By John J. Motzi, Dowingtown PA, USA
The aim of this project was to create a fully passively cooled PC sufficient for use in image processing. Since image processing requires a powerful PC with workstation graphics and a lot of RAM, the passive cooling capability would have to be much greater than for 35W passive PCs. Commercially available “fanless” PC cases which are used most often for up to 35W processors are not optimal in providing convection cooling for motherboards, memory and storage drives. The result when using the closed cases is high temperatures which can lead to premature failure of PC components. These higher case temperatures also increase the ambient temperature of the heatsinks relative to the environment which reduces the efficiency of the heatsink to cool the CPU. After some experience with building 25W and 35W passively cooled PCs, It was decided that a different case design would be necessary to provide optimal cooling for more powerful components. The design criteria were as follows:
• Motherboard capable of 64 GB RAM and at least one NVMe SSD
• Full size power supply
• 300 Series Intel processor I7-8700 @ 65W
• Workstation Graphics card
• Three SSDs, two of which would be combined in a RAID-0 array
• Full passive cooling
Selected Components
• Gigabyte H370M-D3H microATX motherboard
• 2 x G.Skill Ripjaws DDR4 Memory F4-2400C15D-32GVR (64 GB)
• Intel i7-8700 processor (65W)
• PNY Nvidia Quadro P1000 Workstation Graphics Card (47W)
• Seasonic 600W Fanless PSU SSR-600TLR
• 2 x Streacom DB4 GPU Cooling Unit
• 2 x DIYAudio Store Heatsinks 300 x 210 x 40mm 0.18°C/W
• Samsung NVMe SSD 960 Pro 512 GB (as the OS drive)
• EKWB EK-M.2 NVMe Heatsink
• 2 x 2.0 TB Micron 2.5” 6 GB/s SSD MTFDDAK20T0TBN
• 1.0 TB OWC Mercury Electra 6G SSD 7mm
• Thermaltake PCI-E 3.0 Extender 300 mm AC-045-CN10TN-C1
Design & Construction
An open case design was chosen. Actually, it’s no so much a case as a framework to which the components were mounted. The frame was constructed from 20x20 mm aluminum extrusions salvaged from a mining rig. The same components are readily available and even can be custom ordered from internet vendors. The only customization required was some cutting and end tapping.
The motherboard/CPU and graphics card were directly attached to the large heatsinks using the Streacom DB4 GPU Cooling Units. The DB4 GPU cooling units are designed for graphics card use with a specified GPU TDP of 65W nominal, 75W max. A custom bracket was made from 4mm anodized aluminum in order to mount the cooling unit to the motherboard. The supplied spring-loaded screws were attached through the backside of the motherboard directly into threaded holes in the custom bracket. The cooling unit was mounted to the graphics card using the supplied mounts. Each cooling unit was mounted to the heatsinks via bolting into 3mm drilled and tapped holes in each heatsink. Likewise, the standoffs for the motherboard were mounted in 3mm drilled and tapped holes in the CPU heatsink. The SSDs were mounted on a 4mm anodized aluminum plate which was mounted behind the motherboard on standoffs. This method promotes convection cooling by allowing for air circulation around the SSDs.
The heatsinks were mounted to the aluminum framework such with the fins oriented vertically for natural convection cooling. Likewise, there is vertical convection airflow on both sides of the motherboard to promote passive cooling of the chipset, memory and other motherboard components. Also because of the height of the DB4 GPU cooling unit, there is sufficient airflow on both sides of the graphics card. The fanless power supply was mounted on the framework in the recommended orientation and was positioned at the bottom of the chassis. The PSU was mounted to the framework using the universal bracket salvaged from the DB4 cooling units. The mesh construction of the PSU allows for cooling air from all sides to flow over the components. The PC is suitable for placement on the floor or other surface, since the framework elevates all components which promotes cooling airflow.
The orientation of the motherboard on the heatsink is such that all I/O connections are at the top of the PC. This facilitates attaching peripherals when the PC is located next to a desk or on a rack. This orientation also allowed the motherboard PSU connections to be slightly below the heatsink, which allowed easy connection after mounting. The orientation of the graphics card on the heatsink is such that the connections are facing downward. This was necessary due to the length of the previously purchased PCI extender cable. A longer cable would facilitate other orientations.
Overall dimensions of the PC are 276mm (W) x 300mm (D) x 420mm (H) (10.9”W x 12”D x 16.5”H)
The installed operating system is Windows 10 Pro on the NVMe drive.
Performance Test 1 – PRIME95
A stress test was performed using the PRIME95 Torture Test at the default settings for 3 hours. Ambient temperature was 20.5 ± 0.3 °C. The graphs show that the maximum CPU temperature was 64 °C. The steady state temperature of 61 °C was reached after approximately 30 minutes of testing. The maximum VRM MOS temperature was 73 °C. The heatsinks were warm to the touch. The PSU emitted no noticeable heat, so the location of the PSU under the motherboard did not hamper cooling of the motherboard or drives. The test was also conducted on a day with higher ambient temperature (25.0 ± 0.5 °C) resulting in a maximum CPU temperature of 68 °C
Performance Test 2 – ADIA64
A stress test was performed using the AIDA64 Stress Test (CPU, FPU, Cache, GPU) for 2 hours. Ambient temperature was 20.5 ± 0.3 °C. The graphs show that the maximum CPU temperature was 73 °C which was reached within the first hour. The maximum VRM MOS temperature was 80 °C and the GPU reached 65 °C within the first hour and both held there throughout the test. The heatsinks were warm to the touch. The PSU emitted no noticeable heat. Based on these results, it is likely that the graphics card could be increased to a P2000 (75W) and still perform adequately. Both performance tests show that the passive cooling is adequate for this PC.
