Introduction

The miniaturized, densely integrated electronics are in high demand in many fields of technology that require high operating frequencies. By the irradiated electromagnetic (EM) waves from the operating devices interfering with each other, the risk of signal degradation, malfunction, or total system failure increases1,2,3,4. This is especially crucial for applications that need highly reliable devices such as wireless communication, aerospace, and healthcare4,5. Consequently, the electromagnetic interference (EMI) shielding technologies that preserve the functionality and reliability of electronic devices have become important6.

Conventional EMI shielding materials (such as copper, aluminum, and stainless steel) have been used due to their excellent electrical conductivity and shielding performance7,8. Metals are commonly adopted in electronics through electroplating, physical vapor deposition, and lamination techniques9. However, the high density, stiffness, and poor processability of bulk metal have become significant drawbacks in recently appearing wearable, foldable electronics10,11,12.

Nanomaterials have emerged as promising alternatives to address these issues. Carbon nanotubes (CNTs), graphene, and metal nanoparticles offer advantages in being lightweight, solution-processable, and compatible with flexible substrates13,14,15,16,17,18,19. Among them, silver nanoparticles (Ag NPs) are especially attractive due to their superior electrical conductivity and resistance to oxidation. Although copper nanoparticles are more cost-effective, they are highly prone to oxidation during thermal processing, which can degrade electrical conductivity and EMI performance20. In contrast, Ag NPs retain electrical stability without requiring protective atmospheres, making them better suited for low-temperature processing and high-frequency applications where surface conductivity is critical21,22. Ag NPs can be coated onto ambient surfaces using scalable techniques to form conductive thin films well suited for next-generation EMI shielding applications23,24. Their ability to meet both electrical and manufacturing requirements makes them a strong candidate to replace traditional metal-based shielding in electronics25.

Pressureless sintering is a scalable and practical approach for fabricating conductive metal films without the need for external pressure. The porous microstructures made by sintering without pressure can be advantageous for EMI shielding by enhancing internal reflection and scattering of incident EM waves26,27. Also, the low-temperature processability of EMI shielding materials is considered significant for applications involving temperature-sensitive substrates or devices28. Accordingly, the development of the low-energy sintering process is required for applicability to flexible printed electronic platforms29.

In this study, we investigated the pressureless low-temperature sintering process of Ag NPs to obtain a porous Ag layer for the EMI shielding. Solution-processable Ag NPs were blade-coated on a polyimide (PI) substrate, and subsequent pressureless low-temperature sintering was conducted to obtain the porous Ag structures. The porous structure of the Ag layer was extensively observed and analyzed because it was predicted that the high EMI shielding performance was closely related to the presence of inherent pores, resulting in multiple internal reflections of the EM waves. The resulting Ag layer exhibited an EMI SE as high as 56.5 dB across the frequency range of 0.5–18.0 GHz, which means that EM waves were shielded by over 99.99%. This wideband performance demonstrates the suitability of our porous Ag layer for high-frequency EMI shielding applications, including modern wireless communication systems. Moreover, the EMI SE/t of the porous sintered Ag layer was measured to be higher than that of EMI shielding materials reported previously. The EMI shielding performance of practical devices was demonstrated using a PCB antenna coated with a porous sintered Ag layer. In addition to its facile fabrication, the porous Ag layer was effectively applied to shield integrated circuits (ICs) from EM waves generated during device operation. Consequently, solution processability, facile fabrication process, and excellent efficiency make the porous Ag layer highly attractive as an alternative EMI shielding material for electronic devices.

Results and discussion

We fabricated a porous sintered Ag layer without applying external pressure, solely by controlling the temperature and time of the thermal sintering of Ag NPs. The EMI shielding mechanism of the porous sintered Ag layer is illustrated in Fig. 1a. When EM waves are incident on the material, a portion of the waves is reflected at the surface of the layer due to the high electrical conductivity of silver30,31. The abundant free electrons in the silver layer interact with the oscillating electric field component of the EM waves, leading to strong surface reflection30,32,33. The remaining waves penetrate the porous interior, and they are either absorbed by the layer or transmitted through it, or undergo multiple internal reflections caused by the complex pore structure32,33. As shown in Fig. 1a, the porous structure formed by thermally sintered Ag NPs creates an intricate network of interconnected Ag NPs. This structure with high porosity promotes multiple reflections of incident EM waves, which increase the effective propagation path and facilitate energy dissipation. These effects, combined with the high electrical conductivity of silver, contribute to enhanced shielding performance without the need for external pressure during the sintering processes.

