Pointless Circuits

- Programming, Electronics and Breaking Things. -

Soldering Exercise: Hardwiring a microSD Card

For a very cheap hard drive extension, I decided to fit a 32gb microSD card into my Acer C720(p) Chromebook. As there exists no microSD to SD adapter short enough to not stick out of the Chromebook’s card slot, the only option I had was hardwiring the card. (I could obviously have bought a bigger M.2 SSD, but that would have been more expensive and well, where’s the fun in that?)

Opening the Chromebook, one finds a nice separate PCB, on which the card reader is mounted: Untouched reader PCB

Time for some handiwork! I carefully removed a big part of the card slot and soldered in some 0.1mm enameled copper wire. Note that the soldering points are behind the main contact areas of the spring contacts. If I ever remove the wires, the slot should be usable again as originally intended.



Using a fine soldering iron tip, I soldered the wires onto the microSD card’s pads. Trying to not destroy the card by applying too much heat, I touched the pads only for a few moments. I also cooled the card with an air can held upside down before applying solder.

Additionally, the card detection and write protection probes were both shorted to ground (solder blob in the lower right corner of card slot).

After putting some tape on top and reassembling the Chromebook the card was recognized correctly. I was happy until the first errors came up during formatting. The system log was full of “I/O error sector […]” and “Buffer I/O error sector […]”.

Failure!

Although I was pretty sure a maximum of ~50MHz on the clock and data lines whouldn’t be too much for those wires, I added some shielding. Still no luck…

To debug the problem, I removed the card again and put it into a modified microSD to SD adapter. Same issue, so the card was broken. I’m not sure what killed it, but I ordered a new one (only the second cheapest one at 12.99€) and was even more careful this time. That also meant no compressed air (maybe -40°C aren’t that ideal after all). While still not considering it strictly necessary, I put in new wires and trimmed them so they all had the same length. I also added aluminium foil for shielding again.

Putting it all together again… it worked perfectly!

While that was quite a nice challenge, simply buying a bigger SSD would have saved me quite a lot of hassle here. They’re not that expensive after all…

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