Wearable and mobile products come with a number of challenges in controlling temperature and achieving the comfort temperature goal. The following text delves into how thermal simulation observations are concretized into design solutions and ultimately refined into higher quality products from user experience.
The normal way to use wearable and mobile devices is to hold them against the skin for long periods of time. Metal exterior parts feel hotter than plastic parts due to their better thermal conductivity. Wireless charging pads, in turn, may unnecessarily heat the device. The compact size and power density of the devices contribute to raising the bar for design. Simulation can be used to model thermal behavior up to the precision point with respect to the insides of the component.
Safety and comfort are the starting points of thermal design
The basic premise of product and thermal design is that products are designed to be safe for the user and are of high quality for the user. In mobile consumer electronics, such as tablets, smartphones and smart bracelets, the temperature application ranges are rather limited in normal use from 0 to 35°C.
As a rule, no lower limit is specified for equipment surface heat, but the maximum value is often considered to be +48°C (IEC 62368-1:2020). However, for the most common usage cases of equipment, the objective is to remain within so-called comfort heat limits. These comfort heat limits vary, depending on the surface materials of the device and the body part concerned. For example, the ear is more sensitive to heat than the palm of the hand. We have research data on general limits used and surveys conducted according to user surveys.
Thermal design objectives
In principle, the object of monitoring in thermal simulation is surface heat specific to various components and materials. Thermal simulation has two objectives:
To achieve comfortable surface temperatures (thermal comfort) for the most common cases of use.
To ensure that device temperatures remain below the safety limits prescribed by the standards.
Safety limits vary depending on the target audience of the product, the environment of use, and the approvals required by the product. Approvals depend on, for instance, whether what is concerned is a consumer, military or medical device, and what standards relate to the product.
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If you look at temperatures somewhat higher than comfort temperatures, it can be observed how various electronic components start to be affected by disturbances as a result of temperature increases.
For example, processors used in mobile devices start dropping power at +80°C and may have to turn themselves off at +95°C. DDR4 memories may begin to produce errors when they exceed the +75°C temperature limit. For instance, faults occur as errors in writing to memory when, i.e., incorrect information is entered while shooting a video; or the entire processing file can be corrupted.
There are slight differences in temperature limits between battery technologies, but, e.g., a Li-ion battery commonly used in mobile devices is not allowed to be charged above +60°C and must not be discharged at temperatures above +70°C. Excessive heat depletes battery capacity and shortens its life, so it is good to keep the battery-operating temperatures at below +50°C.
Certain display types and camera sensors generate noise to the image at +60°C and are sensitive to large thermal distribution. PCB materials are usually specified for a +85°C performance maximum, though their actual tolerance moves in the range from 100 to 135°C, depending on the insulation material used.