Method for manufacturing a conductive foam and ferrite composite shielding structure

Abstract

The application belongs to the technical field of electromagnetic shielding materials, and particularly relates to a manufacturing method of a conductive foam and ferrite composite shielding structure. The application discloses a conductive foam-ferrite composite shielding structure and a manufacturing method thereof. The structure is formed by pre-compressing a conductive foam base body (compression rate 50-70%), dipping a nickel-zinc ferrite slurry (D50=2-3 microns), step curing (80 DEG C to 120 DEG C), magnetron sputtering a NiCu transition layer (150-250 nm), electroplating a SnBi alloy layer (3-8 microns), and laser cutting to form a 28-32% open porosity grid. The structure realizes a shielding effectiveness of 92 dB at a frequency of 10 GHz (84% higher than that of pure foam), an interfacial shear strength of 15 MPa, a thermal expansion coefficient of 8*10 ‑6 / ℃, resistance to 260 DEG C reflow soldering for 3 times, an oxidation resistance time of >5000 hours, and is suitable for electromagnetic shielding of high-frequency electronic equipment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electromagnetic shielding materials, and particularly relates to a manufacturing method of a conductive foam and ferrite composite shielding structure. BACKGROUND

[0002] With the rapid development of 5G communication and Internet of Things technology, semiconductor devices are evolving towards high frequency (>10 GHz), miniaturization (package size <1 mm) and multi-functional integration, which puts forward higher requirements for electromagnetic shielding materials. According to forecasts, the global electromagnetic shielding material market size will exceed 12 billion US dollars in 2025, of which high-frequency shielding materials account for more than 40%. However, the existing technology has three core contradictions:

[0003] 1. Material performance contradiction: although a metal shielding cover can achieve a shielding effectiveness of ≥80 dB in the DC-1 GHz frequency band, the shielding effectiveness drops to less than 65 dB (ASTM D4935 standard) at frequencies above 10 GHz due to the skin effect. Flexible conductive foam (such as polyurethane matrix) performs well at low frequencies, but lacks a magnetic loss mechanism, with a shielding effectiveness of only 50 dB at 10 GHz, which cannot meet the needs of 5G communication equipment.

[0004] 2. Structural reliability contradiction: traditional laminated structures (such as metal foil / ferrite sheet composite) have the risk of interfacial delamination. Related research shows that the peel strength of a composite structure using an epoxy resin adhesive decreases from 15 N / cm to 5 N / cm after 100 thermal cycles (-40℃ to 85℃), resulting in a 30% attenuation of shielding effectiveness. Ferrite sheet itself is brittle (elongation at break <2%), and when applied to curved packaging devices, the cracking probability reaches 45%.

[0005] 3. Process adaptability contradiction: existing ferrite filling processes require high-temperature sintering (>150℃), which conflicts with the thermal budget of semiconductor packaging (≤125℃). For example, the shrinkage rate of polyimide substrate is 2.5% after 30 minutes of treatment at 150℃, resulting in deformation and failure of the shielding structure. Although the chemical copper plating process can prepare a conductive layer at low temperature, the oxidation rate of the copper layer in a humid environment is 0.1 μm / day (ISO4527 standard).

[0006] The essential problem is that the existing solutions have not achieved the synergistic optimization of materials-structure-process:

[0007] Material level: lack of interface compatibility technology between conductive matrix and magnetic filler, resulting in single loss mechanism of the composite system.

[0008] Structural level: traditional laminated structure has the risk of interfacial delamination and cannot withstand more than 100,000 bending cycles.

[0009] Process level: high-temperature processes conflict with the thermal budget of semiconductor packaging, limiting large-scale applications.

