Electronic – Why aren’t CPUs cooled from below as well as above

coolingcpuheatheatsinkthermal

The transistory bits of an integrated circuit are approximately in the centre of the (plastic or ceramic) package. They sometimes get hot, and we cool them by affixing a heat sink to one side. Sometimes we just blow air over them with a fan. Some of this heat propagates upwards, but some must also go downwards towards the PCB. I don't know the ratio. The following is the underside of a Intel Core i7-7700K CPU dissipating 91W of heat:-

cpu underside

There are many connection pads. Clearly they act as lots of micro heat sinks that transfer some significant proportion of the heat to the socket /PCB. Indeed many surface mounted components dissipate heat through (via stitched) copper layers.

So if cooling is important (as for the CPU overclocking community), why aren't CPUs cooled from beneath the PCB as well, with say a fan?

EDIT:

Whilst the below comments are on the whole negative, there are two new items. One, there is a long thread on Overclock suggesting that a significant number of degrees could be taken off the CPU temperature with a fan on the backplate. And two, I tried it (admittedly with only a Raspberry Pi). I covered the top side with cloth to isolate the Broadcom CPU, whilst cooling the underside only with a 60mm fan. The fan reduced the maxed out CPU temperature from 82 deg. to 49. Not bad, so I think that this idea has legs…

Best Answer

They aren't cooled from below because they have pins on the bottom, and FR4 below that.

Due to having a much lower thermal conductivity, $$ \begin{array}{rrl} \text{Copper:} & 385\phantom{.25} & \frac{\mathrm{W}}{\mathrm{m}{\cdot}\mathrm{K}} \\ \text{Aluminum:} & 205\phantom{.25} & \frac{\mathrm{W}}{\mathrm{m}{\cdot}\mathrm{K}} \\ \text{FR4:} & 0.25 & \frac{\mathrm{W}}{\mathrm{m}{\cdot}\mathrm{K}} \\ \end{array} $$the material on the bottom of the CPU would transport much less heat.

Odds are you wouldn't want to surround the signals with metal which would change the impedance drastically, so metal on the bottom is more of an issue. If you did build a socket out of metal, it would need to be micromachined, which would be many times more expensive than a plastic injection molded socket. These things would prevent you building a processor socket that would wick heat away.

You could put a cooling block on the bottom of the board, but the PCB material (FR4) would reduce the cooling substantially.