A guide for thermal design in new product development

Thermal design plays a critical role in the success of modern products. Integrating this effort into the product development process can lead to considerable time and cost savings.

Most products generate, transfer, and absorb heat, and that heat can lead to several types of challenges including direct thermal material failure, creep and stress ruptures, performance loss, deflections, and thermal safety. In this article we review the issues heat generates, the process for integrating thermal design and building detailed simulations for validating prototypes.

Challenges of Heat in Product Design

Thermal Material Failure

The most obvious issues heat generates is the direct thermal material failure (breakage) of parts which is frequently due to:

Low or high cycle fatigue: like bending a paperclip back and forth or cracking electronic solder ball grid arrays (BGA).

Figure 1: Thermo-mechanical stress cracks in BGA.
Thermal shock where sudden large temperature changes induce high stresses like ice cubes placed in a glass of warm water cracking.

Figure 2: Ice cracking due to thermal shock
Creep and stress rupture when materials are subject to stress at elevated temperatures for extended periods of time.

System failure

The second biggest set of issues heat generates are those that lead to system failures, and these typically involve:

Performance loss due to temperature, e.g., wavelength-temperature coefficient of a laser diode causing loss of useful output as temperature varies.
Stresses or deflections caused by mismatches in thermal expansion such as in optically aligned systems or mechanical systems with larger distances and varying materials like long belt-driven motion systems.
Thermal safety / comfort for humans, e.g., wearable electronics.

Knowing that thermal considerations are important to the success of a product. How do these considerations get incorporated into the product development process?

Thermal Design in Product Development

We endeavor to get our customers to their product development goal with efficiency.  The process is time-critical and requires solid data on which to base a business decision or to implement a plan.

Simulation and testing all play a key role in phases of the product development process: architecture, design, and fabrication, build and test, design verification. Product design and development is a complex cross- functional effort that requires close interdisciplinary coordination and iteration as product understanding increases through the development cycle. Simulation is key because it saves time and can guide the design through various changes quickly. Real world testing of physical parts is needed to truly know the outcome of a given product scenario and make necessary adjustments to meet requirements.

Thermal design process - Early Phases

In early development phases, before a concept is mature, we use reduced order simulation and first principles hand calculations.

Conduction: Q = k · A · (T₂ – T₁) / L

Convection: Q = h · A · (T₂-T₁)

Radiation: Q = ε · σ · A · (T₂⁴ - T₁⁴)

Absorbed Heat: Q = m · c_p · (T₂ - T₁)

Thermal Expansion: ΔL = α · L · (T₂ - T₁)

Figure 3: Basic heat transfer equations commonly used

We typically use rough concept CAD, weight criteria, heat load estimates, and ambient conditions. The output from the first principles hand calculations identifies any errors in the value proposition or feasibility.

The thermal design power (TDP) is then quantified as a function of proposed thermal architectures.  Various thermal modules can be employed in managing the heat such as heat sinks, heat pipes, vapor chambers, thermal interface materials (TIM), fans, pumps, heat exchangers, etc. Often this will trigger the comparison of competing technologies and architectures such as cost vs. performance of various heat spreading technologies.

Figure 4: Example comparison of various heat spreading technologies. Relative cost vs Thermal resistance by heat sink technology.

This phase focuses on requirements development, and the high-level design and simulation needed to provide confidence that the value proposition can be delivered with technology available in the product timeline.

Rinse and repeat this phase with the wider team of electrical, firmware, optical, manufacturing, and product engineering as each subsystem matures.

Thermal design process - Middle Phases

Once the architecture converges, develop a detailed FEA/CFD simulation to gain insights into key outputs, such as chip and contact surface temperatures, while also assessing sensitivity to various assumptions to guide technical risk and requirements.

Temperature contour plot internal components of a smartphone.

Figure 5: Temperature contour plot internal components of a smartphone. Credit: Author

At Sherpa Design, we use Siemens FloEFD to perform CFD simulation and find it a great tool to quickly build and run high quality models. Its approach to Immersed Boundary Cartesian Meshing minimizes calculation error and skips over the time-consuming body conformal mesh approach used by many CFD software packages.

comparison of mesh types Siemens FloEFD

Figure 6: Comparison of various mesh types Credit: Advanced Immersed Boundary Cartesian Meshing Technology in FloEFD^TM

The quality of the calculation and robustness to CAD geometry defects are improved with this approach. Important steps in the overall FEA/CFD model building process include:

Careful de-featuring of design CAD to eliminate unnecessary detail and error sources;
Meshing with sufficient density in critical regions and employing 2D meshes where possible.
Capturing the relevant physics of the problem.
Applying accurate boundary conditions, loads, and material/fluid properties.
Achieving numerical convergence of the model (low RMS error, steady values, imbalances less than 1%).
Verifying mesh/grid independence (finding the smallest mesh size that results in consistent monitored values).

The Human Touch

Many products and equipment are touched or held by a person during use. Will it be safe? Will it be comfortable? Will it find acceptance in the market given its temperature? How long will a user tolerate it? When modeling heat producing devices in contact with human tissue, we can employ a Pennes model for the volumetric tissue heat generated by the skin as well as using physical properties of the tissue for thermal conductivity, heat capacity, density, and blood perfusion rate. This approach gives accurate predictions for touch temperature.

Handheld FBD-Thermal Sim Study - Pruyn

Figure 7: Heat FBD for a handheld device. Credit: Author

Figure 8: Temperature contour plots for human hand using Pennes modeling technique. Credit: Author

Device Throttling

Many wearable devices are thermally limited by their small size, modest natural cooling, and temperature comfort limits. Planning this power usage is central to product design, ensuring smooth device operation without abrupt shutdowns

Building a thermal digital twin into the software control architecture can be used to estimate when an operational limit is approaching and trigger the device to enter a lower power state. This can be accomplished by creating a reduced order model (ROM) from the high-fidelity FEA/CFD model with inputs at observable (thermal sensor) locations and estimating the temperature of the most critical components as a function of past use and available measurements.

Thermal Design – Model Validation

In early development phases, validation often involves peer review of hand calculations. Physical system testing often begins early on evaluating prospective thermal modules like heat sinks, heat pipes, vapor chambers, graphite, and TIMs. This effort usually involves working closely with suppliers and integration teams as early in the process as possible.

In the middle phases, full testing of prototypes and proof of concept hardware should be leveraged to validate the high-fidelity model. Care should be used to focus on the model’s accuracy from the perspective of its intended use. If your most important output is chip temperature your testing should measure chip temperature, input power and the dominant cooling paths with adequate resolution to discover differences in the model so that errors can be driven down to acceptable levels.

Thermal engineering should provide the performance metric that must be achieved. System validation may involve a thermal test vehicle outfitted with heaters, key modules, and other instrumentation to verify the thermal budget. Outputs include answers to: Was the thermal budget achieved? How was the performance of key modules? And were safety and compliance requirements met?

Once those goals are met metrics can be then incorporated into the quality plan for the various components and modules to ensure ongoing success of the product.

Bringing it all together

There are numerous reasons to incorporate thermal development into your product design process, ranging from preventing mechanical failures or system breakdowns caused by thermal changes to ensuring human safety and comfort. We start with the first principles and move onto building detailed models, heat dissipation solutions, and model validation testing of prototypes. By following these steps, you will not only drive the success of your product but also enhance customer satisfaction, making them well worth the effort in your planning and development.