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Chips at the back of the miner will always be very hot and will depend only on air, instead of termal conduction of a large heatsink. Meaning that they will overheat easily and thermal adhesives tend to lose their grip on higher temps. Which can end in heatsink unsticking from the chip.
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May be a good reason to try a new configuration with the S7 ... as in having the fans strapped at the top / bottom top rather than front / rear (that'll require the unit being stripped though as top and bottom are sealed). That way, I suppose, even the fan specs can be downgraded as the distance to exhaust has been more than halved. (just a thought!)