Introduction
Printed circuit boards (PCBs) must dissipate heat generated by components to maintain safe operating temperatures. Excessive heat buildup can degrade performance, reduce reliability, and damage sensitive components. With increasing power densities and device miniaturization, effective thermal management is critical for PCBs used in modern electronics.
This article provides a comprehensive overview of the sources of heat generation in PCBs and techniques to dissipate heat through design approaches encompassing substrate materials selection, trace sizes, component placement, heat spreading layers, thermal vias, and interface materials. Supplementary cooling methods like heat sinks, fans, openings, liquid cooling, and phase change materials are also covered. By understanding the thermal characteristics and optimizing the thermal design, PCBs can dissipate heat efficiently to meet performance and reliability needs.
Heat Generation Sources in PCBs
The primary heat sources in typical PCBs are:
High Power Components
Components like power regulators, amplifiers, CPU/GPUs, and drivers dissipate significant heat depending on current flows and voltage drops across them. Heat is generated both in the silicon die and at contact junctions.
Traces and Layers
Traces carrying substantial currents generate I<sup>2</sup>R heating due to inherent resistivity of the conductors. Inner layers with large high-current planes are prone to heating.
Vias
Plated-through hole vias between layers produce heating caused by conduction through the via barrel resistance in high current paths.
Inductors
Inductor winding resistance and core losses convert electrical energy into heat. Iron powder and ferrite materials have higher core losses.
Thermal Interfaces
Contact resistance across structural and thermal interfaces like ball grid arrays, leads, thermal pads etc. restrict heat flow and create heat spots.
Heat Transfer Mechanisms
PCBs use conduction, convection and radiation to dissipate heat:
Conduction
Conduction transfers heat between solids through direct molecular contact. Copper with high thermal conductivity efficiently conducts heat laterally through PCB traces and layers.
Convection
Convection utilizes airflow over board surfaces to remove heat through fluid exchange. Extended surfaces like fins help enhance convective heat transfer.
Radiation
Radiation emits electromagnetic waves from hot object surfaces. The heat dissipation rate is proportional to the temperature differential between the PCB surface and surroundings. Practically low contribution for most electronics.
PCB Design Considerations for Heat Dissipation
Key aspects of PCB design optimization for thermal management include:
Component Placement
Position high power components with maximum clearance from heat sensitive devices for thermal isolation. Cluster components with high dissipations together for concentrated cooling.
Copper Weights
Increasing copper thickness and widths raises thermal conduction allowing better dissipation through traces and planes. But electrical and mechanical considerations also apply.
Traces
Use short and wide traces for high current paths to reduce I<sup>2</sup>R heating. Avoid neck-downs which raise resistance. 45° bends minimize heating versus 90° bends.
Vias
Minimize via lengths and quantity through smart routing. Ensure via barrels and lands have sufficient cross-sectional area to handle peak currents without overheating.
Flood and Relief Patterns
Addition of thermal relief cutouts, voids and planes help conduct heat away from hot regions and spread it over larger board areas for reduced heat flux density.
Layer Stacking
Strategic placement of power and ground layers internally conducts heat laterally to reduce surface hotspots. Multiple thin layers are better than one thick layer for heat spreading.
Thermal Pads
Exposed thermal landing pads and adjacent ground pads allow supplemental heat removal paths from packages into the PCB structure.
Substrate Materials Selection
The dielectric substrate material properties significantly influence thermal performance:
Thermal Conductivity
Higher thermal conductivity substrates like metal core PCBs and ceramic-filled laminates conduct heat better internally. Standard FR-4’s poor thermal conductivity restricts heat spreading.
Specific Heat Capacity
Higher specific heat capacity reduces temperature rise for a given heat input, allowing safe operation at higher power levels. Ceramics offer high specific heat capacity.
Coefficient of Thermal Expansion (CTE)
Matching substrate CTE to copper and components reduces thermomechanical stresses from CTE mismatches during thermal cycling. This improves reliability.
Glass Transition Temperature
PCB substrate materials should have high glass transition temperatures well above the application environment temperature to avoid softening and mechanical failures.
