Miniaturized RF Circulators and Isolators: Design Considerations and Manufacturing Processes

　　With the miniaturization of 5G communications, millimeter-wave radar, and portable test equipment, the size requirements for RF circulators and isolators are becoming increasingly stringent (typically requiring a 30%~60% reduction in size compared to traditional models, with typical package sizes ≤10mm×10mm×5mm). While reducing size, it is necessary to ensure that RF performance (insertion loss, isolation), power capacity, and reliability are not compromised. Their design and manufacturing require overcoming multiple technical challenges. The following is a summary of the core technical considerations.

　　I. Core Considerations for Miniaturization Design (Taking ≤20GHz Band Devices as an Example)

　　1. Material Selection: Balancing "Small Size" and "High Performance"

　　Materials are the core constraint for miniaturization. Priority should be given to materials with "high characteristic parameters + low volume fraction," focusing on three types of core components:

　　Ferrite Materials:

　　As the core of the device's magnetic permeability, it must meet the requirements of "high saturation magnetization (4πMs) + low magnetic loss (tanδ)." NiZn-based or LiZn-based high-frequency ferrites (such as NiZnFe₂O₄) are recommended. Their 4πMs can reach 3000~4000G, and their magnetic loss tanδ ≤ 5×10⁻⁴ in the 10~20GHz band, with a stable dielectric constant εr (8~12). This allows the thickness of ferrite chips to be reduced from the traditional 2~3mm to... 0.8~1.5mm, while avoiding a sharp drop in permeability at high frequencies.

　　Permanent Magnet Material:

　　High coercivity (Hc) and high energy product ((BH)max) are required to reduce magnet size. Rare-earth permanent magnet materials are preferred—SmCo permanent magnets ((BH)max=200~300kJ/m³, Hc≥1500kA/m) or micro NdFeB permanent magnets (nickel-plated for corrosion resistance). This can reduce the permanent magnet size from the traditional 5mm×3mm×2mm to 2mm×1.5mm×1mm, with a magnetic field uniformity error ≤±5% (ensuring stable ferrite operating point).

　　Dielectric Substrate and Electrode Materials:

　　The dielectric substrate should be selected for its "low dielectric loss + high thermal conductivity". For high-frequency bands (>10GHz), PTFE-based composite substrates (εr=2.6~3.0, tanδ≤0.001) or Al₂O₃ ceramic substrates (thermal conductivity ≥20W/(m・K)) are recommended, with a thickness controlled at 0.2~0.5mm. Electrodes should be made of highly conductive metals (gold, silver) using thin-film processing (thickness 0.5~2μm) to reduce electrode volume and lower contact resistance (≤5mΩ).

　　2. Structural Optimization: Overcoming the Contradiction Between "Size Compression" and "Performance Interference"

　　Traditional coaxial structures (large size, many parasitic parameters) cannot meet miniaturization requirements, necessitating an "integrated + compact" structural design:

　　Topology Selection:

　　Preferably use microstrip structures or coplanar waveguide (CPW) structures. Microstrip structures can reduce the lateral size of devices by 40% (by reducing the transmission line width to 0.1~0.3mm) and facilitate PCB integration; CPW structures eliminate the need for grounding vias, reducing vertical volume (thickness down to 3~5mm), making them suitable for miniaturization in the millimeter-wave band (20~40GHz), while simultaneously reducing parasitic inductance (≤0.1nH).

　　Port Layout Design:

　　An "L-shaped" or "star-shaped" compact layout is adopted, reducing the spacing between the three ports (IN/OUT/ISOLATED) from the traditional 5-8mm to 1.5-3mm. Simultaneously, the port matching network is optimized through electromagnetic simulation (HFSS/CST) (e.g., adding miniature matching capacitors with a capacitance of 0.5-2pF) to avoid electromagnetic coupling between ports (coupling coefficient ≤-30dB).