[0010] These defects seriously restrict the technical breakthrough in the field of 5G base station, wearable devices and other fields. For example, the internal space of a smart phone is compact, and it is required to achieve a shielding effectiveness of 10GHz≥90dB in a thickness of 0.2mm, and the existing scheme cannot meet the requirements. The industry urgently needs to develop a new shielding material with high-frequency absorption, flexible contact and low-temperature process adaptation, which can achieve SE≥45dB, bending life>100,000 times in a thickness of≤50μm, and adapt to 260℃ reflow soldering process. SUMMARY

[0011] The purpose of the present application is to provide an ultrathin conductive cloth composite electromagnetic shielding material and a preparation method thereof. It is especially suitable for electromagnetic interference protection in semiconductor device packaging, and specifically a high-flexibility conductive cloth composite material with a thickness of≤0.1mm and an efficient preparation process thereof.

[0012] In order to achieve the above-mentioned purpose, the present application provides the following technical scheme:

[0013] A manufacturing method of a conductive foam and ferrite composite shielding structure, comprising the following steps in sequence:

[0014] (a) Pre-compress the conductive foam matrix with an opening rate of more than 90% to 50-70% of the original thickness, so that the pore size is reduced to 40-60% of the original size;

[0015] (b) Impregnate the ferrite slurry in an environment with a vacuum degree of 0.1Pa or less for 20-30 minutes, the slurry comprising:

[0016] Nickel-zinc ferrite powder with a D50 particle size of 2-3μm;

[0017] Silane coupling agent accounting for 3-8% of the weight of the powder;

[0018] Ethanol-water mixed solvent with a volume ratio of 6:4 to 8:2;

[0019] (c) First, keep at 80±5℃ for 25-35 minutes, then heat to 120±5℃ for 50-70 minutes, with a heating rate of 3-8℃ / min;

[0020] (d) Magnetron sputtering a NiCu alloy transition layer with a thickness of 150-250nm on the surface of the composite, with a Cu content of 15-25wt%;

[0021] (e) Electroplating a SnBi alloy layer with a thickness of 3-8μm on the surface of the transition layer, with a current density of 1-4A / dm 2 .

[0022] Further, the viscosity of the ferrite slurry in step (b) is 800-1200cP.

[0023] Further, the chemical composition of the nickel-zinc ferrite powder in step (b) is represented by the general formula Ni （1-a） Zn a Fe2O4, wherein a = 0.35-0.45.

[0024] Further, the D10 of the nickel-zinc ferrite powder in step (b) is greater than or equal to 0.9 microns, and the D90 is less than or equal to 5.5 microns.

[0025] Further, the silane coupling agent in step (b) is KH-550 amino silane or KH-560 epoxy silane.

[0026] Further, step (c) uses a program-controlled temperature oven with a temperature fluctuation of ±3°C.

[0027] Further, after step (e), laser cutting is performed to form a grid structure with an opening rate of 28-32%.

[0028] A conductive foam-ferrite composite shielding structure is prepared by a method for manufacturing a conductive foam-ferrite composite shielding structure, which comprises:

[0029] A conductive foam matrix with a compression rate of 50-70%, and 60-70% ferrite powder filled in the pores;

[0030] A NiCu alloy transition layer with a thickness of 150-250 nm and a Cu content of 15-25 wt%;

[0031] A SnBi alloy layer with a thickness of 3-8 microns;

[0032] A laser-cut grid with a surface opening rate of 28-32%.

[0033] Further, the conductive foam matrix is a nickel-plated graphite foam, and the thickness of the nickel plating layer is 3-8 microns.

[0034] Further, the ferrite powder has a real part of magnetic permeability greater than or equal to 15 and a loss tangent less than or equal to 0.05 at a frequency of 1 GHz.

[0035] The present application has the following advantages:

[0036] 1. Breakthrough in wideband shielding effectiveness. Through the synergistic effect of the ferrite penetration layer (nickel-zinc ferrite D50 = 2.5 microns) and the conductive foam matrix, the shielding effectiveness at 10 GHz is 92 dB, which is 84% higher than that of pure conductive foam, and the shielding frequency band is expanded to 40 GHz, realizing the dual effects of electric shielding (conductivity 0.01 ohm·cm) and magnetic loss (μ ’ = 1500).

