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In the design review of the System in Package (SiP) module, a recurring contradiction is that the packaging height has been compressed to less than 1 mm, but the need to suppress radiation leakage has become more urgent as the operating frequency increases. Traditional absorbing materials typically require a thickness of 0.5 mm or more to establish effective impedance matching and loss paths, while in many advanced packages, the space left for absorbing materials is only 0.1-0.3 mm.
This is not a problem that can be easily solved by changing materials. Thin wave absorbing plates face a set of constraints from the fundamental principles of electromagnetics, and breakthroughs in engineering often require finding new design degrees of freedom on the constraint boundaries.
Quarter wavelength constraint: thin physical baseline
The prerequisite for absorbing materials to convert electromagnetic wave energy into thermal energy dissipation is that electromagnetic waves can enter the interior of the material. For metal backplate type absorbers, there is a phase difference between the reflected waves of electromagnetic waves on the front surface of the material and the waves reflected on the back surface after penetrating the material. When the thickness of the material is equal to an odd multiple of the wavelength of the electromagnetic wave inside the material by one fourth, the two columns of reflected waves have opposite phases, resulting in interference cancellation and the minimum reflection loss.
This matching condition can be expressed in a simplified relationship: the matching thickness t ₘ is inversely proportional to the target frequency f ₘ, and inversely proportional to the square root of the product of the material's magnetic permeability and dielectric constant. In other words, to achieve effective absorption at lower frequencies, either increase the thickness or improve the magnetic permeability and dielectric constant of the material.
For thin wave absorbing plates, the boundaries of physical constraints are determined accordingly. In the 1-6 GHz frequency band, the wavelength of electromagnetic waves in free space is 50-300 mm. Even if the equivalent wavelength inside the material is shortened several times due to high magnetic permeability and high dielectric constant, the thickness required to achieve quarter wavelength matching is still in the millimeter range. When the available space is compressed to below 0.3 mm, the traditional quarter wavelength matching path is no longer feasible. The engineering response strategy is not to directly challenge this physical constraint, but to "bypass" it by changing the propagation mode of electromagnetic waves inside the material.
Impedance gradient architecture: allowing electromagnetic waves to be 'digested' within subwavelength thickness
The core design concept of thin absorbing wave plates is to construct a progressive impedance transition and loss path within a thickness far less than a quarter wavelength, so that electromagnetic waves are gradually attenuated during the process of penetrating the material, rather than relying on interference cancellation at a single interface.
A study published in 2026 provides a clear engineering example. The research team prepared ultra-thin absorbing materials with precise impedance gradient nanofiber architecture through synchronous electrospinning electrostatic spraying process. The porous polyvinylidene fluoride hexafluoropropylene nanofiber matrix provides a low dielectric constant to achieve impedance matching with air, while spatially controlled deposition of multi walled carbon nanotubes forms an optimized conductive gradient in the thickness direction.
The key to this design is that the conductive gradient profile effectively suppresses the reflection inside the material, so that electromagnetic waves will not be "bounced back" at a certain interface after entering the material, but will be continuously attenuated along the path of graded conductivity. Finally, the absorbing material with a thickness of only 500 μ m achieved ultra wideband absorption of 35.6-330 GHz, with an effective absorption bandwidth of 294.4 GHz and a relative bandwidth of 161%. For the design of thin absorbing wave plates, the engineering implications of this result are that constructing continuous impedance transitions and loss gradients through structural design within sub millimeter thickness is more effective than simply increasing the intrinsic magnetic permeability or dielectric constant of the material. The impedance gradient architecture transforms the two processes of "electromagnetic wave entering the material" and "electromagnetic wave attenuation" from a competitive relationship at the same interface to a series process along the thickness direction.
Metamaterial unit design: achieving 90% absorption within a thickness of λ/228
In the microwave and millimeter wave frequency bands, metamaterial structures provide another technological path for thin absorbing wave plates. Unlike uniform absorbers that rely on intrinsic electromagnetic parameters of materials, the absorption frequency of metamaterial absorbers is determined by the geometric dimensions of periodic resonant units, rather than the chemical composition of the material.
A study on radiation leakage suppression of SiP modules proposed an ultra-thin absorber plate design based on artificial surface plasmon polaritons (SSPP). By adjusting the length of the metal strips to control their dispersion characteristics, stepped metal strips are arranged in an array on a dielectric substrate to excite SSPP modes at specific frequencies, thereby forming a relatively wide absorption band. According to the electromagnetic field distribution inside the SiP module, SSPP absorbing units are arranged in a circular pattern to form a complete absorbing plate. This design achieves 90% absorption performance in the range of 26.3-31.6 GHz, while the substrate thickness is only λ _ L/228 of the wavelength corresponding to the lowest operating frequency, with a total thickness of λ _ L/134.
