# Thermal Interaction and Cooling of Electronic Device with Chiplet 2.5D Integration
## 4 Near-Chip Cooling
### Experiments and Results  
In this study, the coolant is deionized water, and two different heat modes are examined: (1) chiplet1 is heated and chiplet2 is not heated; (2) both chiplet1 and chiplet2 are heated. The heat flux of the TTC is controlled by the input voltage, which ranges from 9 V to 33 V in increments of 6 V. The heat modes and voltages are shown in Table 2. In the experiment, the volume flow rate of the coolant is changed to 0.2 L/min, 0.5 L/min, and 0.8 L/min. Since the heating resistance value of the Pt serpentine wire drifted with temperature during the experiment, there is a slight difference in the input power of the same input voltage for the same input voltage under different heat modes and flow rates, and the heat flow of the two TTCs is determined by the output current and voltage of the power supply, and the heat flux is obtained.  
By varying the TIM (silicone grease vs. liquid metal with thermal conductivities of 5.2 W/mK and 72 W/mK, respectively), the resistance values of chiplet1 and chiplet2 in different heat modes are examined in Table 2, and the temperature values are determined by Equations (14) and (15). The relationship between chiplet temperature rise and various heat fluxes in heat mode 1 when the TIM is silicone grease (DOW5888) is shown in Figure 13a. When the flow rates are 0.2 L/min and 0.8 L/min, at an input voltage of 33 V, the temperature rises of chiplet1 are 74.6 \({}^{\circ}\)C and 67.4 \({}^{\circ}\)C, respectively, while the temperature rises of chiplet2 are 5.2 \({}^{\circ}\)C and 2.1 \({}^{\circ}\)C. The relationship between chiplet temperature rise and various heat fluxes in heat mode 1 when the TIM is liquid metal is shown in Figure 13a.  
Figure 12: Resistance–temperature curves for Pt temperature measuring resistors. (**a**) Chiplet1. (**b**) Chiplet2.  
When the flow rates are 0.2 L/min and 0.8 L/min, at an input voltage of 33 V, the temperature rises of chiplet1 are 36.5 \({}^{\circ}\)C and 31.1 \({}^{\circ}\)C, respectively, while the temperature rises of chiplet2 are 2.3 \({}^{\circ}\)C and 0.5 \({}^{\circ}\)C. Compared with DOW5888, the liquid metal TIM mitigates thermal interaction by 56% and 76% at the flow rates of 0.2 L/min and 0.8 L/min. Lower thermal interaction is obtained when liquid metal is used as the TIM, which is consistent with the trend predicted by the collective parameter model. Furthermore, the thermal resistance decreases by 24.8 Kmm\({}^{2}\)/W at a flow rate of 0.2 L/min. The variation in the TIM significantly increases the cooling capacity of chiplet 2.5D integration and mitigates thermal interaction, especially for higher-heat-flux chiplets in the future.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **Test Number** & **Chiplet1** & **Chiplet2** \\ \hline \multirow{6}{*}{Mode 1} & 1 & 9 V & 0 V \\  & 2 & 15 V & 0 V \\  & 3 & 21 V & 0 V \\  & 4 & 27 V & 0 V \\  & 5 & 33 V & 0 V \\ \hline \multirow{6}{*}{Mode 2} & 1 & 9 V & 9 V \\  & 2 & 15 V & 15 V \\ \cline{1-1}  & 3 & 21 V & 21 V \\ \cline{1-1}  & 4 & 27 V & 27 V \\ \cline{1-1}  & 5 & 33 V & 33 V \\ \hline \hline \end{tabular}
\end{table}
Table 2: Chiplet loading voltage in two different heat modes.  
Figure 13: _Cont._  
Figure 13c demonstrates the relationship between the temperature rise of the two chiplets and different heat fluxes in heat mode 2 with a flow rate of 0.8 L/min. According to Figure 13c, when the highest allowable temperature rise in the chiplet is 60 \({}^{\circ}\)C and two chiplets are heated at the same time, using DOW5888 as the TIM, the maximum heat fluxes of chiplet1 and chiplet2 are 140 W/cm\({}^{2}\) and 180 W/cm\({}^{2}\), respectively. When liquid metal is used as the TIM, the maximum allowable heat fluxes of chiplet1 and chiplet2 are 330 W/cm\({}^{2}\) and 520 W/cm\({}^{2}\), respectively; the liquid metal can greatly increase the allowable heat flux of the chiplets.