How Hot is too Hot? The Real Maximum Temperature of Your Product
Summer is upon us and it brings some sweltering hot days with it. This is a great time of year to consider your next product and the thermal conditions it may encounter. It also puts us into a mindset where we experience the hotter temperatures our end users may subject our products to. Just think about it: the blazing hot temperatures of your car when you don't leave the windows open, the hot temperatures that drive you to the beach and sit on the sunny sands, or walking barefoot on the pavement.
The summer heat won't stop anyone from using their favorite devices, unless they become too hot to handle. Maximum touch temperature is an important aspect to consider in the end user experience. In the summertime, it'll be much easier for your device to reach that maximum touch temperature. As thermal designers, we need to account for that.
How Do We Determine What Our Maximum Touch Temperature is?
You'll hear this more than once when you're talking about thermal management and that is "it depends." The maximum allowable temperature of your device relies on a whole bunch of different factors such as the environment, your product design and application, and how you expect your users to handle your product. Let's walk through these different parameters to get to the core of the maximum temperature question.
Remember that obscenely hot car? Let's get into it and put our hands on the wheel and buckle up. You're probably flinching just thinking about that, right? The seat and the wheel are much too hot for comfort. You also were ginger on how you handled the belt buckle since you didn't want to get burned by the metal part. These are all examples of how touch temperature is essential to consider for the end user experience of your product.
Touch temperature is based off the limitation of human skin. Human skin can bear some higher temperatures for brief periods of time before pain and cellular damage can set in. These two limitations, pain and damage, are the last things our users want to experience with our products. NASA put out an awesome summary of a new approach to touch temperature. NASA conducted some empirical data and research on previous studies to develop their own mathematical model of what temperatures are allowable for skin constant contact. Essentially, our pain threshold for heat depends on what area of the skin is heated, but in general that ranges between 42C (sensitive skin the chest area) to 45C (on our feet).
Another factor for determining the maximum touch temperature is how long a user might be in contact with the surface. Skin can handle higher temperatures for a few seconds before pain or skin damage occurs. Ecma International, a body responsible for creating standards for Information and Communication Technology and Consumer Electronics, has set some ergonomic limitations for skin contact time and temperature. The Standard ECMA-287 has a table the breaks down allowable temperatures based off use and the expected time of contact.
Ecma International also specified material type in this table since maximum touch temperature is also influenced by material. We'll assume that the belt buckle and the steering wheel were both at the same temperature, since the car was sitting long enough to reach equilibrium. You notice that you're able to tolerate touching the wheel more than the belt buckle. That's because the thermal conductivity of the metal buckle is much higher than the rubber or leather we have on the wheel. This higher conductivity allows heat to transfer to the hot buckle to your relatively cooler skin more quickly than the material on your steering wheel.
Calculating Backwards to Maximum Temperature
Now that we have an external maximum temperature based off what our precious end users can handle comfortably, we can calculate backwards to our maximum device temperature. If your user is in direct contact with your heat sink for your device, your maximum device temperature is easy to determine. You can plug in your conditions into Aavid Genie and figure out the temperature of the surfaces your user will touch.
In some products, your device will be enclosed in a case, and dissipating heat is a little more difficult. You need to cool your device for performance and reliability, but you need to do so safely and not sour the user experience with your product. You might need a more complex study in the case of an enclosure. Aavid Engineers are well-versed in studying thermal transfer within enclosures and can help your out. Reach out to Aavid Design if you're in need of more help.
While you're suffering in the heat this summer, remember how our products might make our users suffer. Take some time with a nice cool beverage and think of how you can keep your electronics devices cool, too.