Fig. 1
figure 1

Pressureless sintering of the Ag NPs for EMI shielding. (a) The EMI shielding mechanism of the porous Ag layers and (b) a schematic illustration of the pressureless low-temperature sintering process of the Ag NPs.

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The pressureless low-temperature sintering process is shown in Fig. 1b. Commercial Ag NPs were dispersed in ethanol at a high concentration and then deposited onto a polyimide (PI) substrate using a blade-coating method. The content in the dispersions was fixed at 70 wt%, which provided a balance between dispersion stability, coating uniformity, and printability. The Ag ink showed no visible sedimentation or phase separation after several days of storage, indicating excellent long-term dispersion stability under ambient conditions (Figure S1). The optimal sintering condition was identified, yielding the highest electromagnetic interference (EMI) shielding efficiency (SE).

The structural property of the Ag NP used in this work was analyzed in Figure S2. The SEM and TEM images (Figure S2a and b) explain that the individual Ag NPs exhibit a spherical morphology. In addition, high-resolution transmission electron microscopy (HRTEM) was conducted to evaluate the crystallinity of the Ag NPs. As shown in Figure S3, the observed lattice fringe spacing was approximately 0.238 nm, which corresponds to the (111) plane of face-centered cubic (FCC) silver. The average size of Ag NPs was about 29.3 nm, and the standard deviation was about 7.4 nm. According to the size effect of metal nanoparticles, the neck and metallization between the Ag NPs sized about 30 nm began at around 150–170 °C34. Due to the small diameter, the Ag NPs used in this work have advantages for low-temperature sintering. In Figure S4, the crystallinity and thermal behaviors of the Ag NPs were analyzed. The XRD pattern of Ag NPs shown in Figure S4a was compared with the standard powder diffraction card of JCPDS No. 87–0597. The diffraction peaks at 2θ values of 38.1, 44.4, 64.5, 77.5, and 81.6° correspond to the (111), (200), (220), (311), and (222) planes, possessing an FCC structure35. Also, the thermal behavior of Ag NPs was investigated. The TGA curve in Figure S4b shows the weight loss of the Ag NPs. The gentle slope in the temperature range from 0 to 100 °C exhibits the desorption of the residual water and organic solvents36. There was a weight loss of less than 3% up to 200 °C, indicating that the content of organic stabilizers was extremely low and most of them were decomposed under 200 °C36. As shown in Figure S4c, a sharp endothermic peak at 154 °C was observed from the DSC curve of the Ag NPs. This peak corresponds to the decomposition of organic layers surrounding the Ag NPs37. From the thermal behaviors of the Ag NPs used in this work, the organic stabilizers could decompose readily even at low temperatures, which is advantageous for the low-temperature sintering processes.

The relationship between EMI shielding efficiency (SE) and porosity of the porous sintered Ag layer fabricated by pressureless low-temperature sintering was investigated according to sintering conditions. As shown in Figure S5, the as-coated Ag NPs layer exhibited individually presented Ag NPs without any necking between the Ag NPs. The sintering behaviors of Ag NPs due to temperature effects were observed first between the temperature range from 150 to 180 °C (Fig. 2). The sintering time was fixed to 6 h. At the sintering temperatures of 150 °C in Fig. 2a, the cross-sectional image showed the different sintering behaviors in one region. The upper side region showed the obvious necking between sintered Ag NPs, forming the porous Ag layer. However, the Ag NPs were insufficiently connected at the bottom side of the Ag layer near the substrate. The low thermal conductivity of the PI substrate (~ 0.12 W/m K) limited heat transfer from the bottom side of the layer38. This thermal barrier induced a vertical temperature gradient across the Ag layer, especially at low processing temperatures. Therefore, the bottom region of the layer near the substrate experienced insufficient thermal energy, leading to under-sintering and poor interparticle necking. Sintering occurred primarily between adjacent particles at the upper surface, where heat was more readily delivered. At sintering temperatures above 160 °C, the thermal energy became sufficient to overcome this substrate-induced limitation, promoting uniform neck formation throughout the layer and resulting in a well-sintered Ag layer with interconnected porous structures.

