- Sep 25, 2018
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I am heavily overclocking an Intel i9-7980XE CPU and decided to use liquid metal. I delidded the CPU and am using Der8auer's Direct Die Frame allowing me to put the cold plate of my custom water cooling loop directly on the chip silicon.
The problem is that the silicon die itself is slightly convex at its top. This apparently is a common issue - the CPU chip itself not being perfectly flat. It is less of an issue with conventional thermal paste, which is much thicker than liquid metal and can be applied thick to make up for any die/heat-sink/cold-plate warping or imperfections.
With liquid metal though, and I am using Thermal Grizzly Conductanaut, the general recommendation is to apply a thin coat to the silicon and a thin coat to the heat-sink. This accepted approach does not work for me. Only an oval portion of the chip, representing maybe 33% of its central silicon area, is coming into contact with the perfectly flat top-notch mirror finished cold plate I am using, an Aqua Computer Cuplex Kryos Next Vario with Vision. The periphery of the silicon, due to the convexity of the chip (i.e., ever so slight central bulge) is not coming into contact with the cold plate under tension (regardless of how much tension).
The result of applying two thin layers of LM to the CPU die and cold plate, respectively, is that some cores, probably the ones along the periphery of the die, are not making contact with the cold plate and jump to the Tj MAX temperature (or close to it) at load. There is an almost 60 degree C disparity between properly cooled cores (near the center of the die) and the overheating ones at the periphery.
I have been able to resolve this on my motherboard which is sitting horizontally on a test-bed by applying a much thicker Conductanaut coat to both the die and cold plate. At this point, I see LM pooling on both surfaces, but the temps are uniform across cores and very very good when mounted.
My concern is that when I actually mount the motherboard vertically in a case, the liquid metal will pool over time to the bottom of the CPU die due to gravity. I have protected the CPU PCB with many layers of nail polish so that any spilled over LM will not short out the tiny resistors and caps on the CPU PCB, but as soon as the LM pools and shifts off of the top facing part of the CPU die, the cores situated at the top of the die will again lose contact with the cold plate and begin to overheat at load.
How can this be resolved, short of sanding down the silicon to make it perfectly flat and risking damage to a $2000 CPU (especially over time, as the protective layer at the top of the silicon will be sanded off)? Will the pooling be an actual issue or will capillary action and the surface tension of the Conductanaut be enough to prevent pooling due to gravity over time, when the motherboard is mounted into a common vertical position?
The problem is that the silicon die itself is slightly convex at its top. This apparently is a common issue - the CPU chip itself not being perfectly flat. It is less of an issue with conventional thermal paste, which is much thicker than liquid metal and can be applied thick to make up for any die/heat-sink/cold-plate warping or imperfections.
With liquid metal though, and I am using Thermal Grizzly Conductanaut, the general recommendation is to apply a thin coat to the silicon and a thin coat to the heat-sink. This accepted approach does not work for me. Only an oval portion of the chip, representing maybe 33% of its central silicon area, is coming into contact with the perfectly flat top-notch mirror finished cold plate I am using, an Aqua Computer Cuplex Kryos Next Vario with Vision. The periphery of the silicon, due to the convexity of the chip (i.e., ever so slight central bulge) is not coming into contact with the cold plate under tension (regardless of how much tension).
The result of applying two thin layers of LM to the CPU die and cold plate, respectively, is that some cores, probably the ones along the periphery of the die, are not making contact with the cold plate and jump to the Tj MAX temperature (or close to it) at load. There is an almost 60 degree C disparity between properly cooled cores (near the center of the die) and the overheating ones at the periphery.
I have been able to resolve this on my motherboard which is sitting horizontally on a test-bed by applying a much thicker Conductanaut coat to both the die and cold plate. At this point, I see LM pooling on both surfaces, but the temps are uniform across cores and very very good when mounted.
My concern is that when I actually mount the motherboard vertically in a case, the liquid metal will pool over time to the bottom of the CPU die due to gravity. I have protected the CPU PCB with many layers of nail polish so that any spilled over LM will not short out the tiny resistors and caps on the CPU PCB, but as soon as the LM pools and shifts off of the top facing part of the CPU die, the cores situated at the top of the die will again lose contact with the cold plate and begin to overheat at load.
How can this be resolved, short of sanding down the silicon to make it perfectly flat and risking damage to a $2000 CPU (especially over time, as the protective layer at the top of the silicon will be sanded off)? Will the pooling be an actual issue or will capillary action and the surface tension of the Conductanaut be enough to prevent pooling due to gravity over time, when the motherboard is mounted into a common vertical position?
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