Abstract
The computational capabilities of central processing unit (CPU) and graphics processing unit (GPU) are rising steadily with every generation thanks to advancement in electronics and packaging techniques. However, this brings about issues in effective heat dissipation as the thermal design point (TDP) of the chips increases as well. In view of the higher TDPs, traditional air-cooling solutions, such as air-cooled CPU heat sinks and built-in fan modules on the GPU card are no longer sufficient in addressing this problem. This causes the CPU and GPU temperatures to hover very close to their maximum allowable limit. When this occurs, the performance of the chips degrades due to thermal throttling. Thus, the introduction of liquid cooling to high-performance computing (HPC) servers is inevitable due to the above issue. Liquid-cooled HPC servers using finned cold plates not only effectively dissipates higher heat flux but also reduces power consumption of the chip by significantly lowering the chip operating temperature. In this work, the oblique-fin cold plate was fabricated using a hybrid manufacturing technique, whereby the fins were additively manufactured (AM) directly on a solid machined base plate. The fins were fabricated by selective laser melting using copper chromium zirconium, CuCrZr powder, which is a high thermal conductivity copper-based alloy. The base plate, on which the fins were printed, was machined from solid copper. Hybrid printing reduced fabrication time and saves cost, as bulk of the printing time and cost came from the base plate. On the other hand, the benchmark oblique-fin cold plate was machined using wire-cut (WC) electro-discharge machining (EDM), which is a form of subtractive manufacturing. The AM oblique fins reduced material wastage compared to the machined fins. Liquid cooling experiments were conducted on a 1U server running on a 32-core AMD CPU with a TDP of 180 W. Water flow rates were varied between 400 ml/min to 2000 ml/min with an inlet temperature of 25°C. Thermal and hydraulic performance analyses revealed that the hybrid printed oblique-fin cold plate managed to maintain lower CPU temperature compared to the WC oblique-fin cold plate. However, the associated pressure drop penalty was slightly higher due to higher surface roughness of the AM fins. Nevertheless, the pressure drop penalty incurred was justified due to its better cooling performance, as the AM oblique-fin cold plate exhibited higher overall performance when evaluated using the thermal resistance vs pumping power curve. As a result, AM, in particular hybrid printing, can be considered as an alternative to fabricate finned cold plates, especially irregular fin designs that are otherwise not realizable using conventional machining techniques, without compromising cooling performance.
