Detailed Explanation of Key PCB Parameters
1. Electrical Performance Parameters
The electrical properties of a PCB directly affect signal integrity, especially in high-frequency and high-speed circuits.
- Dielektrisk konstant (Dk) – Measures a material’s ability to store electrical energy. Lower Dk values (e.g., PTFE with Dk≈2.2) enable faster signal transmission, making them ideal for 5G and millimeter-wave applications.
- Dissipation Factor (Df/Loss Tangent) – Indicates signal energy loss. High-frequency applications (e.g., radar, satellite communication) require Df < 0.005.
- Surface/Volume Resistivity – High insulation resistance (>10¹² Ω·cm) prevents leakage currents, crucial for high-voltage PCBs (e.g., power modules).
- Breakdown Voltage – Standard FR4 withstands ≥20 kV/mm, while ceramic substrates can handle up to 50 kV/mm.
- Impedans-kontrol – High-speed PCBs (e.g., DDR5, PCIe 6.0) require tight impedance tolerance (±5%) to minimize signal reflections.
2. Thermal Performance Parameters
A PCB’s heat resistance determines its reliability in high-temperature environments, especially for lead-free soldering and long-term stability.
- Glass Transition Temperature (Tg) – Standard FR4 has Tg≈130°C, while high-Tg PCBs (Tg≥170°C) are used in automotive and military electronics.
- Thermal Decomposition Temperature (Td) – Materials with Td > 325°C (e.g., Isola 370HR) are preferred for lead-free soldering.
- Termisk ledningsevne – FR4 has low thermal conductivity (~0.3 W/m·K), whereas metal-core PCBs (e.g., aluminum) can reach 10 W/m·K, making them ideal for LED cooling.
- Coefficient of Thermal Expansion (CTE) – Z-axis CTE should be <50 ppm/°C to prevent delamination in multilayer PCBs (IC substrates require CTE≈6 ppm/°C).
3. Mechanical Performance Parameters
Mechanical strength impacts assembly processes and long-term durability.
- Bøjningsstyrke – Standard FR4 ranges from 400-600 MPa, while flexible PCBs (polyimide) require >200 MPa.
- Peel Strength – Copper adhesion must exceed 1.0 N/mm (IPC standard) to prevent foil detachment during soldering.
- Water Absorption – Low moisture absorption (<0.2%) prevents blistering; high-frequency laminates typically maintain <0.1%.
4. Structural Characteristics
Manufacturing precision is critical for high-density interconnect (HDI) and miniaturized designs.
- Copper Thickness Tolerance – Standard 1 oz copper has ±10% tolerance, while precision circuits require ±5%.
- Layer-to-Layer Registration Accuracy – HDI PCBs demand <25 μm alignment, whereas standard multilayer boards allow <50 μm.
- Minimum Trace/Space (L/S) – Standard PCBs use 0.1 mm/0.1 mm, while advanced IC substrates achieve 20 μm/20 μm.
5. Reliability Testing Metrics
PCBs must pass rigorous tests to ensure long-term stability.
- Conductive Anodic Filament (CAF) Resistance – Evaluates short-circuit risks under humid conditions (85°C/85% RH for 1000 hours).
- Surface Insulation Resistance (SIR) – Must exceed 10⁸ Ω (per JIS standards).
- Test af termisk cykling – Survives 100 cycles (-55°C to 125°C) without cracking (automotive PCBs require stricter tests).
6. Environmental & Process Compliance
Environmental regulations (e.g., RoHS, REACH) drive PCB material advancements.
- Comparative Tracking Index (CTI) – Medical devices need Class 3 (400-600V), while industrial controls require Class 2.
- Halogen-Free – Chlorine/bromine content must be <900 ppm to reduce toxic emissions.
- Flame Retardancy (UL94) – V-0 is the highest rating, mandatory for aerospace applications.
PCB Board Classification and Selection
1. Common PCB Materials
- FR4 – Standard epoxy-glass laminate for consumer electronics.
- CEM-3 – Composite substrate, cost-effective for simple double-sided PCBs.
- High-Tg PCBs (Tg≥170°C) – Heat-resistant for automotive and military use.
- High-Frequency Laminates (e.g., Rogers RO4003C) – Low Dk/Df for 5G/radar applications.
- Metal-Core PCBs (Aluminum/Copper) – Excellent thermal management for LEDs and power modules.
2. PCB Material Grades Comparison
| Material Grade | Karakteristika | Typiske anvendelser |
|---|
| 94HB | Paper-based, non-flame-retardant | Low-cost consumer electronics |
| 94V0 | Flame-retardant paper substrate | Home appliance control boards |
| CEM-1 | Single-sided fiberglass | Simple circuits |
| CEM-3 | Double-sided semi-glass | Low-cost double-layer PCBs |
| FR4 | Standard fiberglass | Consumer electronics, industrial controls |
| High-Tg FR4 | High-temperature resistant | Automotive, aerospace |
3. How to Choose the Right PCB Material?
- Højfrekvente anvendelser → Low-Dk/Df materials (Rogers, Taconic).
- High-Temperature Environments → High-Tg (≥170°C) or high-Td (>325°C) materials.
- High-Power Heat Dissipation → Metal-core or high-thermal-conductivity FR4.
- Eco-Friendly Requirements → Halogen-free, RoHS-compliant materials.
The selection of PCB parameters directly affects product performance, reliability, and cost. Engineers need to select appropriate board materials (such as FR4, CEM-3, high Tg PCB, etc.) based on the application scenario (such as high frequency, high temperature, high power), and optimize the design to improve signal integrity, heat dissipation capabilities, and mechanical strength.