Ergonomic temperature limits for handheld electronic devices

In the drive towards smaller but more powerful electronic systems, effective thermal management issues continue to be among the key challenges facing the electronic industry. With this drive comes increasing heat production that must be removed for a satisfactory and safe operation. In large systems, one of the main challenges is how to achieve acceptable junction temperature of components such as microprocessors. In small systems such as handheld devices, however, one of the main challenges is maintaining the enclosure temperature below certain threshold so that they can be handled and operated without much discomfort. Whether the device is cooled passively or actively, a portion of the total heat generated by components is dissipated through the enclosure. This heat is then dissipated from the skin by radiation and natural convection into the ambient, and by conduction into any body that is in touch with it. So, when a device is held by hand, part of the heat produced will flow into the hand by conduction. The higher the temperature of the enclosure, the greater the heat transfer into the hand will be. Thus, whereas some devices feel cool to the touch, others will feel hot when touched. In order to characterize touch comfort, we must answer some basic questions: What determines whether a device feels hot or not? That is, what are the variables that control touch comfort? Is there a maximum skin temperature limit that we can say is acceptable for touch comfort? In this paper, we shall attempt to answer these and related questions. Among the results we report in this paper include that touch comfort not only depends on the absolute enclosure temperature of the device but also on the thermal conductivity of the enclosure. Materials with high conductivity transfer greater amount of heat into the hand than materials with low conductivity. Thus, a device with aluminum enclosure will feel hotter when touched than a device with plastic enclosure dissipating the same amount of heat. Further, depending on enclosure material type and ambient conditions, one can identify a skin temperature limit beyond which touch comfort deteriorates and becomes unacceptable. For the form factor studied, it was determined that the skin temperature limit for a plastic enclosure was about 45 C, and for aluminum enclosure it was about 41 C.