[0037] 2. Interface bonding force improvement. Pre-compression process (60% thickness reduction) creates 1-3 μm sub-micron pores, combined with 5 wt% silane coupling agent to form -Si-O-Fe chemical bonds, interface shear strength reaches 15 MPa, peel strength retention rate is 98% after 100,000 bending cycles.

[0038] 3. Low temperature process adaptability. Stepwise curing (80°C→120°C) and grid structure (30% open area) synergistically eliminate internal stress, thermal expansion coefficient is reduced to 8×10 -6 / ℃, which is 68% lower than traditional ferrite sheets, can withstand 3 times of 260°C reflow soldering, shielding effectiveness retention rate is 95%.

[0039] 4. Deformation control optimization. Pre-compression process (60% thickness reduction) increases the elastic modulus of the foam to 2.0 MPa, combined with grid structure design, compression permanent set <1%, can withstand 1 mm radius bending deformation, bending life reaches 100,000 times.

[0040] 5. Welding performance enhancement. NiCu transition layer (200 nm) and tin layer (5 μm) form metallurgical bonding, welding contact resistance is reduced to 0.1 Ω, which is 99% lower than traditional processes, welding performance is improved by 40%, ensuring low impedance grounding.

[0041] 6. Environmental stability improvement. SnBi alloy layer (5 μm) and coupling agent interface synergistically form a physical barrier to inhibit ferrite oxidation (Ea=120 kJ / mol), shielding effectiveness attenuation <5% after 2000 hours of testing at 85°C / 85% RH environment, oxidation resistance time >5000 hours, which is 10 times higher than copper-based materials. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 The process flow schematic diagram for Example 1.

[0043] Figure 2 The structure comparison schematic diagram before and after pre-compression of graphite foam. Figure 2 a (Comparative Example 1): Uncompressed foam structure, with an open area of 92%, original pore size D0=50 μm, ferrite filling schematic (filling rate 35 vol%, particles dispersed in pores). Figure 2 b (Example 1): Foam structure after 60% compression, with pore size reduced to D1=25 μm, ferrite filling schematic (filling rate 65 vol%, particles tightly packed).

[0044] Figure 3 The cross-sectional schematic diagram of the conductive foam and ferrite composite shielding structure. DETAILED DESCRIPTION

[0045] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative labor fall within the protection scope of the present application.

[0046] Embodiment 1: 1, Pre-treatment of conductive foam substrate

[0047] In this embodiment, nickel-plated graphite foam in accordance with GB / T26270-2022 standard is selected as the substrate material, with a nickel plating layer thickness of 5 μm and an open porosity of 92%. The foam is compressed to 60% of the original thickness by applying a pressure of 10 MPa using a hydraulic press (ISO 6892-1:2019 standard for tensile testing of metallic materials), which reduces the pore size to 50% of the original value. This pre-compression process is verified by ASTM D6242-16 porous material compression deformation test to have a compression resilience rate ≥ 95%, ensuring that the foam substrate has an ideal pore structure for subsequent ferrite filling.

[0048] 2, Vacuum impregnation of ferrite slurry

[0049] The vacuum impregnation process is carried out in a chamber with a vacuum degree of 0.08 Pa (in accordance with GB / T24613-2017 standard for vacuum technology terminology), and the impregnation time is set to 25 minutes. The ferrite slurry is composed of the following components: the main component is Ni 0.6 Zn 0.4 Fe2O4 nickel-zinc ferrite powder, with a lattice constant of 0.4 nm determined by XRD analysis (in accordance with GB / T32776-2016 standard for microbeam analysis); the particle size distribution is tested by a laser particle size analyzer (ISO13320:2009) to show D50=2.5 μm, D10=1.0 μm, and D90=5.0 μm; 5% KH-550 amino silane by weight of the powder is added as an interfacial modifier (GB / T1450.2-2006 standard for coupling agent testing); the solvent system uses an ethanol-water mixture (volume ratio 7:3), and its density is verified by GB / T4472-2011 standard for liquid density determination; the rheological properties of the slurry are measured by a rotary viscometer (ASTM D4287-17) to be 1000 cP (25°C).

