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I'm facing the issue that an OLED display develops a short over time and stops working. The short always occurs at the solder point where the flat flex cable gets soldered to the PCB. Touching the shorted pins with the soldering iron fixes the short and I can use the OLED again but after at least a few hours the pins are shorted again. How can this happen? The OLED was hand soldered. Is there anything specific that has to be done to properly solder these connectors?

The short also always appears at the Chip Select and Reset line. This problem occurred on 3 different PCBs. So it's a bit weird that its never different pins.

Datasheet of the OLED module: http://www.buydisplay.com/download/manual/ER-OLED0.91-1_Series_Datasheet.pdf

enter image description here

Here a picture of the surrounding circuitry: enter image description here

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  • \$\begingroup\$ You should put them under a microscope and look what actually is causing the short, like moving around solder drops (like those seen in the image above the contacts) or other kinds of just messy solder jobs. I am not sure if its just the light and out of focus, but the pic looks like quite bad soldering. \$\endgroup\$
    – PlasmaHH
    Commented Oct 4, 2016 at 15:14
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    \$\begingroup\$ It was retouched many times. It got quite icky over time. There were no solder drops before the last retouch. If it was bad soldering, why always these two pins, not single other short ever occured. \$\endgroup\$
    – timonsku
    Commented Oct 4, 2016 at 15:39
  • \$\begingroup\$ Check the layout under the connector - are there tracks routed under the flex PCB that we can't see? Is there damage to the solder resist, that could short to the flex PCB? Kapton tape over the affected area before mounting the flex PCB... Or solder paste you can't get at without removing the flexPCB? A hot air gun may persuade it to move... \$\endgroup\$
    – user16324
    Commented Oct 4, 2016 at 15:46
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    \$\begingroup\$ I soldered it with tin and an iron, no paste used. The soldermask looks fine, so far I have not been able to damage the soldermask even when prying around with 380°c+ for a long time. Could a via maybe produce a short? I will try to place kapton around the pads for the next one. \$\endgroup\$
    – timonsku
    Commented Oct 4, 2016 at 16:14
  • \$\begingroup\$ I added a picture of the surrounding circuit. \$\endgroup\$
    – timonsku
    Commented Oct 4, 2016 at 16:19

2 Answers 2

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A few things that I can see that are probably not helping:

  1. Spacing between the pads is not consistent because of the traces taking some of the space between the pins. Between pins 8-9 and 11-12 the spacing is quite narrow

  2. The PCB CAD does not show the copper pours up to and between the pins that are present on the physical board, the pad-to-pour distance might have been too small for the board manufacturer to do well

  3. It's not a good idea to create an acute angle when connecting to a pad because this may not etch well, although usually not a source of shorts.

  4. The vias on the ribbon actually make the effective pad width wider, so it may be easy for them to contact the adjacent pin, especially if the space between the pads is narrow.

It's hard to tell what is going on underneath the ribbon- I would assume that the majority of the shorts are actually due to solder spilling between pins under the ribbon. It's difficult to solder well, so that's where a proper connector is your friend. You could try desoldering the whole thing, stripping out any copper pours that went near or between the pads, trimming back the fatter pads, and trying to solder the ribbon a bit higher off the board so that the ribbon isn't possibly squishing solder between the pins. Use lots of flux so that the solder really just wants to be on the pads.

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    \$\begingroup\$ "majority of the shorts" its just one short though and always the same. Thats why I am so confused about this. Thanks for the other advise, I already fixed some of that stuff up and waiting for a new prototype to arrive. \$\endgroup\$
    – timonsku
    Commented Oct 4, 2016 at 21:17
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Flux does help you from getting gaps in the joint which are susceptible to oxidation, If it is due to overheating and you are able to use a thicker flex cable, that should help with spreading the heat from the load and dissipating heat through a larger area at the joints.

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