heat sinks on PCIe cards for NVMe SSD

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It is all down to design. If you install a PCIe card, there will be not much space around it to keep itself cool. Nearby components such as RAM, CPU and motherboard chips will keep the temperature up. Synology have placed their NVMe slots far away from other components at the bottom. This helps them to keep it at reasonable temperature levels.

I hope this helps.