Are smartphones getting too hot?

Designing a premium smartphone has been a delicate balance between performance and battery life for generations. However, continued increases in performance cause growing concerns around heat.

The increased heat generated by advanced SoCs has led to thermal issues during benchmark testing of several of the latest premium smartphones using high-performance SoCs, such as testing of the HTC One M9 and LG G Flex 2 by Ars Technica. These premium smartphones use the Snapdragon 810, one of the most popular 3rd party chipsets on the market.

Despite the claims, however, no one has identified a specific issue with the Snapdragon or any other chipset that would cause the SoC and phone to overheat. To attempt to clear up this matter, Tirias Research has investigated the issues around smartphone overheating.

Design issues
The primary tool for silicon vendors to increase performance efficiency has been through the use of an increasing numbers of compute cores because increasing clock frequencies increase power even more. In premium handsets smartphone SoCs, the number of central processing unit (CPU) and graphics processing unit (GPU) cores has been doubling the past few generations to the point where we have up to 10 CPU cores and hundreds of GPU cores in the highest-end smartphone SoCs.

In addition, many smartphone SoC utilise highly specialised compute cores for image processing, video encoding and decoding, audio processing, signal processing for cellular communications, and sensor hubs. This all leads to more power sources that increase performance and can create thermal issues.

The problem is further exacerbated by the foundation of silicon technology—Moore's Law. Each process generation results in smaller design features the further condense these compute cores and the space between the cores. This has the potential for creating hot spots when cores are operating near their peak potential so close to other cores.

To address the issue of higher power consumption and to extend battery life, SoC vendors have turned to increasingly complex designs that allow for separate power planes for different cores and increasingly minute levels of clock controls, referred to as clock gating.

The finer the power solution, the more of the SoC can be disabled when it's not required. While this also aids in reducing thermals to some extent, it does not eliminate the potential problem because the SoC is not the only component contributing to thermal issues.

There are other components that can lead thermal issues when being driven to perform at or close to peak rates, including the power management IC (PMIC), the power amplifier, the image sensor, and the display. Many are the result of applications or operating conditions.

Calling or transmitting data from the edge of the cell network forces the power amplifier to or at peak levels; on-line gaming can drive the display, power amplifier, and SoC are high levels; and capturing and encoding 4k video can drive the image sensor, display, SoC, and memory at high-levels.

In all of these cases, the components temperature can rise to high levels causing the case or skin of the smartphone to heat-up excessively. And, as smartphone vendors push for thinner designs and more rigid materials, such as metal, the potential for thermal issues increases because these materials more readily transmit heat to the surface of the phone. A simple change or error in design or assembly can alter the thermal characteristics of a device and increase the potential for thermal issues.

There are two key technologies that prevent these premium smartphones, and most other electronic devices for that matter, from entering thermal runaway, or essentially catching fire or melting – thermal sensors and the smartphone software. The thermal sensors are hardware components positioned in areas of a chip or device to measure the temperature.

These temperatures are monitored in the system software according to specification of the various components. Typically, the silicon vendor provides recommendations and/or settings to the smartphone OEM, but it is up to the OEM to determine what that final settings will be and this may change over time as testing or customer feedback requires modifications. If the component temperature exceeds recommended levels, that system software reduces performance until acceptable temperatures are reached.

Putting the smartphones to the test
All premium smartphones face thermal design challenges, especially when using a leading-edge SoC. However, testing for the difference in designs and software settings requires testing smartphones with both the same components and those with different components.