Friday, 1 June 2012

Micro surgery

So here is how one repairs a flex PCB, well at least I think it is repaired, the proof will be in trying it on the 3D printer. Photos are from my phone peering into the eye piece of a stereo microscope.

Well, there is one very obvious break! The line above was also broken at the point where the Kapton covering ends. I made two light cuts through the tape and peeled it back and trimmed it. Then I scraped any remaining adhesive from the tracks.

Now the tracks were tinned with a lead/tin/silver solder, it is the thinnest I have at 0.5mm and a lower melting point than most.

To make the repair I took a single strand of tinned copper wire from a piece of multicore hook up wire and flattened it in smooth pliers, then I tinned it and soldered it in place using a flux pen to ensure the solder flowed even if it took me a while. I flattened the wire because I was worried that it might sit higher than the little nipples that form the contacts. So that is two done, finished! Yeah right.

On higher mag I could see that this poor line had a further two breaks!

Much clearer when the Kapton is peeled back

This time I used a thinner wire from the multicore that is found in mice leads. It is really nice wire for many projects so save those mice cables! Here you can see how I made a little handle using the wire, if the wire and tracks are tinned you just need to press down with the iron and they bond, extra flux does help. The wire was trimmed by placing the scalpel on it before wiggling it a little, remember this is super thin stuff!

The other repair was on a bend and does not look so pretty. This joint and the others where carefully shaved with a scalpel to reduce their height, probably not needed but did not hurt.

All done! Well not quite, when I found someone to assist I did electrical tests from the other end of the ribbon to this end and found two more breaks, again always at the boundary of the Kapton, this is simply where stress concentrates. I hope it works after all this effort. If it does I can start looking for an old printer to ravage.

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