Duet 0.6 - CPU cooking

Hi folks,

Any ideas what could cause the microcontroller to get too hot to touch within 30 seconds of applying power. It does this even with no other connections made to the board, the SD card removed and running on USB power alone.

3.3V rail is getting pulled down to 2.9V

Cheers,

Steve

Edited 1 time(s). Last edit at 06/01/2017 07:13AM by stevelup.

Does it do the same if you hold the "erase" button at power-up? If not, then it is probably a short on one of the I/O pins, but if it still draws too much current when it is not initialised by the firmware, then the CPU is most likely damaged

Dave

Yes, it cooks even when in the bootloader. You can start uploading a firmware but it bails out shortly after commencing the verify.

Dead CPU then... Weird thing is it works for a short while - even brings the ethernet up and loads the web GUI!

You could try disconnecting everything from the duet except the USB just to be certain, but it sounds like something on the silicon has shorted and is drawing too much current. In many cases such a fault will not stop the rest of the silicon working until it gets too hot and goes into thermal shut-down.

Replacing the CPU is possible but difficult, and a new Duet is probably the best option.

Dave

It warms up the same when fed with the external power and USB disconnected.

I'm fine with replacing the MCU - have a hot air rework station. What is causing me trouble is finding anywhere to buy the chip from!

That said, this is a good excuse to invest in the new Duet

Got some...

Slightly excessive packaging for two chips

IMG_2767 by Steve Lupton, on Flickr

IMG_2768 by Steve Lupton, on Flickr

They probably have a sealing machine that only works on a full tray. The vacuum seal and desiccant is to prevent "popcorning" during soldering - ideally should only be unpacked on the day of use. If left out, water vapour can enter around the legs and condense inside the chip package. A microscopic amount of water can flash boil during soldering and crack the chip inside the package, sometimes with no outward sign of damage. Not all that likely during hand-soldering, but I've had it happen in reflow ovens or hot-air rework, in one case caused 70% failure of a batch of boards. Can be prevented by baking the chip for several hours before use.

Dave

Good luck with replacing the chip! The tricky part is removing the old chip without lifting any of the traces. Do not attempt to prise the chip off, but use a cheap vacuum pen to pick it up when the solder is molten. It helps to have the board on a hotplate heated to around 120C, that way you don't need to use so much hot air.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

It lives!

I didn't have any problem getting the old chip off:-



... but I did manage to bend the first three pins on one side when installing the new one creating three unresolvable solder bridges.. That was a genuine bugger to sort out - had to heat it with the hot air gun and poke the individual pins with a needle to get them back in alignment.

All up and running again with the latest firmware.

This is great work! I know that the board is not cheap but to replace the cpu just like that makes the rest of us look bad

I took it as a challenge - I've never changed a fine pitch SMD device like that. I brought a load of scrap boards home from the office to experiment on first until I thought I had the knack!

Part of me thought 'excuse to buy a new Duet', the other part thought 'yeah, you can change that chip'!

Well done! For solder bridges, I find "Solder wick" is easiest to suck off the solder. e.g. [uk.rs-online.com]

Dave

I didn't explain well. I'd bumped the first three pins on one side so they were actually bridging the pads. No amount of solder wick would sort that

Congratulations! You have succeeded at doing something that I failed to do twice last year. I have better equipment and more experience now, so I hope it will be third time lucky.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I bought a second hand stereo bench microscope - best gadget I’ve ever acquired. Did a final inspection with that before powering on and spotted another pair of bridged pins that were simply not visible to the naked eye.

Prior to this, the smallest stuff I’d done was SOIC and 0603 - both fine with the naked eye and a steady hand, but (for me at least) this was a step too far!

I already had a decent soldering iron (Hakko) but my hot air station was the cheapest one CPC did (think it was £44) - yet it seems perfectly adequate. I’ve spent way more than a new Duet would have cost, but acquired some skills, not to mention gear for my workshop, so all for the greater good!