For 5G millimeter wave modules and vehicle mounted radar modules, this thickness level means that the absorbing plate can be integrated onto the chip surface or inside the package cover without increasing the packaging height. The XJY series high-frequency absorbing ultra-thin design of Advanced Institute Technology also provides an engineering solution in this direction, with a thickness of 0.3 mm covering 26-40 GHz and an absorption rate of ≥ 95%, used to suppress inter antenna coupling and cavity resonance.
Selection and judgment of thin absorbing plate: from "thickness priority" to "loss path priority"
For engineers who need to achieve electromagnetic noise suppression under strict spatial constraints, there are systematic differences in the selection logic of thin absorbing wave plates compared to traditional absorbing materials.
Firstly, confirm the matching relationship between the target frequency band and the available thickness
There is a direct correlation between the effective operating frequency band and thickness of thin wave absorbing plates. In the ultra-thin range of 0.1-0.3 mm, the effective absorption of current engineered products mainly covers frequency bands above 2 GHz; In the low frequency range of 30 MHz to 1 GHz, even with a high permeability formula, it is difficult to achieve effective broadband absorption with a thickness of less than 0.5 mm. When selecting, the first step should be to confirm whether the target noise frequency and available thickness fall within the effective working window of the material. The XJY series ultra-thin products of Advanced Institute Technology cover a thickness range of 0.05-0.3mm, with key frequency bands including 2.4/5/6 GHz, Sub-6 GHz, and 24/77 GHz.
Secondly, distinguish between "peak reflection loss" and "actual attenuation capability"
The peak reflection loss of thin absorbing wave plates may occur at specific frequencies and under specific testing conditions, but in the actual cavity environment of the module, the incident angle and polarization state of electromagnetic waves are often different from laboratory tests. For SiP modules and millimeter wave modules, suppliers should be required to provide attenuation values or shielding effectiveness data of materials within the target frequency band, rather than relying solely on reflection loss curves. The consistency between the measured and simulated results of SSPP absorbing plate in SiP module indicates that the feasibility of this technology route in actual packaging environment has been verified.
Thirdly, evaluate the combined effects of thinning on thermal conductivity and mechanical properties
Compressing the thickness of the absorbing plate to below 0.3 mm means that the space available for thermal conductive fillers and reinforcing phases is synchronously reduced. For scenarios where it is necessary to absorb electromagnetic noise while exporting heat, the thermal conductivity of thin wave absorbing plates may be lower than that of thick products. When selecting, the thermal conductivity data of the material and the influence of the backing adhesive system on the heat dissipation path should be confirmed simultaneously - low thermal conductivity pressure-sensitive adhesives will significantly weaken the heat transfer efficiency from the absorbing layer to the heat dissipation substrate. Advanced Institute Technology's thin absorbing wave plate offers a thermal conductivity option of 1-5 W/mK and supports thermal conductive adhesive backing to adapt to application scenarios that require synchronous management of electromagnetic noise and heat.
Fourth, focus on maintaining performance under bending and compression conditions
In mobile phones, wearable devices, and foldable screen terminals, thin wave absorbing plates often need to be attached to curved surfaces or undergo bending during assembly. The mechanical strength of ultra-thin waveplates is lower than that of thick sheets, and surface microcracks after bending may change the impedance matching window, resulting in a silent decrease in absorption performance. When selecting, it is necessary to confirm the data of the reflection loss of the material at the minimum bending radius, as well as the measured results of the number of bending cycles. The XJY series broadband flexible absorbing wave plate from Advanced Institute Technology has a bending resistance of over 100000 times and a minimum thickness of 0.1 mm. It is suitable for small spaces such as mobile phones, laptops, wearable devices, and optical modules.
Advanced Institute (Shenzhen) Technology Co., LtdThe research platinum brand XJY series electromagnetic wave suppression absorbing plate covers a customizable frequency range of 10 MHz – 40 GHz, with a thickness range of 0.05-3 mm, including an ultra-thin type of 0.05-0.3 mm. The product can achieve RL ≤ -10 dB in the target frequency band, peak value better than -20 dB, and corresponding absorption rate ≥ 90%.
Temperature resistance range of -40 ° C to 150 ° C, flame retardant grade UL94 V-0, supports multiple backing options of acrylic adhesive, silicone adhesive, and thermal conductive adhesive, with forms covering sheet, coil, and precision die-cutting shaped parts. This series adopts gradient impedance matching and porous structure technology, and can customize parameters according to specific frequency bands and spatial constraints. For more technical information, please visithttp://www.avanzado.cn/.