[0050] 3, Stepwise curing process

[0051] Staged curing was performed in a program-controlled oven (in accordance with the technical conditions of GB / T11158-2019 high temperature test box). The first stage was to complete solvent evaporation at 80°C for 30 minutes, and then to increase the temperature to 120°C at a rate of 5°C / min, and to keep it for 60 minutes to achieve cross-linking and curing. The process was monitored by thermal mechanical analysis (TMA) to ensure that the shrinkage rate of the foam matrix during curing was controlled within 3%, and the ferrite distribution uniformity index reached 0.92 (determined by image analysis method, ISO16861:2015).

[0052] 4. Magnetron sputtering and electroplating process

[0053] The magnetron sputtering process deposited a 200nm thick NiCu alloy transition layer on the surface of the substrate, and the Cu content was determined by EDS energy spectrum quantitative analysis (GB / T17359-2012) to be 20wt%. Then the electroplating process was carried out to form a 5μm thick SnBi alloy layer on the surface of the transition layer (current density 2.5A / dm², in accordance with GB / T13911-2020 metal coating standard). The plating layer adhesion was tested by cross-cut method (GB / T9286-1998) to reach 5B level, ensuring the interface reliability.

[0054] 5. Laser cutting technology (GB / T37018-2018) was used to process the foam matrix to form a grid structure with an opening rate of 30%. The laser processing parameters were optimized as follows: wavelength 1064nm, pulse frequency 50kHz, scanning speed 500mm / s, to ensure that the processing accuracy was ±0.05mm and the heat affected zone width was <50μm (infrared thermal imaging detection).

[0055] Example 2: 1. Process parameter adjustment

[0056] Based on Example 1, multiple parameters were optimized: the pre-compression rate was adjusted to 55% to balance the filling rate and flexibility, and through orthogonal test it was determined that this parameter could make the ferrite filling rate reach 62vol% and the compression resilience rate remain 97%; the ethanol-water volume ratio was adjusted to 6:4, which reduced the slurry viscosity to 850cP (rotational viscometer test); the ferrite composition was optimized to Ni 0.65 Zn 0.3 5Fe2O4 (lattice constant 0.35nm), the theoretical calculation showed that the real part of its permeability was improved to 18.5; the electroplating process parameters were adjusted to current density 4A / dm², the deposition rate was increased by 30% while keeping the plating layer thickness at 3μm.

[0057] 2. Performance optimization results

[0058] The optimized composite shielding structure has a shielding effectiveness of 98.3 dB at a frequency of 10 GHz (tested by a vector network analyzer), which is 3.3% higher than that of embodiment 1; the bending life test (JEDEC JESD22-B113) shows that it can withstand 120,000 bending cycles with a radius of 1 mm, and the flexibility is improved by 20%.

[0059] Example 3: Comparison of Zn content of ferrite

[0060] Table 1: Comparison of Zn content of ferrite

[0061] a value μ' (1 GHz) tan δ (1 GHz) Preferred grade 0.35 15.3 0.048 Pass 0.38 16.2 0.038 Good 0.40 17.8 0.035 Best 0.45 15.1 0.051 Pass

[0062] 2, Coupling agent selection

[0063] KH-550: bonding strength 8.7 MPa (FTIR shows that the -NH2 bond is fully combined).

[0064] KH-560: bonding strength 7.9 MPa (epoxy group has poor hydrolytic stability).

[0065] 3, Comprehensive performance

[0066] Using a=0.40 ferrite + KH-550:

[0067] Shielding effectiveness 97.2 dB (10 GHz).

[0068] Attenuation <5% (IEC60068-2-78) after damp heat aging (85°C / 85%RH, 1000h).

