How to Do Thermal Simulation of PCB ?

Thermal management is a critical aspect of printed circuit board (PCB) design. The temperature profile of a PCB depends on the power dissipation and distribution of heat generating components and the overall board construction. Overheating can lead to performance issues, lowered reliability and even complete failure.

Performing thermal simulation enables predicting the temperature distribution across the PCB and identifying hot spots. This allows optimizing the design to maintain components within their safe operating temperature range. This article provides a comprehensive guide on how to carry out thermal modeling and simulation for PCBs.

Importance of PCB Thermal Simulation

Thermal simulation of PCBs during the design stage is important for the following reasons:

• Prevents overheating damage – Simulating temperature profile helps avoid thermal issues that can damage sensitive components.
• Optimizes cooling provisions – Heat sink sizes, location and other cooling parameters are determined through simulation.
• Improves reliability – Maintaining safe operating temperatures enhances long term reliability of the board.
• Saves cost and time – Thermal issues found late can require expensive board spins. Simulation prevents this.
• Gains insights early – Thermal behavior is understood upfront before building prototypes.
• Allows design trade-offs – Simulating different scenarios facilitates design trade-off decisions.
• Validates improvements – Verify enhancements like thicker copper, thermal vias, etc. through simulation.

Types of PCB Thermal Simulations

There are two main types of thermal simulation used for PCB analysis:

Steady-State Thermal Simulation

Steady-state simulation models the board under equilibrium conditions, with sustained power dissipation in components. It reveals the overall temperature distribution across the PCB when heat flow stabilizes after sufficient time.

Steady-state analysis is quick and provides a good understanding of the general thermal performance. It is commonly used in early design stage.

Transient Thermal Simulation

Transient simulation models the dynamic thermal response of the board to power cycling over time. The temperature profile is analyzed as heat varies when devices switch on-off during actual operation.

Transient analysis can capture peak temperatures and time-dependent effects. It provides more detailed understanding and is done at later design phases.

Steps for PCB Thermal Simulation

The typical workflow for performing thermal simulation on a PCB design involves the following steps:

1. Build the PCB Model

The first step is to build the complete PCB model with all mechanical and material details in the simulation software. This includes board shape, layer stackup, component footprints, copper weights, finishes, etc.

2. Define Material Properties

Materials like FR4, copper, solder mask, etc. have inherent thermal conductivity and heat capacity. These parameters need to be defined in the simulator library.

3. Identify Heat Sources

Determine all components that dissipate significant heat like ICs, regulators, LEDs, etc. Estimate their power consumption and define heat generation for each.

4. Apply Boundary Conditions

Specify the external thermal conditions the board will be subject to such as ambient temperature, air flow, contact surfaces, etc.

5. Generate Thermal Mesh

The software automatically meshes the PCB model into small nodes and elements to enable thermal calculations through finite element or finite difference techniques.

6. Run Thermal Simulation

Execute the thermal solver to calculate the steady-state or transient temperature profile based on the defined parameters and conditions.

7. Analyze Simulation Results

The software produces thermal maps showing the temperature distribution. Analyze hot spots, gradients, spreads and compare components against temperature limits.

8. Refine the Design

If any components exceed safe limits, modify the design with heat sinks, vias, copper weights, airflow, etc. and rerun simulation until results are satisfactory.

PCB Features Relevant for Thermal Simulation

To perform accurate thermal modeling, the PCB parameters that significantly influence heat flow need to be properly captured in the simulation software. These include:

• Layer Stackup – Thickness and dielectric material used in the PCB layer structure impacts heat conductive and capacitive properties.
• Copper Weight – Amount of copper in layers, landing patterns, plane fills determine heat spreading.
• Board Size – Physical dimensions constrain how heat can dissipate laterally across the PCB.
• Board Shape – Shape factors like cutouts, notches, openings affect airflow and heat concentration.
• Component Footprints – Size, shape and distribution of component footprints influence localized heating.
• Traces – Copper traces act as heat conduction paths to dissipate heat between pads.
• Vias/Holes – Number, size, spacing and copper plating of drilled holes impacts vertical heat transfer.
• Solder Mask – Openings in solder mask determine heat exposure and spreading from copper surfaces.
• Silks and Legends – Non-copper areas where legends are marked affect underlying heat density.
• Fiducials – Metallic fiducials can act as localized heat sinks.
• Thermal Pads – Exposed thermal pads provide conduction path from hot components.
• Heatsinks – Any attached heatsinks or devices to dissipate heat need to be modeled.
• Thermal Vias – Copper vias under pads conduct heat between layers.
• Buried Copper Planes – Internal copper planes distribute heat laterally within layers.
• Multi-board Assembly – Any enclosure or adjacent PCBs influence airflow and heat transfer.

