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A project log for ELLO 2M

The DIY laptop built entirely from PCB

KnivDKnivD 11/12/2016 at 21:0310 Comments

Well, it's time for the first bad report... :-(

Received the production samples. Excellent build. In fact so much excellent that it has created a problem on it own.

In the earlier prototypes I used to work with a different PCB factory which had more limited production capacity and less precise equipment. The result of that - working prototypes. This time the new factory has equipment which is apparently much better and has managed to run soldermask between the teeth of the keyboard comb pads. Result - keyboard not working unless some other techniques are used, such as additional rubber pads, etc. The reason for that is because the soldermask is taller than the exposed copper in the pads and the panel contacts can't create the needed short in the comb. Grrr... :-(

I have fixed the PCB file now to force soldermask expansion (should have done that long time ago!), but now the big question in front of me - what to do with all those first batch manufactured systems with damn non-operation keyboard combs? Trying to think of a quick solution to patch them so people can receive them sooner. So far my ideas are circling around additional self-adhesive conductive rubber pads (tried them with success), or some sort of thin elastic conductive sheet to stick under the keyboard panel (no idea if such thing exists at all).

It is very frustrating problem that needs a quick and clever solution...

Discussions

jareklupinski wrote 11/13/2016 at 07:09 point

is there any chemical solution that you can apply carefully to the soldermask with a q-tip or something that would melt or eat away the soldermask?

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David H Haffner Sr wrote 11/13/2016 at 07:13 point

Hey jarek319, yeah, apply some Acetic acid (30% solution,) and it will dissolve in about 20 seconds, (don't worry, it won't hurt the PCB or any other component.)

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jareklupinski wrote 11/13/2016 at 07:48 point

@KnivD maybe using some acetic acid on a qtip or similar cotton applicator to remove the solder mask?

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K.C. Lee wrote 11/13/2016 at 08:05 point

Is the now exposed copper is covered by ENIG or just bare copper?  My guess it is plain copper which can get oxidize and eventually get bad contacts which is not what you would want.

The more active metal (copper, tin etc) oxidizes when another dissimilar less active one (gold) are in contact under moisture and contaminants.  Back in the days they tell you that you should not mix SIMM with different contacts type than the socket.

Contrary to popular believe, solder is not a good candidate for reliable long term protective coating for contacts either.  That's why ENIG is used for reliability instead of HASL.  It form a black tarnish oxide just on its own.  Just rub the surface of solder coated material e.g. resistor leads, lead solder ec and you'll see that for yourself.

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KnivD wrote 11/13/2016 at 08:59 point

Shoud be ENIG. That is by specs of the PCB. I will double check though

Thanks about the acetic acid hint. I will check that possibility too.

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esot.eric wrote 11/12/2016 at 22:37 point

Just received some of this stuff: https://www.adafruit.com/products/1656 Haven't tried it yet.

Erm, just opened it, it is double-sided "tape", (more like a thin/clear film) that's adhesive on both sides, so might be a bit more difficult than I thought... I thought it was more like that conductive stuff they use to attach e.g. watch LCDs to PCBs. 

Though, it's thin enough, if combined with a thin layer of copper and squeezed really-tight to malform around the solder-mask, it could probably do the job. Can't vouch for how long it would remain so-formed.

What about blobbing solder on 'em? :)

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KnivD wrote 11/13/2016 at 09:02 point

Blobbing solder will create permanent shorts - a big no-no. Solder itself doesn't seem to be a good contact material. Tried two coated pads and they were very unreliable in making contact with each other.

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K.C. Lee wrote 11/12/2016 at 21:29 point

>some sort of thin elastic conductive sheet to stick under the keyboard panel (no idea if such thing exists at all)

https://www.adafruit.com/product/1361

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KnivD wrote 11/12/2016 at 21:42 point

Interesting... Thanks!

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K.C. Lee wrote 11/13/2016 at 08:28 point

FYI, they use a pressure sensitive conductive film on the original xbox controller for reading off analog X/Y/A/B buttons.  (Games don't bother using it.)  See black patch on right hand side.

http://blog.fictiv.com/posts/xbox-controller-teardown

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