1. Enlarging Copper Area & Large‑Area Copper Pour
Core Principle: Copper has a thermal conductivity of approximately 401 W/(m·K), thousands of times higher than FR‑4 substrate (~0.3 W/(m·K)). Expanding copper foil area around heat‑generating components creates a larger cross‑section for heat spreading and effectively reduces junction‑to‑ambient thermal resistance.
Engineering Key Points:
Power and ground pins of heat‑intensive ICs shall connect directly to large copper planes; avoid narrow traces that restrict heat flow.
Pour copper on both top and bottom layers and thermally couple them via vias to achieve equivalent copper thickness doubling.
Larger copper pour does not always deliver proportional benefits. Thermal gain diminishes beyond 3‑5 times the chip size. Balance thermal performance against EMI and signal integrity.
For BGA / QFN devices with bottom thermal pads, maintain continuous copper without segmentation directly underneath the chip.
2. Thermal Vias Array
Core Principle: When a component's thermal pad is on the top layer while major heat dissipation occurs on bottom or inner layers, vertical thermal resistance is dominated by FR‑4 dielectric. Thermal vias form copper channels across PCB layers, transferring heat rapidly from top‑side pads to bottom‑layer copper or heat sinks. They represent one of the most effective ways to lower vertical thermal resistance.
Engineering Key Points:
Quantity matters more than single‑via diameter. Multiple small vias outperform one large via. Typical implementation: 0.3‑0.5 mm diameter vias arranged in arrays with 1.0‑1.5 mm pitch underneath thermal pads.
Prevent solder wicking: vias landing directly on pads may draw away solder. For mass production, adopt VIPPO (Via‑in‑Pad Plated‑Over) with via filling and copper capping.
Pair thermal vias with solid inner‑layer ground or power planes to build a 3‑D heat dissipation structure combining vias and planes.
For devices above 5 W power consumption, use at least 9 thermal vias and validate results through thermal simulation.
3. Thermal Pad & Exposed Pad (EPAD)
Core Principle: Packages such as QFN, DFN and SOIC‑EP feature an exposed metallic die pad at the bottom directly bonded to the chip die. A matching copper pad designed on PCB allows heat to escape from the chip backside rather than only through component leads.
Engineering Key Points:
PCB EPAD dimension shall follow datasheet recommendations, usually slightly larger (0.1‑0.25 mm margin on each side) than the package exposed pad.
Route EPAD to bottom or inner‑layer large copper planes through thermal via arrays; top‑layer copper alone provides limited cooling capability.
Design stencil with multiple small openings (instead of one solid large aperture) over EPAD to control solder paste volume and prevent component floating or bridging.
Well‑implemented EPAD can achieve thermal resistance as low as 1‑2 °C/W, delivering high cost‑performance for thermal design.
4. Metal‑Core PCB (MCPCB / Aluminum‑Base PCB)
Core Principle: Standard FR‑4 substrates have poor thermal conductivity. MCPCB uses aluminum or copper as the core with a thin thermally conductive dielectric layer (50‑100 μm). Its overall thermal conductivity reaches 1‑8 W/(m·K), many times higher than FR‑4. Heat transfers from component pads through dielectric layer to metal core then to enclosure.
Engineering Key Points:
Typical applications: high‑power LED lighting, automotive electronics, power modules. LED junction temperature control heavily relies on aluminum‑base PCBs.
Dielectric layer is the main thermal bottleneck. Prioritize dielectric thermal conductivity rather than only focusing on base metal material during component selection.
Higher cost than FR‑4. Complex via processing is required due to conductive metal core; multi‑layer MCPCB is technically challenging.
Alternative: partial copper coin / copper inlay embedded inside FR‑4 boards for balanced cost and thermal performance.
5. External Heat Sinks & Thermal Interface Materials (TIM)
Core Principle: When PCB‑intrinsic heat dissipation reaches limits, heat must be conducted to larger external heat sinks. Heat‑generating components or back‑side PCB copper make contact with heat sinks via TIMs, filling microscopic gaps and displacing air (air thermal conductivity ~0.026 W/(m·K), a poor heat conductor).
| Type | Thermal Conductivity | Thickness | Application Scenarios |
|---|---|---|---|
| Thermal Grease | 2‑8 W/(m·K) | Very thin (<0.1 mm) | IGBT, CPU; applications allowing reassembly with controlled pressure |
| Thermal Gap Pad | 1‑6 W/(m·K) | 0.5‑5 mm | Height difference compensation, electrical insulation, shock absorption |
| Two‑Component Thermal Adhesive | 1‑3 W/(m·K) | Application‑dependent | Scenarios requiring both thermal contact and mechanical bonding |
| Phase‑Change Material | 3‑6 W/(m·K) | 0.1‑0.25 mm | Melts under high temperature for gap filling, good long‑term reliability |
6. Component Layout Optimization & Air Duct Design
Core Principle: The previous five methods focus on heat conduction. Ultimately heat must dissipate to ambient through convection and radiation. Proper layout and airflow design can substantially reduce system thermal resistance without additional BOM cost.
Summary: Systematic Thinking for Thermal Design
PCB thermal design is not simply stacking individual tricks, but systematic optimization across the full thermal‑resistance chain:
Chip Junction → Package → Solder Pad → PCB Copper → Vias / Metal Core → Heat Sink → Ambient Environment
Each segment's thermal resistance needs quantitative evaluation. Recommended workflow:
Estimate power consumption and maximum allowable junction temperature for each heat‑generating device.
Calculate allowable total thermal resistance: θ_JA = (T_Jmax − T_Ambient) / P_max.
Allocate thermal resistance budget for package, PCB and heat sink and identify bottlenecks.
Combine above‑mentioned methods. EPAD plus thermal vias plus large bottom‑layer copper offers the most cost‑effective baseline. Add heat sinks or metal‑core boards for high‑power scenarios.
Validate by thermal simulation (FloTHERM, ANSYS Icepak etc.) and calibrate with infrared thermal imaging measurements.