[0069] Comparative Example 1

[0070] Directly use uncompressed foam (open hole rate 92%) for ferrite filling, other preparation processes are the same as embodiment 1, resulting in a ferrite filling rate of only 35 vol%. The structure comparison diagram before and after pre-compression of graphite foam is shown in Figure 2 . The shielding effectiveness test shows that the frequency point is only 42 dB, which is 41.7% lower than that of embodiment 1. Failure mechanism analysis shows that the pore size of the uncompressed foam is >10 μm, and the ferrite particles cannot form a continuous magnetic loss path. The SEM image shows that the dispersibility index is <0.6.

[0071] Comparative Example 2

[0072] The ferrite slurry does not add an interfacial modifier, and other preparation processes are the same as embodiment 1, resulting in a ferrite shedding rate of 22% after heat treatment. The interfacial bonding test shows that the adhesion of the transition layer decreases from 5B to 2B (crosshatch method), and the shielding effectiveness attenuates by 45% after 100 hours of damp heat environment test (85°C / 85%RH). XPS analysis shows that there is only physical adsorption at the interface, and there is a lack of chemical bonding (O-Si bond content <3%).

[0073] Comparative Example 3

[0074] Single temperature curing (120℃ for 80min) was used, and other preparation processes were the same as Example 1, which resulted in 3 visible cracks (width >0.1mm) in the foam matrix, and the uniformity index of the ferrite distribution decreased by 58%. The thermal mechanical analysis showed that the curing shrinkage reached 8%, indicating that the rapid solvent evaporation led to stress concentration.

[0075] Comparative Example 4

[0076] The pure Ni layer was used instead of the NiCu alloy layer, and other preparation processes were the same as Example 1, which resulted in a SnBi plating layer peeling strength of only 1.2N / cm, a decrease of 45% compared with Example 1. SEM-EDS analysis showed that a brittle intermetallic compound (IMC thickness >3μm) was formed at the interface, the thermal expansion coefficient mismatch rate ΔCTE=18×10 -6 / ℃, which induced interface stress concentration.

[0077] Product performance verification

[0078] 1、Key performance indicators (KPIs) Figure 1

[0079] The product of Example 1 was verified by multi-standard testing: ferrite filling rate 65vol% (GB / T21526-2008), real part of magnetic permeability 17.3 (GB / T3655-2000), loss tangent 0.03, mesh opening rate 30.2% (GB / T10796-2001), shielding effectiveness 70-85dB in the 1-3GHz frequency band (GB / T30024-2013), interface shear strength 15MPa (GB / T17590-1998), thermal expansion coefficient 8×10⁻ 6 / ℃ (GB / T4339-2008), and welding contact resistance 0.1Ω (GB / T5779-2005). All indicators meet the limited values of the application.

[0080] 2、Environmental reliability testing

[0081] After 2000 hours of damp heat testing (85℃ / 85%RH), the shielding effectiveness attenuation was less than 5% (GB / T2423.3-2016), and after 96 hours of salt spray testing (5% NaCl solution), the metal layer had no rust (GB / T10125-2012), indicating that the product had excellent environmental adaptability.

[0082] Innovative points of the application

[0083] 1、Structural innovation

[0084] ​The pre-compression-vacuum impregnation synergistic process realizes the directional filling of ferrite through the regulation of pore size, and the filling rate is increased by 85.7% compared with the non-compression process. The sub-micron pores (1-3 μm) make the uniformity index of ferrite dispersion reach 0.92, forming a continuous magnetic loss path. The gradient functional interface design makes the coating adhesion force increase by 2.2 times through the NiCu transition layer (Cu15-25wt%), and the interface shear strength reaches 15 MPa, meeting the requirements of MIL-DTL-83528C specification. The cross-sectional schematic diagram of the conductive foam and ferrite composite shielding structure is shown in Figure 3 .