Fig. 2
figure 2

EMI shielding performances of the porous sintered Ag layer according to sintering temperatures (sintering time: 6 h). (a) Cross-sectional SEM images and (b) EMI SE graphs of the porous sintered Ag layers.

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Grayscale binarization was performed to evaluate the porosity of the porous sintered Ag layer (Figure S6). The cross-sectional SEM images in Fig. 2a were binarized into grayscale using the image processing program ImageJ. Pores appeared as white regions in the image, and their area fraction was calculated. The average porosity values were obtained with 27.9%, 24.6%, 19.1%, and 13.2% when the sintering temperatures were 150 °C, 160 °C, 170 °C, and 180 °C, respectively. There was quite a significant decrease in porosity between the Ag layers sintered at 170 °C and 180 °C because the melting of the surface of the Ag structures was severe, covering the pores when sintered at 180 °C.

We investigated the EMI shielding performance of the porous sintered Ag layer in Fig. 2b. The coaxial holder measurement equipment was used to obtain ratios between transmitted and incident EM waves. The EMI SE of the porous sintered Ag layer with different sintering temperatures was measured in a frequency range of 0.5–18.0 GHz (Fig. 2b). As the sintering temperature increased to 150, 160, 170, and 180 °C, the average EMI SE values increased to 20.3, 34.7, 56.5, and 56.3 dB, respectively. From the above results, a sintering temperature of 170 °C yielded the highest EMI shielding performance, which no significant improvement was observed at higher temperature.

The sintering behavior of the porous Ag layer was investigated at a fixed temperature of 170 °C by varying the sintering time. As shown in Fig. 3a, the cross-sectional morphologies of the porous sintered Ag layer with varying sintering times were observed. When the sintering process was performed for under 6 h (1 and 3 h), most of the coated Ag NPs were isolated into small particles. The Ag layers sintered for over 6 h showed the sintered appearance with clear necking between Ag NPs.

Fig. 3
figure 3

EMI shielding performances of the porous sintered Ag layer according to sintering time (sintering temperature: 170 °C). (a) Cross-sectional SEM images and (b) EMI SE graphs of the porous sintered Ag layers.

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The porosities of the sintered Ag layer with the varied sintering time were calculated using the binarization method explained in Figure S6. Grayscale binarized images of the cross-sectional SEM images are shown in Figure S7. The average porosity values of the sintered Ag layer were 24.4%, 23.6%, 19.1%, and 18.7% when varying the sintering time to 1, 3, 6, and 12 h, respectively. A significant decrease in the average porosity had occurred between 6 and 12 h of sintering for the same reason as a result of the varying sintering temperature described above.

The EMI shielding performance of the porous sintered Ag layer with different sintering times is exhibited in Fig. 3b. The average EMI SE values were 44.9, 50.1, 56.5, and 52.7 dB when the sintering process was treated for 1, 3, 6, and 12 h, respectively. The EMI shielding performance was enhanced as the sintering time increased from 1 to 6 h. Although the Ag layers sintered at 150 °C and for 1 h retained high porosity, they lacked sufficient interparticle connectivity. Since EMI shielding in metals relies primarily on reflection, which requires a continuous conductive path, the insufficient necking under these conditions led to weak performance despite favorable porosity12,39. This highlights the need for a balance between porosity and conductivity in achieving optimal shielding efficiency. However, the average EMI SE value slightly decreased with the excessive processing time of 12 h. The decreased porosity of the Ag layer, sintered for 12 h, affected the reduction of multiple internal reflections of incident EM waves, thereby lowering the EMI shielding performance.

We measured the electrical conductivity of the porous sintered Ag layers to investigate the additional effect of sintering temperature and time on EMI shielding performance. The theoretical EMI SE (SET) values can be calculated by the following equations40,41,42:

$$\begin{aligned} {\text{SE}}_{{\text{T}}} & = - {\text{2}}0{\text{ log }}\left( {{\text{P}}_{{\text{T}}} /{\text{P}}_{{\text{I}}} } \right) = {\text{ SE}}_{{\text{A}}} + {\text{ SE}}_{{\text{R}}} \\ & = 20\left( {{\text{log e}}} \right)\frac{{\text{t}}}{{{\delta }}} + 20\,{\text{log}}\left( {\frac{{{\text{Z}}_{0} {{\delta \sigma }}}}{{4\sqrt 2 }}} \right) \\ \end{aligned}$$
(1)

(PT and PI is the intensity of transmitted and incident EM waves, e is the natural constant, t is the thickness of the porous sintered Ag layer, Z0 is the characteristic impedance of air (120π), and is the skin depth (δ = (\(\:\sqrt{\pi\:f\mu\:\sigma\:}\))−1), where f, µ, and σ are the frequency of microwave, the magnetic permeability, and the electrical conductivity of the material, respectively).

