Thermal Management: Heat in Product Design

Authors: Jeff Swanson, Brandon Fichera, and Ted Santos

Heat Sink Thermal Imaging

There are many factors to consider when beginning a new product design effort. Hardware and software features, materials, interfaces, industrial design, size, and color to name a few. All of these need to be defined up front in order to deliver a compelling and viable product to market.

Oftentimes, thermal performance of the product is an afterthought in the design process and is commonly only viewed as a reliability issue. However, with many mobile processors, including Qualcomm’s Snapdragon™ product line, thermal protection is built into the system. That is, the processor’s voltage and frequency are throttled when silicon die temperatures reach specified threshold levels. Effective thermal system design is needed not for long-term reliability but rather to improve performance, efficiency, and, ultimately, the perceived quality of the product.

Conversely, inadequate thermal system design can cripple the performance of the sleekest products, sabotaging all the hard work that went into the design. Poor thermal design can also result in surface hot spots, impacting the user experience and creating potential regulatory compliance concerns. These challenges are only exacerbated as product sizes decrease and processor power increases.

Often, a basic assumption of thermal design is that getting the heat out of the product as quick as possible is the optimal solution. This method can be effective – and is very common – but is not always the most appropriate method. Thermal management techniques that immediately exhaust heat often have attributes which could be considered a negative in product design, such as requiring forced convection (fans) or resulting in hot spots on the outer surfaces of the product. In most products using high performance mobile processors, neither fans nor hot spots are acceptable due to the handheld nature of the product, compact size, and/or limited power available to drive a fan.

Fortunately, most products have a significant amount of thermal mass inside. With good thermal system design and proper selection of materials, this internal mass can be used to absorb energy from the heat generating electronics, evenly spread it, and eventually dissipate it out of the product through the relatively larger external surface area.

Defining thermal system requirements

Before starting on thermal management solutions for a product, understanding the system requirements which impact thermal system design are critical. Areas to consider include:

1. Product application and resulting processor load

What is the primary application of the product? Will this result in a steady state loading of the system, or is the use intermittent or transient in nature?

2. Operating environment

Are there heat sources other than the processor in the product that need to be considered in the thermal system design? What is the operating temperature range? In what type of environment will the product be used? Does the device run off of a battery or external power? Is active cooling with use of a fan a consideration?

3. User experience

Is this a handheld product? What sort of materials will the user contact? Plastic, metal, elastomers, etc? Are there concerns about hot spots?

4. BOM Cost and Manufacturability

How sensitive is the product to BOM cost? How does the thermal solution integrate with the rest of the assembly process?  Are there supply chain constraints driving component selection?

Addressing thermal system design early in the development process provides the best opportunity for an optimized solution and ensures that any potential costs are factored and designed into the overall budget from the start.

High Processor Load Test Case

In the development of the Open-Q™ 820 µSOM (micro System on Module) development kit, we simulated and tested many thermal management solutions. Let’s dive deeper into one specific scenario to illustrate how effective thermal design can benefit a product.

The objective outlined for this study is to maximize processor performance under high load using passive thermal management (ie. no forced convection/fans). As previously discussed, dynamic voltage and frequency scaling (DFVS) are built into the Snapdragon ™ 820 processor to manage die temperature.  A good thermal design will provide a conduction path out of the die, into the product’s internal thermal mass, and eventually dissipate it out of the product’s external surfaces, enabling the processor to run at higher frequencies with resultant better performance. Performance criteria which were evaluated include:

  • Higher average processor speed over the test duration
  • Increased input power which is required for higher processor speeds without affecting steady state die temperatures.
  • Extended thermal time constants (ie. increased time to reach steady state)

Test setup:

Our test setup used in this study included an Open-Q™ 820 µSOM development kit with a processor stress test application. Temperature data was logged at the device and processor level.  Processor frequency data and SOM input power were also tracked.

Test procedure:

Using the Open-Q™ 820 µSOM development kit test setup previously described, we logged temperature, power and processor frequency data. The test duration was 360 seconds to ensure devices reached thermal steady state conditions.

Configurations evaluated in this study:

1. Baseline development kit without any thermal management
2. Development kit with 2800 mm2 x 2 mm thick Aluminum heat spreader coupled to the Snapdragon® 820 processor
3. 1 cm3 Thermal Interface Putty (TIP) dispensed between the SOM Board and Carrier Board
4. Both the Aluminum heat spreader + TIP used

Thermal Interface Putty IOT device board

Figure 1. Thermal Interface Putty shown between uSOM and carrier board.

Summary of results:

Thermal Interface Putty shown between uSOM and carrier board

Table 1. Test Results

The data also shows that a heat spreader on top of the processor or thermal interface putty between the SOM and Carrier Board are similarly effective in dissipating heat from the system. Furthermore, it shows that a conduction path into the mating carrier board can be as effective as pulling heat out of the top of the processor, and that dissipating heat from both the top of the processor and through the SOM into the carrier board further improves performance. Thermal time constants are increased as thermal management solutions are added. Increased time constants benefit use cases where the power draw is intermittent in nature.

Average Die Temperature Vs Time

Figure 2. This graph shows the impact thermal management has on the thermal time constant of the uSOM

This sort of data provides product designers with options for system level thermal design—for instance, the thermal interface putty will not impact device thickness and may help to reduce the risk of surface hot spots by spreading heat into a larger circuit board before dissipating to the environment. Use of a heat spreader may enable a designer to re-purpose an existing part of the product assembly for heat dissipation, eliminating the need for additional thermal components or processes. The difference in heat dissipation between the various options is graphically represented in the thermal simulation data shown in the figure below.

Thermal Interface Heat Mitigation Diagram

Figure 3. Simulation data showing heat dissipation for different configurations tested.

These results were achieved by incorporating management solutions on a finished circuit board assembly. In a commercial product, thermal system design must be considered concurrently as part of the overall product design effort to ensure system level requirements can be achieved. Attempting to integrate thermal solutions after the initial product design is completed may result in compromised system performance.

The results discussed only touch upon a few of the many management materials and configurations we have explored at Lantronix. We have completed thermal characterization of SOM based systems using a variety of thermal management solutions and have also run accompanying thermal simulations using Solidworks Simulation tools.

This characterization and simulation data can be used as a starting point for concurrent development of a thermal solution in parallel with your overall product design.

There are many methods for managing heat in your device. We’ve summarized a few of them here. However, since each product has its own unique requirements, it’s best to assume your device will require its own unique thermal solution. You’ll be better off if you plan for it up front and design the thermal solution in parallel with the rest of the product design. Intrinsyc’s engineering staff is available to lend their expertise and experience in helping you develop a thermal management solution optimized for your design requirements.

Authors: Jeff Swanson, Brandon Fichera, and Ted Santos lead Intrinsyc’s mechanical engineering product design team. Their team has wide-ranging experience in the development and commercialization of mobile and IoT products. Their design experience spans a breadth of product types, with a variety of thermal and structural engineering challenges, high density device packaging and ruggedization.