How Google made the Chromecast cool

May 02, 2016 // By Tom Gregory
Google’s decision to jump into the world of streaming with its Chromecast proved to be a smart move. Since the release of the first generation of the Chromecast, they have sold over 20m units to customers looking to make their tired old TVs a little smarter.

Despite its popularity, however, the first generation of Chromecast did have its sticking points. Some users reported that the device ran hot, resulting in a loss of functionality and constant automatic reboots.

In October 2015, Google announced the second generation of Chromecast, giving the company a chance to start from scratch and fix the overheating issues of the original device. So, did Google manage to fix the issue second time around?

Thermal management for low power devices

You might be surprised that thermal management is even an issue for a device like the Chromecast. After all, it’s a relatively simple device: effectively a PCB in a box with just two connections - HDMI, and a USB port for power. However, while the Chromecast is a low power device (about 1W), you’d be mistaken to think that the designers don’t need to worry about thermal management.

For TV streaming devices such as the Chromecast, there is a general set of requirements that need to be fulfilled: the device must be small, low cost (made of plastic), and quiet. As such, the 2015 Chromecast is completely self-contained, with no vents or fans. This means the air inside the device is either still or moving very slowly, presenting a thermal challenge. Moving air is in fact very effective at transferring heat through convection. However, in a small, sealed device this is not possible.

When air remains still within an enclosure box such as the Chromecast, heat can only be transferred through the air by conduction. The thermal conductivity of still air is approximately 0.02 w/mk. In comparison, a typical plastic (a poor conductor of heat) has a thermal conductivity of approximately 0.33 w/mk. Therefore, to avoid overheating issues, the components must have a high conductance path to the outside case where the heat is dissipated to the ambient air through natural convection.

 

Under the hood

A look inside the Chromecast reveals that this is exactly what Google has set out to achieve. Each side of the PCB is also covered with an EMI shield. These shields are used both to reduce electromagnetic interference and to spread heat away from the components to the outside case. They are designed to make direct contact with the most powerful components, such as the ARM processor, to ensure that the components are kept within their thermal limits.   

Thermal paste is used to increase the thermal conductivity between two interfacing objects. On the Chromecast, thermal paste is used to improve the heat transfer from the memory and processor to the EMI shield. Without the thermal paste, the heat dissipation to the shield would be dramatically reduced.