Common Substrate Materials Comparison
| Material | Thermal Conductivity | Specific Heat | CTE | Max Temp |
|---|---|---|---|---|
| FR-4 | 0.3 W/mK | 1200 J/kgK | 14-20 ppm/K | 130°C |
| IMS | 1-4 W/mK | 800 J/kgK | 17-25 ppm/K | 150°C |
| Polyimide | 0.12 W/mK | 1090 J/kgK | 20-70 ppm/K | 240°C |
| Aluminum | 237 W/mK | 900 J/kgK | 23 ppm/K | 660°C |
| Alumina | 24 W/mK | 765 J/kgK | 6.5 ppm/K | >1000°C |
| BeO | 270 W/mK | 1925 J/kgK | 7.5 ppm/K | >1000°C |
Thermal Via Design
Vias provide low thermal resistance conduction pathways between layers in multi-layer boards:
Quantity
Higher via quantity reduces conduction path lengths in the in-plane direction to minimize lateral spreading before heat reaches a via. But more vias increase cost.
Layout Pattern
Gridded arrays offer better heat spreading from hot spots versus peripheral distribution. Mixed patterns are also possible.
Size and Density
Wider via diameters and higher via densities improve vertical heat conduction at the expense of routing space. Laser micro-vias help increase densities.
Depth
Full depth vias spanning the complete board thickness conducts heat more efficiently to inner layers compared to partial depth vias.
Barrel Plating Thickness
Thicker plating on via barrels reduces conduction loss through the via. It also allows higher current rating.
Anti-pad Size
Larger anti-pads or clearance areas around vias provide better heat spreading into plane layers. But it also reduces routing space on signal layers.
Thermal Relief Pads
Exposed thermal pads around plated through-hole vias avoid trapping heat under soldermask. The improved heat transfer lowers via temperatures.
Filling
Filled vias using conductive inserts help transfer heat to inner layers faster than hollow vias relying only on barrel plating.
Heat Spreading Layers
Thin sheets with high in-plane thermal conductivity are inserted to spread heat between hot components and heat sinks:
Materials
Copper and aluminum provide excellent lateral heat spreading capability though aluminum oxide coating is needed to insulate aluminum from PCB traces.
Thickness
Thinner spreads minimize thermal impedance but are more challenging to fabricate. 0.1 mm to 0.3mm sheets are commonly used.
Placement
Adjacent to hot components on same side or at board center plane allows equal heat spreading in both directions.
Patterning
Solid sheets work best, but relief features and cutouts can tailor flexibility and reduce weight if needed. Clearances are required around vias and component pads.
Bonding
Good bonding to dielectric layers using thermally conductive adhesives ensures low contact resistance for efficient heat transfer.
Thermal Interface Materials (TIMs)
TIMs enhance heat transfer from components to heat spreaders and heat sinks:
Thermal Pads
Dispensable adhesive-backed pads provide compliant thermally conductive mounting for uneven surfaces. Phase change compounds improve performance.
Thermal Greases/Gels
Silicone-based pastes or gels fill microscopic surface roughness and air gaps improving thermal coupling between surfaces. They need containment around the placement area.
Phase Change Materials
Wax-like compounds melt during heating to conformally coat surfaces, then re-solidify as temperatures reduce to continue transferring heat without pump-out issues.
Elastomeric Gaskets
Silicone rubber heat sink gaskets avoid messy applications needing containment, while providing electrical isolation if required. Thermal conductivity is lower than pastes.
Thermal Tapes
Tapes offer clean handling and defined thicknesses to control bond line thickness between parts. Foil or fiber fillers enhance conductivity.
External Cooling Techniques
Heat Sinks
Heat sink fins attached to high power components enhance surface area for convection and airflow cooling. Base and fin materials, shape, surface coatings and interface materials influence performance.
Fans
Fans force or induce airflow over PCB surfaces transporting heat away through convection. Blower or centrifugal fans are commonly used to cool board mounted heat sinks.
Openings
Cutouts, windows and ventilation slots in enclosure walls and PCB allow hot exhaust air to escape improving convection cooling.
Liquid Cooling
Liquid cooling channels carrying water or refrigerants remove heat through forced convection and sensible/latent heat transfer. Cold plates, immersion cooling and heat pipes provide high performance cooling.
Phase Change Materials
Encapsulated paraffin waxes absorbing heat during melting transition help buffer transient heat loads. The phase change limits temperature rise.
Thermal Design Process
A structured thermal design process ensures PCB cooling needs are met reliably:
- Power Analysis: Determine power dissipation distribution across PCB from components in application
- Heat Flow Analysis: Model heat transfer paths through board, interfaces and external sinks
- Thermal Simulation: Verify temperatures at components are within safe limits through simulation
- Guideline Application: Implement design guidelines like trace widths, stacking, placement etc.