As shown in Eq. (1), the electrical conductivity of the shielding material is a key factor in the reflective loss of EMI shielding. High electrical conductivity enhances the surface reflection of incident EM waves through increased interaction with free electrons, improving shielding efficiency. Electrical conductivity can be calculated by the following equation:

$${\text{S}} = {\text{1/}}\rho = {\text{1}}/R_{S} {\text{t}}$$
(2)

(S is the electrical conductivity, ρ is the electrical resistivity, \(\:{R}_{S}\) is the sheet resistivity, and t is the thickness of the porous sintered Ag layer)

To measure the electrical properties of the porous sintered Ag layers, the thickness and sheet resistance of each layer shown in Figs. 2 and 3 was measured. Figure S8 shows their cross-sectional SEM images and the average thicknesses, and Fig. 4 shows their sheet resistance and electrical conductivity of the layers prepared under different sintering conditions. The cross-sectional SEM images in Figure S8 reveal that sintering temperature and time did not lead to significant differences in layer thickness.

Fig. 4
figure 4

Electrical properties of the porous sintered Ag layers under different sintering conditions. Sheet resistance and electrical conductivity of the porous sintered Ag layers at varying (a) sintering temperatures and (b) sintering times.

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According to Eq. (1), the absorption loss (SEA) and reflection loss (SER) can be expressed in logarithmic functions of electrical conductivity when the thickness of the layer is constant and the overall EMI SE increases with increasing conductivity. In Fig. 4, the electrical conductivity increased as the sheet resistance gradually decreased when the sintering temperature increased from 150 to 170 °C and then remained nearly constant between 170 and 180 °C. The negligible change in average EMI SE between 170 and 180 °C is likely due to the minimal variation in electrical conductivity within this temperature range. On the other hand, the electrical conductivity gradually increased as the sheet resistance gradually lowered when the sintering time was up to 6 h, but slightly decreased at 12 h, indicating a saturation with prolonged sintering. The decrease in EMI SE (~ 3.8 dB) at 12 h in Fig. 3b could be attributed to the slight reduction in electrical conductivity as described by Eq. (1). In addition, we conducted deconvolution of the EMI SET values in absorbance (A), reflectance (R), and transmittance (T) components of porous sintered Ag layers to obtain the dominant EMI shielding effect (Figure S9). For all sintering conditions, the porous Ag layer exhibited the reflectance coefficient over 0.95, which means the reflectance dominantly affects the EMI shielding property than the absorption.

To compare the EMI shielding performance of the porous sintered Ag layers developed in this study, we compared them with previously reported materials, as summarized in Table S128,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62. This includes EMI shielding data of various metal-based candidates. Although the measured EMI SE and EMI SE per unit thickness (EMI SE/t) of the porous sintered Ag layer were lower than some other Ag-based materials, both its EMI SE/t (Fig. 5a) and absolute EMI SE (Fig. 5b) outperformed all other reported alternatives. This highlights the superior efficiency of the porous sintered Ag layer, even at low thickness.

Fig. 5
figure 5

Comparative chart showing the EMI shielding performances of the porous sintered Ag layer compared with those of the results in representative previous reports. (a) Average EMI SE and (b) EMI SE/t values.

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To demonstrate practical applicability in package-level EMI shielding, a PCB antenna was coated with the porous sintered Ag layer using the blade-coating method explained above. Near-field radiation measurements were conducted on both bare and Ag-coated PCB antennas (Figure S10). In the near-field SE mapping images, red represents regions of high EM radiation intensity, while whitish yellow indicates lower intensity. The bare PCB antenna displayed broad color variation in the near-field mapping with a peak radiation intensity of 100.0 dB µV−1 (Fig. 6a). In contrast, the Ag-coated antenna exhibited a uniformly yellow distribution except for slight leakage near the antenna-connector junction with intensity reduced to 40.0 dB µV−1 (Fig. 6b). This indicates that the porous sintered Ag layers substantially improved near-field EMI shielding performance.