Defining Boundary Conditions for PCB Thermal Simulation

The boundary conditions specify the thermal environment the PCB will operate in. Key boundary parameters to define are:

• Ambient Temperature – The temperature of air surrounding the PCB during operation.
• Air Flow – Speed and direction of forced air cooling over the board if any fans or vents are present.
• Adjacent Objects – If PCB is enclosed, temperature of walls and other objects should be provided.
• Mounting Surfaces – Thermal properties of surfaces on which PCB is mounted like chassis, enclosure etc.
• External Heating – Any external heat sources apart from the PCB affecting air temperature.
• Operating Conditions – Whether normal room conditions or any harsh environment like vacuum, pressure, humidity etc.
• Steady State vs Transient – For steady state, equilibrium temperature is given. For transient, temperature cycle profile over time is input.
• Thermal Interface Materials – Any TIMs used between components, PCB and external surfaces along with their conductivity properties.
• Radiation Effects – Radiative heat transfer can be additionally defined if relevant.
• Convection Mode – Specify nature of convection heat transfer as natural or forced based on air flow.

Sources of Heat Generation in a PCB

The components generating heat dissipation need to be accurately identified and modeled in the thermal simulator. Typical heat sources in a PCB include:

• ICs – Digital chips like microcontrollers, FPGAs, ASICs dissipate heat which increases with clock speed and gate density due to switching.
• Regulators – Voltage regulators including linear and switching types convert and manage electrical power resulting in heat.
• Optoelectronics – LEDs, laser diodes, photodiodes and lighting elements generate heat during operation and need heatsinking.
• Transistors – Discrete transistors used as switches or amplifiers in power circuits heat up at higher currents.
• Resistors – Resistors used for power applications like current sense, battery charging etc. exhibit thermal rise.
• Inductors – Inductors designed to handle large currents heat up due to winding resistance and hysteresis losses.
• Connectors – Connectors carrying higher currents like USB, HDMI, Ethernet etc. may dissipate heat.
• Test Points – High power testpoints provide access to measure voltages/currents and result in thermal dissipation.
• Physically Large Components – Bigger size discrete parts like capacitors or magnetics have more heat capacity.
• High Density Areas – Localized regions containing multiple heat sources in close proximity require analysis.

Estimating Power Dissipation and Temperature Rise of Components

The power dissipation of heat generating components needs to be calculated to define heat sources for simulation. The following methods can be used:

• Datasheet Values – Maximum power dissipation is often provided in component datasheet. These can be used for worst case estimates.
• Calculated from Voltage and Current – Using Ohm’s law, power can be calculated as:P = V * I

where P is power in Watts, V is voltage across part in Volts and I is current through part in Amperes.

• Inferred from Electrical Simulations – Power profiles obtained from circuit simulators like SPICE can provide dissipation data.
• Assumed as Percentage of Total Board Power – Based on experience, assume power proportion of each component from total expected board consumption.

The heat and temperature rise of components can be related using thermal resistance:

ΔT = RθJA * P

where ΔT is temperature rise in °C, RθJA is junction to ambient thermal resistance in °C/Watt, and P is power dissipation in Watts.

Thermal Simulation Output and Results Analysis

The thermal simulation software generates outputs in the form of detailed thermal maps. The results should be carefully analyzed as follows:

• Check peak temperatures reached at components and compare against their maximum rating.
• Identify any hot spots where temperature exceeds safe limits and by how much.
• Review temperature gradient between closely placed components and across board.
• Examine spreading of heat laterally across board layers and impact of any heat sinks.
• Check if temperature is lower at board edges showing heat dissipation paths are effective.
• In transient analysis, evaluate temperature fluctuation over time and with cycling.
• Verify if densely packed regions exhibit significant thermal rise and spreading.
• Compare components with same rating to see if thermal response is similar.
• Overlay electrical constraints like voltage levels, current density with thermal profile.
• Determine impact on temperature due to changes in air flow or ambient conditions.
• Correlate results with thermal metrics like thermal resistance, capacitance and time constants.

Using Thermal Simulation to Improve PCB Design

Based on the thermal simulation results, the PCB design can be optimized to enhance heat dissipation and cooling. Typical improvements are:

• Resizing Components: Increase clearance around heat generating parts or shift location to spread heating.
• Adding/Reshaping Heat Sinks: Use heat sinks more strategically to divert heat from hot components.
• Thermal Vias: Add more vias under hot parts to transfer heat vertically between layers.
• Copper Area Fill: Increase copper fill around critical components for lateral heat spreading.
• Thicker Copper: Use thicker or heavier copper weights to improve thermal conduction.
• Layer Stack Changes: Modify layer count or stackup to improve heat transfer to external layers.
• Board Shape Modification: Consider cutouts, slots and openings to facilitate airflow and cooling of internal regions.
• Component Grouping: Reposition components to avoid heating concentration in localized board regions.
• Routing Tuning: Modify trace routing to improve conduction between heated component pads.
• Solder Mask / Legend Tweaks: Adjust solder mask openings to expose more ground plane for cooling nearby parts.
• Material Changes: Consider low loss PCB materials or insulating substrates with higher thermal conductivity.
• Land Pattern Shapes: Adjust individual land shapes and sizes to steer heat away through copper shapes.