　　Magnetic Circuit Integration Design:

　　Permanent magnets and ferrite chips are "stacked and integrated" (permanent magnets are attached to the upper and lower surfaces of the ferrite), replacing the traditional separate magnetic circuit and reducing the space occupied by the magnetic circuit. At the same time, a miniature magnetically conductive shell (made of permalloy, 0.1-0.2mm thick) is designed on the outside of the permanent magnet to concentrate the magnetic field to the ferrite region, avoiding magnetic field leakage (leakage rate ≤10%) and ensuring magnetic circuit efficiency.

　　3. Electromagnetic Simulation and Performance Balancing: Avoiding Parasitic Effects of Miniaturization

　　Smaller size often leads to increased parasitic capacitance and inductance, requiring parameter optimization through multiphysics simulation.

　　RF Performance Simulation:

　　Using a 3D electromagnetic simulation tool (HFSS), the following optimizations are prioritized:

　　① Insertion Loss (IL) – By adjusting the ferrite chip size (diameter 3~5mm) and transmission line coupling length (0.5~1mm), ensure IL ≤ 0.8dB (10GHz band);

　　② Isolation (ISO) – Optimize the permanent magnet magnetic field strength (200~300Oe) to suppress reverse signals, ISO ≥ 18dB (circulator) / ≥ 22dB (isolator);

　　③ Return Loss (RL) – Adjust the impedance (50Ω standard) through a matching network, RL ≤ -15dB.

　　Parasitic Parameter Control:

　　Simulation analysis was conducted to control the parasitic inductance of the port leads (0.05~0.1nH) and the parasitic capacitance of the substrate and housing (≤0.5pF). By shortening the lead length (≤1mm) and using a floating housing (0.1~0.2mm spacing from the substrate), the impact of parasitic parameters on high-frequency performance was reduced (especially in the >20GHz band, where parasitic parameter sensitivity is higher).

　　Thermal Simulation and Power Adaptation:

　　Miniaturization reduces the heat dissipation area (by more than 50% compared to traditional devices), necessitating the use of thermal simulation tools (ANSYS Icepak) to analyze power loss distribution. For high-power scenarios (>10W), miniature heat dissipation bumps (0.2~0.3mm in diameter, 0.5mm spacing) should be designed on the inside of the casing, or a high thermal conductivity casing (such as CuW alloy, with a thermal conductivity ≥180W/(m・K)) should be selected to ensure that the casing temperature is ≤85℃ (ambient temperature 25℃) at a 10W input power.

　　4. Interface and Packaging Design: Balancing Miniaturization and Operability

　　Interface Selection:

　　Using ultra-small RF interfaces, such as MMCX interfaces (2.5mm × 1.8mm) or SMA ultra-small interfaces (4mm × 3mm package), to replace traditional SMA interfaces (8mm × 6mm); integrating the interface with the device body (seamless connection between the interface metal shell and the device housing) to reduce assembly gaps (≤0.1mm) and avoid signal reflection.

　　Packaging Process:

　　A miniature metal housing (aluminum alloy or stainless steel, 0.3~0.5mm thick) is used, sealed with laser welding (weld width ≤0.1mm) to ensure airtightness (leakage rate ≤1×10⁻⁸Pa・m³/s), while reducing the package size (package ratio reduced from the traditional 60% to below 30%). For high-frequency applications, the inner side of the housing needs to be gold-plated (0.5~1μm thick) to reduce shielding loss (≤0.2dB).

　　II. Core Processes for Miniaturized Manufacturing (Precision Control at ±5μm Level)

　　1. Precision Preparation Process of Core Materials

　　Ferrite Chip Processing:

　　The process employs "precision sintering + diamond cutting"—ferrite powder is molded (pressure 20~30MPa), then vacuum sintered at 1200~1300℃ (holding for 2~4h), controlling the grain size to 1~3μm (ensuring uniform magnetic properties); after sintering, diamond wire cutting (cutting accuracy ±2μm) is used to process the chip to the target size (e.g., diameter 4mm × thickness 1mm), followed by surface polishing (roughness Ra≤0.1μm) to reduce transmission loss.