- Prototyping: Fabricate and measure temperature on prototype under operational loads
- Design Refinement: Improve cooling through heat sink additions, airflow modifications, substrate changes etc. based on validation tests
- Qualification Testing: Confirm board operates safely across environmental and lifetime testing
Conclusion
Eliminating excessive heat generation and facilitating efficient heat dissipation are crucial to the reliability of modern high-density PCBs. Following fundamental heat transfer mechanisms coupled with thermal design guidelines and practical cooling methods described in this article will enable electronics engineers to develop boards that meet both thermal and electrical performance needs for their applications. Advancements in substrate materials, TIMs, fabrication techniques and cooling products continue to expand the possibilities for thermal management in next-generation electronics.
Frequently Asked Questions (FQA)
Q1: What are some common overheating problems caused by poor PCB thermal design?
A1: Excessive heat can lead to improper operation, reduced lifetime, or permanent failure of components like ICs, capacitors, magnetics etc. Heat spots also create thermomechanical stresses and substrate delamination or warping. High temperatures may violate safety standards as well.
Q2: How can I estimate the heat dissipated by traces and planes on a PCB?
A2: Use the I<sup>2</sup>R resistive power loss formula, where I is the root-mean-square (RMS) current through the trace/plane and R is the end-end resistance. Wider, shorter traces have lower resistances and hence dissipate lower heat for a given current.
Q3: What are some typical thermal interface materials used in electronics cooling applications?
A3: Thermal greases, gels, phase change materials, elastomeric pads, thermally conductive tapes and epoxies are commonly used to reduce contact thermal resistance between components, heat spreaders and heat sinks or enclosures.
Q4: How does liquid cooling help lower PCB temperatures compared to air cooling?
A4: The higher heat capacity and heat transfer coefficients of liquids allow significantly higher heat dissipation rates compared to air cooling. Cold plates and immersion cooling are the typical approaches used for direct liquid cooling of PCBs.
Q5: What tools are available for estimating PCB temperatures during design stage?
A5: Thermal modeling and simulation tools like finite element analysis, computational fluid dynamics, and thermal equivalent network modeling help predict board temperatures by accounting for design parameters. Measurements on instrumented prototypes validate simulations.
How to Improve Heat Dissipation with PCB Design
For electronic devices, a certain amount of heat is generated during operation, which causes the internal temperature of the device to rise rapidly.
If the heat is not dissipated in time, the device will continue to heat up, the device will fail due to overheating, and the reliability of the electronic device will decrease.
Therefore, it is very important to perform a good heat dissipation process on the board.
1. Add heat-dissipating copper foil and copper foil with large-area power supply.
As can be seen from the above figure, the larger the area connecting the copper, the lower the junction temperature.
According to the above figure, it can be seen that the larger the copper area, the lower the junction temperature.
2. Thermal Vias
Thermal vias can effectively reduce the junction temperature of the device and increase the uniformity of the temperature in the thickness direction of the board, which makes it possible to adopt other heat dissipation methods on the back side of the PCB.
Through simulation, it is found that compared with the non-thermal via, the thermal via of the device with a thermal power consumption of 2.5W, a pitch of 1mm, and a central design of 6×6 can reduce the junction temperature by about 4.8°C, and the temperature difference between the top and bottom of the PCB. Reduced from the original 21 ° C to 5 ° C. After changing the thermal via array to 4×4, the junction temperature of the device is 2.2°C higher than that of 6×6, which is worthy of attention.
3. Theexposed copper on the back of the IC reduces the thermal resistance between the copper and air.
4. PCB layout
High power, thermal device requirements.
a. The heat sensitive device is placed in the cold air area.
b. The temperature sensing device is placed in the hottest position.
c. The devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) should be placed. The uppermost flow (at the inlet) of the cooling airflow, the device that generates a large amount of heat or heat (such as a power transistor, a large-scale integrated circuit, etc.) is placed at the most downstream of the cooling airflow.
d. In the horizontal direction, the high-power devices are placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board, so as to reduce the temperature of other devices while the devices are operating. Impact.
e. The heat dissipation of the printed circuit board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area. The same problem should be noted in the configuration of multiple printed circuit boards in the whole machine.
f. Temperature sensitive devices should be placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. Multiple devices are preferably staggered on a horizontal plane.
g. Place the device with the highest power consumption and maximum heat generation near the best position for heat dissipation. Do not place a device with a higher heat on the corners and peripheral edges of the printed board unless a heat sink is placed near it. When designing the power resistor, choose a larger device as much as possible, and have enough space for heat dissipation when adjusting the layout of the printed board.
h. electronic component spacing recommendations:
