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Product Description
High-frequency signals are easily affected by the dielectric properties and losses of the substrate. Material selection is the core of high-frequency PCBs.
| Material Type | Key Parameters | Recommended Material / Specification | Applicable Scene |
|---|---|---|---|
| Base material |
Dielectric constant (Dk) Dielectric loss tangent of angle (Df) Coefficient of thermal expansion (CTE) |
High-frequency dedicated substrate — PTFE (polytetrafluoroethylene, Dk=2.1–2.6, Df≤0.0009) — Rogers series (e.g. RO4350B, Dk=3.48, Df=0.0037) — Modified epoxy resin (FR-4 high-frequency version, Dk=3.8–4.2, Df≤0.01) |
PTFE/Rogers: Millimeter wave, radar, 5G base station High-frequency FR-4: Consumer electronics (mobile phones, routers) |
| Copper foil | Surface roughness | Low profile copper foil (LP) / very low profile copper foil (VLP), roughness ≤1.0μm | Reduce the transmission loss of signals on the surface of copper foil (skin effect) |
| Solder mask layer | Dielectric loss | High-frequency special solder mask oil (Df≤0.01) | Avoid increasing signal attenuation by the solder mask layer |
- The core of high-frequency PCBs needs to control the characteristic impedance (such as 50Ω single-ended impedance, 100Ω differential impedance), with deviation ≤±5% (±10% for ordinary PCBs).
- Impedance calculation: Combine base material Dk, plate thickness, line width/line spacing, and copper foil thickness. Simulate through professional software (e.g. Polar SI9000).
- Example: RO4350B substrate (thickness 0.254mm), with a single-ended impedance of 50Ω corresponding to a line width of approximately 0.8mm.
- Impedance compensation: Considering the influence of etching side etching and solder mask coverage, a compensation amount of ±0.02mm should be reserved for the line width.
- Traces should be as short and straight as possible, avoiding right angles or acute angles (prone to reflection at high frequencies); fillet radius should be no less than 0.5mm.
- Differential traces should be of equal length and spacing (length difference ≤5mil), with ground wrapping treatment (ground wire to differential wire distance ≤2× line width) to reduce EMI interference.
- Skin effect response: High-frequency current is concentrated on the surface of the copper foil. Silver-plated copper foil can be used (to reduce surface resistance), or traces can be widened (to reduce current density).
- Complete ground layer: High-frequency PCBs need a complete GND Plane to avoid ground layer segmentation and reduce signal return path length. Distance between ground layer and signal layer should be no more than 0.2mm.
- Shielding measures: For sensitive high-frequency areas (e.g. RF chips), design metal shielding covers with reliable grounding (multiple grounding points, spacing ≤λ/20, where λ is signal wavelength).
- Heat dissipation: Design heat dissipation pads and metallized via arrays (via diameter ≥0.3mm, spacing ≤1mm) for high-frequency devices such as power amplifiers.
- Device packaging: Prioritize miniaturized, low-parasitic parameter packaging (e.g. QFN, LGA) to reduce signal reflection introduced by the packaging.
- Line width/line spacing tolerance: ≤±0.01mm (±0.02mm for ordinary PCBs), using high-precision laser direct imaging (LDI) technology to replace traditional film exposure.
- Lamination accuracy: Interlayer alignment deviation ≤0.02mm to avoid impedance offset; bonding temperature and pressure must match base material characteristics (e.g. PTFE requires high-temperature and high-pressure bonding).
- Etching process: Micro-etching technology is adopted to reduce surface roughness of copper foil and avoid increasing transmission loss. Lateral erosion ≤0.01mm.
- Surface treatment: Electroless nickel gold plating (ENIG) or silver plating is preferred over OSP (organic film increases contact resistance). Gold layer thickness should be ≤0.1μm to avoid increased loss.
A high-frequency PCB is generally defined as one designed to handle signals above 500MHz, or digital signals with rise times less than 1 nanosecond. These boards require special substrate materials and precision manufacturing to maintain signal integrity at such speeds.
Standard FR-4 has a relatively high dielectric constant (Dk≈4.5) and dielectric loss (Df≈0.02), which causes significant signal attenuation at high frequencies. For millimeter-wave, radar, or 5G applications, low-loss materials such as PTFE or Rogers series substrates are required to minimize signal degradation.
Impedance control ensures that transmission lines on the PCB maintain a consistent characteristic impedance (e.g. 50Ω single-ended or 100Ω differential). At high frequencies, any impedance mismatch causes signal reflection and power loss, degrading circuit performance. High-frequency PCBs require impedance tolerance of ≤±5%.
PTFE (e.g. pure PTFE laminates) offers the lowest dielectric loss (Df≤0.0009) and is ideal for extreme high-frequency applications like radar and satellite communication. Rogers materials (e.g. RO4350B) offer a balance of performance and processability, with slightly higher Dk/Df values but easier handling during PCB fabrication, making them popular for 5G base stations and automotive radar.
At high frequencies, the skin effect causes current to flow mainly on the surface of conductors. A rough copper surface or thick gold plating (>0.1μm) increases surface resistance and transmission loss. ENIG (Electroless Nickel Immersion Gold) or silver plating with controlled gold thickness is recommended over OSP for high-frequency boards.
High-frequency PCBs are widely used across multiple industries including 5G/6G telecommunications infrastructure, automotive radar and ADAS systems, aerospace and defense (radar, electronic warfare, satellite terminals), high-speed data centers (400G/800G switches), and precision test & measurement equipment such as network analyzers and high-frequency oscilloscopes.