Thermal Simulation Tips and Best Practices

Here are some tips to follow for effective PCB thermal modeling:

• Use the simplest model possible that represents the heat flow physics accurately. Overly complex models take longer to solve without much added benefit.
• Leverage board symmetry and repeating patterns to reduce modeling size for faster simulation.
• Apply fine mesh only in critical high gradient regions. Use coarser mesh in larger copper areas.
• Start with steady-state simulation to quickly gauge overall thermal performance before doing transient analysis.
• Focus simulation on the hottest components identified from power estimation and electrical design.
• Validate simulation settings and results with measuring actual temperatures on prototype boards.
• Adjust ambient conditions, air flow direction and interface materials across repeated simulation runs to evaluate sensitivity.
• Simulate extreme use scenarios like heavy workloads, worst case environment, high duty cycles etc. to confirm robustness.
• Compare simulation temperature scales and distribution with infrared thermography heat maps from real measurements.
• Document all simulation assumptions, parameters and boundary conditions to enable correlation and reproduce results.

Role of PCB Thermal Simulation at Different Design Stages

Thermal modeling and simulation plays an evolving role as the PCB design progresses:

• Concept Stage: Simulation used for feasibility study of cooling approaches and high level trade-off decisions.
• Block Diagram Level: Simplified models analyze heating distribution between various functional blocks.
• Detailed Schematic Design: Ascertain temperature rise across sections based on electrical power estimations.
• PCB Layout: Analyze thermal performance of early layouts; identify hotspots and influence of board geometry.
• Pre-Tapeout: Simulation with all placement, routing and stackup details finalized to validate temperature limits.
• First Article: Correlate simulation results with measurements on initial fabricated boards.
• Product Release: Confirm thermal management of PCB across use cases and environmental conditions.
• Ongoing Improvements: continuing to utilize simulation to evaluate any design changes impacting thermal performance.

Thermal Simulation Applications and Use Cases

Some examples of practical use cases where thermal modeling provides significant value are:

• High power PCBs with power devices, regulators, converters and amplifiers.
• Densely packed digital boards with many heat generating ICs at high clock speeds.
• RF and microwave PCBs checking impact on amplifier junction temperatures.
• LED board cooling taking into account ambient light intensity driving power usage.
• Automotive PCBs subject to under hood high temperature environments.
• Handheld and portable electronics with thermal management challenges due to size constraints.
• Multi-board enclosures confirming temperature rise within sealed chassis with ventilation.
• Checking placement feasibility of high power components like FPGAs and GPUs.
• Evaluating thermal performance impact of PCB construction variations between 2-layer, 4-layer, 6-layer, etc.
• Analysis of buried copper planes and effectiveness of thermal vias in transferring heat between layers.

Thermal Simulation Tools and Software

Some of the commonly used software tools for performing PCB thermal analysis are:

• ANSYS Icepak: Powerful general purpose CFD based thermal simulation tool with automated optimization.
• Siemens Simcenter Flotherm XT: Specialized electronic design thermal simulator with extensive component libraries.
• Cadence Celsius: Tightly integrated thermal solver for rapid electro-thermal simulation and co-design.
• Altium Designer®: Built-in finite-element based thermal analyzer to simulate temperature and heat flow.
• Mentor Graphics FloEFD: CFD analysis addon embedded in Mechanical CAD for electronics cooling.
• Keysight Totem-SC: Multiphysics electro-thermal analysis with customized modeling and simulation.
• COMSOL Multiphysics: FEA simulation environment capable of modeling complex thermal characteristics.
• Autodesk CFD: General computational fluid dynamics software applied for electronics thermal management.

Conclusion

Thermal simulation enables assessing the temperature distribution across a PCB design and identifying issues early before costly prototypes are built. Steady state and transient analysis provide insights into overall heating levels, hotspots and adequacy of heat dissipation provisions. The simulations must accurately account for all thermal design parameters and operating conditions expected. Thermal modeling is an indispensable part of the modern electronics design process allowing thermal validation from concept stage through to final product release.

Frequently Asked Questions on PCB Thermal Simulation

Q1. Why is thermal simulation important for PCB design?

Thermal simulation helps predict temperature rise across PCB components. This allows identifying and resolving overheating issues before prototype manufacturing avoiding expensive re-spins.

Q2. What type of components mainly contribute to heat generation in a PCB?

Components like ICs, voltage regulators, power transistors/MOSFETS, LEDs, RF amplifiers, and magnetics like inductors and transformers are typical heat sources due to power dissipation.

Q3. How are power values for components determined for thermal simulation?

Power dissipation values can be obtained from datasheets, electrical simulations, calculated from voltage and current, or approximated as a ratio of total board power based on experience.

Q4. What impacts accuracy of PCB thermal simulation?

Accuracy is influenced by correctly capturing materials, layer stackup, board geometry, thermal vias, copper weights, airflow conditions and power sources. Validating with measured temperatures on prototypes also helps.

Q5. How can thermal simulation results be used to improve PCB design?

Based on hot spots identified, design can be optimized by resizing components, adding heat sinks/fans, increasing airflow, using more thermal vias, modifying routing, and changing board shape or stackup.