Replacing the CPU on an existing motherboard

I made passing reference in a previous post about being careful when upgrading the CPU on an existing motherboard, and I was recently reminded that I never got round to expanding on it.

So here is a quick post about it, especially things to watch out for and gotchas you may encounter.

Skip to the TL;DR summary if you prefer, otherwise strap yourself in for the long version.

The long version

I had decided to upgrade the CPU on an old PC.

Being quite savvy (although not savvy enough, as it turns out), instead of simply buying the fastest CPU for the socket architecture used by the motherboard, I actually took the time to google the specs of my motherboard to find the maximum CPU it supported.

This is very important, as although a CPU might fit in your motherboard, that is no guarantee that it will work. It has to fit and the motherboard has to support it.

A suitable CPU was sourced second-hand on eBay (since it was an old motherboard and an end-of-life CPU) and after a little while it arrived.

I disconnected the PC, moved it to the workbench, laid it flat on its side (ie. motherboard horizontal), stripped it down (whilst wearing an earthed anti-static wristband, of course), removed the old CPU, swapped in the new CPU, applied new thermal paste, re-fitted the heatsink and fan, closed up the case, took the PC back to my desk, connected everything up, and booted up, ready to enjoy increased speed.

Nothing. Not even a POST beep.

Hmmm. Was the new CPU a dud?

Further googling (on another PC) revealed that although my motherboard supported the CPU, it required an upgraded BIOS to do so. And, annoyingly for me, the motherboard didn’t support FlashBIOS so the only way to update the BIOS was from the BIOS, and I was faced with a chicken & egg situation of not being able to get into BIOS with the new CPU. The only solution was to swap back to the old CPU in order to be able to boot into BIOS.

XKCD image


I browsed through to my motherboard manufacturer’s website, downloaded the new BIOS and updater, followed the instructions for creating a USB stick for upgrading, and then set about swapping the CPU back to the old one.

I should point out that by this point it was getting on in the evening and by now I was a little “tired and emotional“, so I made the catastrophically bad decision to do the swap in situ with the case in a vertical orientation.

Yes, I know.

I got away with it on the swap back to the old CPU, successfully booted to BIOS, applied the updated BIOS, powered down, and set about swapping the CPUs again to re-install the new CPU.

It was at this point that it all went horribly wrong.

I slipped installing the new CPU, and it clattered to the bottom of the case. No harm done, I thought – just pick it up and shove it in again.

Yes, I know.

So, it went in on the second attempt, I reinstalled the heatsink, and then booted up.

Nothing. Not even a POST beep.

Hmm. So, another CPU swap to go back to the old CPU. At least I knew that worked.

Nothing. Not even a POST beep. By now, the sharks were circling.

I unplugged the PC, moved it to the workbench, carefully removed the CPU (or as carefully as I could under the circumstances), and had a good look.

Now, in the old days, CPUs had sturdy pins on them and the motherboard socket had a matrix of individual holes into which the pins went. If you bent a pin, then it could usually be straightened.

However, modern sockets have a network of hundreds of tiny sprung contacts, which touch little conductive pads on the CPU, and which are aligned so as to touch the correct pad. A raised ridge around the socket makes sure the CPU is accurately aligned so that the correct contacts on the socket are touching the correct pads on the CPU. A clamp then holds the CPU in the socket. This is a Zero Insertion Force socket.

Some of the contacts on the socket were bent and misaligned. The angle of the misalignment was consistent with the direction of the CPU slipping downwards in a vertical plane.  🙁

That’s why it is important to install CPUs with the motherboard horizontal, as then gravity is working with you not against you.

At this point I really should have come to my senses and called it a night and gone to bed.

But, instead, I set about trying to re-align the bent contacts. Somewhat predictably, I failed, and one of the contacts snapped clean off.

So that was that motherboard toasted, and I would have to order a new one. Time for bed.

Once the replacement motherboard had arrived, and I had very carefully installed the new CPU into it (on the workbench, laid flat, and properly observing anti-static procedures, like I should have done in the first place), and carefully installed it into the case, everything worked fine and I had a working system again, albeit at extra expense.


I put the damaged motherboard up for sale on eBay as “For Spares or Repair”, and the person who ended up buying it was kind enough to later email me and tell me that he had managed to get it working again, albeit with one bank of memory inaccessible, so that was nice.

I also sold the old CPU on eBay and got a little back for it, although not much.


TL;DR summary

  • Check that your motherboard explicitly supports the new CPU.
  • Check whether your motherboard needs an updated BIOS before you swap the CPU.
  • Always install a CPU with the motherboard horizontal, so that gravity is your friend not your enemy.
  • Always undertake these things when well-rested and sober, preferably in daylight.



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About DataHamster

The Data Hamster stores facts and information in its capacious cheek pouches and regurgitates them from time to time.

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