Fig. 6
figure 6

Practical application of the porous sintered Ag layer for packaging-level EMI shielding. Near-field SE mapping images of (a) bare and (b) the porous sintered Ag layer-coated PCB antennas.

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Conclusions

In summary, we successfully demonstrated the production of the porous sintered Ag layer with pressureless low-temperature sintering for EMI shielding. The porous structure for the multiple internal reflections of EM waves was controlled by sintering temperature and time. Adjusting the sintering temperature and time promoted interparticle connections and enabled control over electrical conductivity and porosity, which improved the EMI shielding performance. The optimized sintering condition at 170 °C for 6 h resulted in the highest EMI shielding efficiency of 56.5 dB. The porous sintered Ag layer showed superior EMI shielding performance compared to previously reported materials. PCB antenna tests confirmed reduced electromagnetic radiation and demonstrated the material’s suitability for package-level EMI shielding. This study provides valuable insights and significant potential for the future development of advanced electronic shielding technologies.

Methods

The silver nanoparticles (Ag NPs) were purchased from BKChem Co., Ltd. (Suwon, Korea). Ethanol was purchased from DAEJUNG Chemicals & Metals Co., Ltd. (Siheung, Korea). Polyimide film (PI, 0.125 mm) was purchased from YOUNGWOO FINETECH Co.Ltd. (Hwaseong, Korea).

Silver nanoparticles were dispersed in ethanol for 70 wt%. The dispersion was blade-coated onto PI substrates using the coating device (3000VH, Kipae E&T Co., Ltd., Korea) and a blade applicator with a 120 μm gap. The coated Ag layer was sintered in the oven for 6 h without external pressure. The cross-sections of the sintered Ag layers were prepared and analyzed using focused ion beam scanning electron microscopy (FIB-SEM) to examine their internal microstructures. Sintered Ag films were cut into a circular shape with outer and inner diameters of 12.2 and 2.1 mm, respectively, to measure EMI shielding performance. EMI shielding efficiency in the frequency range of 0.5–18.0 GHz was measured using an Agilent E5071C network analyzer in conjunction with a coaxial sample holder.

As-prepared Ag dispersions were coated on a commercially available printed circuit board (PCB) antenna (P000CTJ, Kims Mobile) for application to package-level EMI shielding. An Ag layer with a porous structure was coated on a PCB antenna using the blade coating with a 120 μm gap of blade applicator and pressureless sintering methods described above. A PCB antenna was employed as a radiation source of EM waves and a spectrum analyzer (E4407B, Agilent) combined with a Langer probe and PCB scanner was used to measure the near-field EMI shielding performance. The mapping of the EM radiation was conducted on the surface of the PCV antenna at 2.4 GHz frequency, and emitted EM waves were detected by moving the axis of the probe at intervals of 2 mm, according to the near-field scan test methods.

The nanostructure and morphological properties of the Ag nanoparticles were analyzed by using a field effect-scanning electron microscope (FE-SEM, JSM-7600 F, JEOL Ltd., Japan) and Field emission transmission electron microscopy (FE-TEM, JEM-F200, JEOL Ltd., Japan). The structure and phase were identified by a Powder X-ray diffractometer (XRD, SmartLab, Rigaku, Japan) in the range of 5–90° at a scan rate of 10 °/min. The thermal properties and content of the surface stabilizer were measured by thermogravimetric analysis (TGA, Seiko Exstar 6000, SEICO, Japan). The temperature ranged from 25 to 350 °C, and the heating rate was 10 °C/min. The thermal decomposition temperature of the surface stabilizer could be confirmed by differential scanning calorimetry (DSC, Nexta DSC 600, Hitachi, Japan). The temperature ranged from 25 to 350 °C, and the heating rate was 10 °C/min. The cross-section of the sintered Ag layer was prepared and analyzed using focused ion beam scanning electron microscopy (FIB-SEM, SMI3050TB, Seiko Instruments, Japan). The electrical conductivity of the sintered Ag layer was measured using the four-point probe measurement system (CMT-SR2000N, AIT, Korea). The porosity value was derived from cross-sectional image analysis. The cross-sectional SEM images were grayscale-binarized using the image processing program (ImageJ). The white-colored region in the binarized images represents the pores in the sintered layer. The porosity value was calculated by the area fraction of the white-colored region to the total area.