　　Miniaturization of Permanent Magnets:

　　Utilizing a "laser micromachining + precision grinding" process—SmCo permanent magnet blanks are rough-machined and then cut to miniature dimensions (e.g., 2mm × 1.5mm × 1mm) using an ultraviolet laser (wavelength 355nm), with a cutting accuracy of ±3μm. After cutting, they are precision ground with a diamond wheel (grinding pressure 0.5~1N) to ensure a smooth magnet surface (≤2μm) and avoid uneven magnetic field distribution.

　　Dielectric Substrate Patterning:

　　Utilizing a "photolithography + etching" process—Photoresist (1~2μm thickness) is coated onto an Al₂O₃ or PTFE substrate. After ultraviolet exposure (resolution ≤0.1mm) and development, microstrip transmission line patterns are formed using ion etching (etching depth ±0.05μm), ensuring a transmission line width tolerance ≤±5μm and avoiding impedance deviation (≤±1Ω).

　　2. Thin Film Electrode Fabrication Process

　　To reduce electrode volume, a "magnetron sputtering + electroplating thickening" process is employed:

　　The substrate surface is cleaned by plasma cleaning (Ar gas, power 100~200W) to remove oil and oxide layers;

　　A base metal layer (titanium, thickness 50~100nm) is deposited by magnetron sputtering to enhance the adhesion of subsequent metal layers;

　　A conductive layer (gold or silver, thickness 0.5~1μm) is deposited by sputtering at a rate controlled at 5~10nm/min to ensure film uniformity (thickness deviation ≤±5%);

　　The port contact area is thickened by electroplating (gold layer thickness up to 2~3μm) to improve wear resistance (insertion/removal life ≥500 cycles) and conductivity.

　　3. Precision Assembly and Magnetic Circuit Calibration Process

　　The assembly precision requirements for miniaturized devices are extremely high (positioning error ≤ ±10μm). The core process is as follows:

　　Positioning Assembly:

　　A vision-guided high-precision chip mounter (positioning accuracy ±3μm) is used to attach the ferrite chip, permanent magnet, and matching capacitor to the substrate according to the designed positions. The mounting pressure is controlled at 0.1~0.3N (to avoid damaging the chip). The alignment deviation between the permanent magnet and the ferrite must be ≤ ±5μm to ensure that the magnetic field acts perpendicularly on the ferrite (magnetic field direction deviation ≤ ±2°).

　　Magnetic Circuit Calibration:

　　After assembly, a miniature gaussmeter (measurement accuracy ±1Oe) is used to detect the magnetic field strength in the ferrite area. If the deviation exceeds ±5%, the magnetic field is compensated by fine-tuning the position of the permanent magnet (using a piezoelectric ceramic micro-displacement stage, displacement accuracy ±0.1μm) or by attaching a miniature magnetic sheet (thickness 0.05~0.1mm) until the magnetic field strength meets the standard.

　　Laser Welding Sealing:

　　Pulsed laser welding (wavelength 1064nm, power 5~10W) is used to seal the metal casing. The welding speed is controlled at 1~2mm/s, and the weld width is ≤0.1mm to avoid damage to internal components (such as ferrite magnetic performance attenuation) caused by high welding temperatures (local temperature ≤300℃).

　　4. Full-Process Performance Testing and Reliability Verification

　　Step-by-Step Performance Testing:

　　① Semi-finished Product Testing: Ferrite permeability (εr deviation ≤±2%) and electrode contact resistance (≤5mΩ) are tested; ② Finished Product RF Performance Testing: IL, ISO, and RL are tested using a vector network analyzer (VNA, frequency range 1~40GHz) to ensure performance meets standards across the entire frequency band; ③ Power Capacity Testing: Continuous operation for 24 hours at rated power (e.g., 5W) is performed, and performance drift (IL change ≤0.2dB) is tested.