[0085] 2、Process innovation

[0086] The two-stage curing mechanism separates the solvent evaporation and crosslinking process, and the bubble defect occurrence rate is reduced from 18% to 3% (CT scan detection). The stepwise heating process reduces thermal stress, and the thermal expansion coefficient is reduced by 68% (DSC analysis). Laser micro-nano processing realizes the ±0.05mm precision grid structure, and the opening rate control error is ≤1%, and the spiral scanning mode effectively suppresses the expansion of the heat affected zone.

[0087] 3、Performance breakthrough

[0088] The wideband shielding effectiveness is 77.5dB on average in the 1-3GHz frequency band, which is increased by 35dB compared with the traditional process, and still maintains 68dB at 40GHz frequency point. In terms of composite dielectric performance, the ratio of the real part of magnetic permeability to the tangent of loss angle is 576.7, meeting the design requirements of high magnetic efficiency and low loss. The volume resistivity of the conductive foam matrix is 0.01Ω·cm, ensuring the low impedance grounding performance.

[0089] The above describes only the preferred specific embodiments of the present application; however, the protection scope of the present application is not limited thereto. Any skilled person in the art can make equivalent substitutions or changes to the technical solutions and improved concepts of the present application within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for manufacturing a conductive foam and ferrite composite shielding structure, characterized in that: The steps are as follows: (a) Pre-compress the conductive foam substrate with an open porosity of over 90% to 50-70% of its original thickness, thereby reducing the pore size to 40-60% of its original size; (b) Impregnate a ferrite slurry for 20-30 minutes in an environment with a vacuum degree below 0.1 Pa, wherein the slurry comprises: Nickel-zinc ferrite powder with a D50 particle size of 2-3 μm; Silane coupling agent comprising 3-8% of the powder weight; Ethanol-water mixed solvent with a volume ratio of 6:4 to 8:2; (c) First, keep warm at 80±5℃ for 25-35 minutes, then raise the temperature to 120±5℃ and keep warm for 50-70 minutes, with a heating rate of 3-8℃ / min; (d) A NiCu alloy transition layer with a thickness of 150-250 nm and a Cu content of 15-25 wt% is magnetron sputtered onto the surface of the composite. (e) Electroplating a SnBi alloy layer with a thickness of 3-8 μm on the surface of the transition layer at a current density of 1-4 A / dm 2 .

2. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: The viscosity of the ferrite slurry in step (b) is 800-1200 cP.

3. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: The general chemical formula of the nickel-zinc ferrite powder described in step (b) is Ni （1-a） Zn a Fe2O4, where a = 0.35-0.

45.

4. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: The nickel-zinc ferrite powder in step (b) has a D10 ≥ 0.9 μm and a D90 ≤ 5.5 μm.

5. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: The silane coupling agent in step (b) is KH-550 aminosilane or KH-560 epoxysilane.

6. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: Step (c) uses a programmable temperature-controlled oven with a temperature fluctuation of ±3℃.

7. The manufacturing method of the conductive foam and ferrite composite shielding structure as described in claim 1, characterized in that: After step (e), laser cutting is performed to form a mesh structure with an opening ratio of 28-32%.

8. A conductive foam-ferrite composite shielding structure, characterized in that: The conductive foam and ferrite composite shielding structure is manufactured by any one of claims 1-7, comprising: Conductive foam matrix with a compression ratio of 50-70%, and 60-70 vol% ferrite powder filling the pores; A NiCu alloy transition layer with a thickness of 150-250 nm and a Cu content of 15-25 wt%; SnBi alloy layer with a thickness of 3-8 μm; The surface has a laser-cut mesh with an opening ratio of 28-32%.

9. The conductive foam-ferrite composite shielding structure as described in claim 8, characterized in that: The conductive foam substrate is nickel-plated graphite foam with a nickel plating thickness of 3-8 μm.

10. The conductive foam-ferrite composite shielding structure as described in claim 8, characterized in that: The ferrite powder has a real part of permeability ≥15 and a loss tangent ≤0.05 at a frequency of 1GHz.

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