## 2 Experimental Setup  
A schematic of the experimental setup used is illustrated in Figure 1. A photograph of the setup is visible in Figure 2. The setup is made of a discrete heat source, a 25.4 mm diameter copper block with a length of 31.75 mm mounted in the middle of the base. A 6.35 mm diameter, 25.4 mm long, cartridge heater (Nordic Sensors, custom made), which can provide up to 12 W of power, is inserted in the middle of the block. This heat source mimics heat dissipation from a tablet computer's SoC. The copper block is in the middle of the setup and flush with the Teflon base that simulates a tablet computer's housing. The square Teflon base has a side length of 178 mm and a thickness of 12.7 mm. A square cavity with a depth of 2 mm and a side length of 152.4 mm is poured on the upper surface of the base. The base forms a small cavity enclosed by a cover made of nylon with a thickness of 3.1 mm. This small gap is filled with the thin PCM package with a mean thickness of less than 2 mm. Finally, the cylindrical heat source is surrounded by insulation (\(k\) = 0.06 W/m\(\cdot\)K), which forces generated heat to move through the base and the tested PCM package. The dimensions of the entire setup are: 177.8 mm on each square side and 115 mm in height. The thermal properties of nylon, Teflon, and copper are given in Table 1.  
A DC voltage supply (type Statco Energy 3 PN501B) is used as a power source. An analog output NI9263 (0-10 V) module controls the power source via LabView 2014. During each experiment, the voltage input of the DC supply was measured using an analog input NI9201 module. The current through the heater was determined by measuring the voltage drop over a shunt resistor RShunt (type HSA25R05J, 0.05 \(\Omega\)), which is placed in series to the heater. Finally, the power input was determined \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Properties** & **Copper**[35] & **Teflon**[35] & **Nylon**[35] \\ \hline \(\rho\) (kg/m\({}^{3}\)) & 8960 & 2140 & 1120 \\ \(k\) (W/m\(\cdot\)K) & 401 & 0.25 & 0.32 \\ \(\mathcal{C}_{p}\) (J/kg\(\cdot\)K) & 381 & 960 & 1380 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical properties of the materials used for the experimental setup. by multiplying the current and the voltage input. The expanded measurement uncertainty of the power input was \(\pm 7\%\). Figure 1: Exploded view of the experimental apparatus showing the thickness of the individual components. Figure 2: Photograph of the experimental setup (**a**) without any package, (**b**) with the thin PCM packet, and (**c**) with the thermal spreader; (**d**) one of the used PCM packages (dimensions in mm). The temperatures of the PCM package, the temperature of the heater and the ambient (\(T_{\infty}\)), as well as the electrical power (\(P\)) input were measured during all experiments. Five T-type thermocouples (\(T_{C}\), see Figure 3) were used to measure the temperatures at different locations of the top of the nylon cover. Four T-type thermocouples (\(T_{U1}\)-\(T_{U4}\)) were mounted on the upper surface and five T-type thermocouples (\(T_{L1}\)-\(T_{L5}\)) on the lower surface of the PCM package. The heater block temperature, \(T_{H}\), was measured using a 1.6 mm (1/16 in) diameter steel T-type thermocouple probe. Figure 3 illustrates the location of the temperature sensors mounted on the PCM package. The thermocouples were mounted using thin high-temperature kapton (polyimide film) tape (0.07 mm, \(k\) = 0.2 W/m\(\cdot\)K). The thermocouple manufacturer's stated accuracy was verified to be within the range of \(\pm\)0.5 \({}^{\circ}\)C using a Fluke Micro-Bath 7102. During the experiments, continuous data acquisition was accomplished at 1 Hz with the use of an NI cDAQ-9174 and an NI 9213 16-port thermocouple module operated via LabView 2014.