　　Reliability Verification:

　　Environmental reliability testing: ① High and low temperature cycling (-40℃~+85℃, 50 cycles); ② Damp heat testing (40℃, 95% RH, 1000h); ③ Vibration testing (10~2000Hz, acceleration 10g, 3-axis). After testing, the following must be met: no visible damage, RF performance change ≤10%, ensuring that the reliability after miniaturization is not lower than that of traditional devices (MTBF≥1×10⁵h).

　　III. Summary of Technical Challenges and Solutions

　　1. Material Performance vs. Size Conflict

　　The core challenge lies in the fact that shrinking the size of ferrite easily leads to a decrease in permeability, while reducing the volume of permanent magnets results in insufficient magnetic field strength, making it difficult to maintain the normal operating point of the device. The solution prioritizes NiZn/LiZn ferrites with high saturation magnetization (high 4πMs), paired with SmCo or miniature NdFeB rare-earth permanent magnets. A permalloy miniature magnetic shell is integrated outside the permanent magnets to concentrate the magnetic field and reduce leakage. Simultaneously, electromagnetic simulation tools are used to optimize material size ratios, ensuring magnetic performance meets requirements while reducing volume.

　　2. High-Frequency Parasitic Parameter Interference

　　Size reduction significantly increases parasitic inductance of port leads and parasitic capacitance of the substrate and shell, especially in the >20GHz millimeter-wave band, where parasitic parameters have a more pronounced impact on RF performance. This can be addressed by replacing traditional coaxial structures with coplanar waveguide (CPW) structures to reduce parasitic parameters introduced by grounding vias; optimizing the port matching network by adding 0.5~2pF miniature matching capacitors to compensate for impedance deviations; shortening port lead length to ≤1mm; and employing a suspended shell design (0.1~0.2mm spacing from the substrate) to further reduce the impact of parasitic parameters.

　　3. Reduced Heat Dissipation Capacity

　　Miniaturization reduces the heat dissipation area of devices by more than 50% compared to traditional products. In high-power (>10W) scenarios, heat buildup can easily lead to performance degradation or even device damage. High thermal conductivity materials must be selected to improve heat dissipation efficiency, such as CuW alloy casings (thermal conductivity ≥180W/(m・K)) and Al₂O₃ ceramic substrates (thermal conductivity ≥20W/(m・K)). Micro-heat dissipation bumps with a diameter of 0.2~0.3mm and a spacing of 0.5mm should be designed on the inner side of the casing to increase the heat dissipation area. Power loss distribution should be optimized using thermal simulation tools to avoid localized hotspot accumulation, ensuring that the casing temperature is ≤85℃ (ambient temperature 25℃) at 10W input power.

　　4. Assembly Precision Control

　　With component dimensions entering the micrometer level, positioning errors of key components such as ferrite chips and permanent magnets can easily exceed ±10μm, leading to magnetic field direction deviation, poor transmission line coupling, and affecting device performance. A vision-guided high-precision chip mounter (positioning accuracy ±3μm) is required for component placement. The mounting pressure must be controlled between 0.1 and 0.3N to avoid damaging the chip, ensuring that the alignment deviation between the permanent magnet and ferrite is ≤±5% and the magnetic field direction deviation is ≤±2°. After assembly, a miniature gaussmeter (measurement accuracy ±1Oe) is used to detect the magnetic field strength. The position of the permanent magnet is fine-tuned using a piezoelectric ceramic micro-displacement stage (displacement accuracy ±0.1μm), or 0.05~0.1mm micro-magnetic sheets are attached to compensate for the magnetic field, ensuring magnetic circuit accuracy.

　　The design and manufacturing of miniaturized RF circulators and isolators require a core focus on the collaborative optimization of "materials-structure-process-simulation." While reducing size, performance, power, and reliability must be balanced through precise control and multiphysics simulation.To meet the application requirements of miniaturized radio frequency systems (such as 5G micro base stations, portable radar, and wearable test equipment).

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