A.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.
Dielectric Constant (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.
Impedance Control – 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.
Thermal Conductivity – 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.
Flexural Strength – 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).
Thermal Cycling Test – 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.
B.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