Repairing Flexible PCBs

[Paul Hovnanian P.E.]

I have one with a broken trace (a very fine crack). Any ideas about
repairing these things? Do those 'repair pens' with the silver-based
conductive ink work on these?

This PCB is the contact grid in a Fluke ScopeMeter, so continuous
flexing isn't as big a problem as it would be with a flex cable in a
hinge, for example.

Where are these pens available on line?

Thanx in advance.

 

[Robert Baer]

I have one with a broken trace (a very fine crack). Any ideas about
repairing these things? Do those 'repair pens' with the silver-based
conductive ink work on these?

This PCB is the contact grid in a Fluke ScopeMeter, so continuous
flexing isn't as big a problem as it would be with a flex cable in a
hinge, for example.

Where are these pens available on line?

Thanx in advance.

As long as the copper is exposed, that might work.
However, if the crack is near a connector, then it likely will be
flexed and stressed when re-connected, and possibly breaking the
conductive coating.
Does not hurt to try.
Possible sources: DigiKey, Mouser, Newark, Allied.

 

[Guy Macon]

Can you find the signal on whetever is at each end of the Flexible PCB?
If so, you can runs a wire in parallel with the bad trace.

 

[Paul Hovnanian P.E.]

Can you find the signal on whetever is at each end of the Flexible PCB?

This is the contact grid for the meter pushbuttons. The buttons are
connected in an X-Y grid. The break is between one button contact (which
is a part of the flexible PCB) and the end connector.

If so, you can runs a wire in parallel with the bad trace.

I'd have to solder on the flexible substrate, which won't stand up to
the heat.

 

[Guy Macon]

connected in an X-Y grid. The break is between one button contact
(which is a part of the flexible PCB) and the end connector.

I'd have to solder on the flexible substrate, which won't stand up to
the heat.

This is a long shot, but what color is the flexible substrate?
If it's Kapton (it probably isn't) it *might* be solderable.

I think that your best chance is to solder a wire on the PWB
that the end connector goes to, strip an inch or two on the
other end, lay the bare wire on it on the flexible PCB trace,
and glue it down with a series of small dots made from the best
conductive adhesive you can find.

 

[Joerg]

Hi Paul,

As Guy said, conductive epoxy and soldering are the only techniques that
have worked for me in flex circuit cases. But both have risks, epoxy in
that it can dissolve some stuff and soldering in terms of too much local
heat. Kapton should be able to take that heat though.

Check the button area. Is the material flexing when a button is pressed?
If so that might be the problem that caused the crack. Possibly you
could backfill to avoid that the repaired section fractures again..

Regards, Joerg

 

[Jim Yanik]

This is the contact grid for the meter pushbuttons. The buttons are
connected in an X-Y grid. The break is between one button contact (which
is a part of the flexible PCB) and the end connector.


I'd have to solder on the flexible substrate, which won't stand up to
the heat.

How about a low-melting point solder? Like Chip-Quik(160degF) or similar.

 

[Paul Hovnanian P.E.]

This is a long shot, but what color is the flexible substrate?
If it's Kapton (it probably isn't) it *might* be solderable.

Its clear with a slightly bluish tint.

I think that your best chance is to solder a wire on the PWB
that the end connector goes to, strip an inch or two on the
other end, lay the bare wire on it on the flexible PCB trace,
and glue it down with a series of small dots made from the best
conductive adhesive you can find.

I went with the 'PCB repair pen' option (the silver paint stuff). It
doesn't appear that the PCB moves very much since it lies against a flat
backing. Its nothing like trying to repair the flex cable at a laptop
hinge. The crack in the trace is nearly impossible to see. I had to
diagnose it with an ohmmeter.

I figure if this paint stuff fails, I can go to plan 'B', some low temp
solder. There is plenty of real estate on this PCB where I can do a melt
test on a corner first.

--
Paul Hovnanian mailto[email protected]
note to spammers: a Washington State resident
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