## 4 Discussion  
Figure 13 shows the temperature history for the sensors that measured the highest peak temperature (\(T_{in\text{-}1}\), \(T_{B\text{-}1}\), and \(T_{F\text{-}1}\)) on the inside, back cover, and front screen, respectively. Measurements from the experiment with the heat spreader and TIM are compared with those of the heat spreader, TIM, and LHTES module.  
The inclusion of the LHTES module reduced the peak temperatures measured inside and on the screen of the mock tablet, while having little impact on the temperatures measured on the back cover. This indicates that the addition of the LHTES increased the rate of heat transfer towards the back cover (lowering the temperature of the SoC and the front screen) while maintaining the back cover at the same temperature through the storage of the additional heat through the PCM.  
Figure 13: Maximum temperatures measured for the best two temperature control solutions at 4.5 and 7 W: (**a**) inside the mock tablet \(T_{in\text{-}1}\), (**b**) on the back surface of the mock tablet \(T_{B\text{-}1}\), and (**c**) on the front surface of the mock tablet \(T_{F\text{-}1}\) (the dashed line represents the relevant temperature limit).  
However, peak temperature is not the best measure of the effectiveness of a temperature control solution. There are two core objectives of a temperature control solution in handheld electronics. The first is to extend the operating time before the device reaches a temperature limit and is required to throttle the processor speed. The second is to reduce the mean operating temperature of the major sources in the system, increasing the life of these components. The LHTES module improved both these aspects in the mock tablet computer.  
The cooling phase of the experiment was also impacted by the LHTES module. The inflection in the temperature profile due to latent heat storage reduced the cooling rate, keeping the tablet near the transition temperature as the PCM solidified. However, after the PCM solidified, the temperature quickly approached the temperature profile of the experiments without latent heat storage. Additionally, all experiments showed the tablet cooling to ambient in just under 60 min.  
Table 4 summarizes the time required for the device to reach its maximum allowed operating temperature, either at the heater or the front surface. Entries in the table showing dashes indicate that the feature did not reach its limiting temperature during a 60-min experiment. Longer times show superior performance since the tablet will be able to operate with thermal throttling for a longer period.  
At a heat generation of 4.5 W, the two temperature control solutions that utilize TIM are by far the best solutions as neither reached their temperature limits within the 60-min time. Without the TIM, the device's SoC overheated in less than a minute when a heat generation rate of 7 W was used. The use of the TIM greatly reduced the thermal contact resistance between the SoC and the heat spreader, moving the location of the temperature limit reached to the front cover.  
At 7 W, the solution using the heat spreader, TIM, and LHTES led to an approximately 30% increase in operating time before the critical temperature (on the front surface) was reached, compared with the case without a LHTES.