Confronting Design Challenges With Smaller Thermal Envelopes

As users demand more from each generation of smartphone, thermal budgets are shrinking.

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For design engineers, physics giveth but physics can also taketh away.

Consider leading-edge smartphones. Outside the improved performance that each generation gives consumers, the handsets themselves get thinner, sleeker, lighter. The reduction in the Z height is effectively a given with each new generation.

In 2010, the HTC Nexus One was 11.5mm thick; this year, the Mate 8 is 7.9mm. Remember those numbers because I’m going to come back to them in a moment.

Additionally, bezels shrink to the point where they’ve almost disappeared.

Overall mechanical volume, therefore, is diminishing every year, and the thinner the handset, the more challenging becomes the ability to dissipate heat from inside the device. Even losing just 1 millimeter can have a major impact on a designer’s thermal envelope. Adding to the heat-dissipation challenge is the greater use of glass as a percentage of the overall surface volume.

Battery sizes are increasing, a bigger battery form factor doesn’t help because the battery is not a good heat dissipater; it’s a heat generator.

Performance’s price tag 

OK, that’s just the mechanics of a smartphone. What about the changing nature of the electronics? This publication has been diligent in charting many of the challenges in recent years, such as this overview of issues from Ann Steffora Mutschler.

Smartphones make greater use of camera subsystems, which means more sensors, which means greater demands on processing.

Think about memory subsystems: 4K video and higher-resolution digital imagery strain memory subsystems, which are taking up more of the overall power budget.

Now consider modems. Their complexity is just plain scary these days! Carrier aggregation means phones are effectively running three modems at once, so even though you’re getting efficiencies on system design and SoCs, you’re now running three front ends on your radio.

So, we’re squeezing more into smaller form factors and delivering higher CPU performance. Recall those thickness numbers above: the CPU performance of the Mate 8 is 16 times the performance of the HTC Nexus One from 2010.

This increase has helped bring new use cases such as augmented and virtual reality to the handset, object-based audio processing, AAA gaming and 360-degree video.

Chilling effect

To date, thermal throttling has been the way to manage the system and mitigate potential damage, but that cripples performance. In the context of ARM’s big.LITTLE technology, consider the peak performance of an ARM big CPU running at 1.5GHz at 69 degrees C: Thermal saturation that might trigger throttling would dial that back to 62 degrees C and 1.229 GHz. That’s a 20 percent frequency hit coupled with a 25% system performance impact.

To keep up this type of SoC design cadence, partners need to figure out how to enable newer use cases when their thermal budgets are shrinking. Some of that can come in the process nodes, but you can’t rely on that alone.

If the thermal envelope is getting tighter, the efficiency of SoCs and IP has to be better.

In ARM’s latest big CPU, the ARM Cortex-A73, engineers—with an eye toward optimizing peak performance and efficiency—tuned the microarchitecture for efficiency by optimizing the pipeline and implemented performance optimizations including branch prediction, L1 and L2 cache prefetching and memory output.

The idea was to yield peak performance and the best efficiency in the mobile environment, and it’s an example of ways designers can confront the thermal envelope.

Ultimately users yearn for better battery life with each new generation of smartphone, but they expect increased performance and sleeker, ever more stylish physical designs.

What’s that mean for designers who need to find ways to deliver peak, sustained performance in space-constrained devices? Robust employment prospects for years to come.