the non-magnetic double-layer structure has more advantages in thickness; For scenarios with higher power or more stringent requirements for batch consistency, the process maturity and loss controllability of multi-layer stacking of fragmented magnets are more reliable.

After ultra-thin technology, thermal management has become a new constraint variable

When the thickness is compressed to below 0.1 mm, the heat capacity and heat dissipation path of the magnetic separator decrease synchronously, and the heat generated by eddy current losses is more likely to accumulate locally. In the smart watch scenario, the thickness of the magnetic separator is usually 0.05-0.1 mm, and it comes into direct contact with the skin. The surface temperature rise needs to be controlled within 3K. This constraint goes beyond the scope of simple "low loss" and requires system level coordination between magnetic circuit design, adhesive thermal conductivity, and assembly bonding of the magnetic separator.

An easily overlooked detail is the impact of the adhesive system on the heat dissipation path. If the magnetic separator is attached with a pressure-sensitive adhesive with low thermal conductivity, even if the intrinsic material loss is low, heat is still difficult to conduct from the magnetic separator to the metal shell or heat dissipation substrate. When selecting ultra-thin magnetic separators, the solution of thermal conductive adhesive backing or direct composite heat dissipation layer is more engineering meaningful in temperature sensitive scenarios than simply pursuing lower material loss factors.

The following table summarizes the key constraints and material path matching of ultra-thin magnetic separators in different scenarios:

Three engineering judgments in selection

The selection of ultra-thin magnetic separators cannot be limited to the layer of "thickness meets standards and magnetic permeability is high enough". The following three dimensions can better distinguish the usability of materials under actual working conditions.