Sub-ambient direct-on-chip liquid cooling is an emerging technology in the data center industry. The risk of an electrically conductive liquid leaking out to the electrical components and damaging the servers has been the major factor in holding back the use of liquid cooling historically. This technology effectively removes that risk. A direct-on-chip liquid cooling system, where average system pressure and average CPU temperatures can be fixed for a range of server computing loads and coolant supply temperatures for data center servers has been designed and constructed. This has been used to determine what impact pressure has on a small-scale liquid cooled server system in terms of CPU power consumption and CPU temperatures. The cooling system was only able to work with one server connected. Experiments with different values for the CPU temperature setpoint, coolant supply temperature setpoint, server computational load, and server pressure were executed to verify that the system works as intended. Applying a range of CPU computing loads works well, maintaining fixed average CPU temperatures works, with differences between the CPUs at higher temperatures and failure to reach average CPU temperatures when the difference between these and the coolant supply temperature is small. Maintaining fixed average pressure before the server works well, while pressure after the server is heavily affected by coolant flow. However, this effect is not seen as important for the experimental goals of the thesis. Maintaining a fixed coolant supply temperature works well with some slow fluctuations around the setpoint. No noticeable effects from pressure on CPU power consumption and CPU temperatures were seen. However, lower flow resistance was seen by the circulating pump when negative system pressure was lower which implies that less pump energy is needed to pump at lower negative pressure. The pressure was not in the region where the coolant could phase change during the experiments.