How to solder flexible printed circuit?

September 22, 2006 2:43 PM

Tips for hand soldering leads to printed flexible circuits?

I'm trying to solder thin, thin wires to a 4 conductor "printed" flexible circuit. Any tips?

I'm already working with a magnifying visor and a headlamp and some very small tools/cutters, but I can't seem to get solder+wire to bond to the solder-pads of the printed flexible ribbon cable/circuit, and I'm really worried about scorching/melting the ribbon/flexible circuit.

Should I try a cold-weld system? Is JB Weld conductive?


( Specifically, I'm trying to fix an old 3 series palm pilot. I managed to get moisture inside the digitizer pane for a Palm IIIC, and I'm sick of waiting for it to evaporate. And since I'm now in the Pacific Northwest, evaporation is unlikely. However, I have an older Palm digitizer, but of course the printed flexible circuit leads are the wrong width and on the wrong side of the glass pane. )

posted by loquacious to Technology (11 answers total)

Best answer: If you want solder to stick, flux is your friend (but don't use the acidic stuff used to solder pipes, because it will corrode electronics). It also sounds like maybe the pads are not getting hot enough.

If you want to dry the thing out, a few minutes with a blow drier should work.
posted by exogenous at 2:48 PM on September 22, 2006

Best answer: You can get conductive epoxy. I expect the impedance of random adhesives will be just high enough to not work, and just low enough to cause you trouble. Check this previous question: Superglue conductive?

I've never tried to solder on flexible circuits.. I wonder if they rely on the details of reflow soldering to make flexible circuits manufacturable at all - solder paste is a little glue like, and the oven produces very even heat, with none of the side forces that your iron will be causing. DIY reflow soldering isn't completely impossible, but it probably isn't very practical..

Oh ya.. A solution for easy SMT at home which sounds promising, glue the part down, and then solder it. If you get solder shorting pins, suck it or wick it away - enough solder to make the electrical connection will stay put.
posted by Chuckles at 3:38 PM on September 22, 2006

Best answer: i think chuckles' epoxy suggestion is the best thing. jb-weld or cyanoacrylate is not the right thing, you want to get a silver based epoxy.

i use this for making electrical connections to copper ribbon tape which is nigh-impossible to solder onto, and it works great. be careful, use a magnifying glass or microscope and a toothpick so you don't short between adjacent conductors.
posted by sergeant sandwich at 4:10 PM on September 22, 2006

be careful, use a magnifying glass or microscope and a toothpick so you don't short between adjacent conductors.

Ya.. Rework will be close to impossible. You should probably practice on something first.
posted by Chuckles at 4:14 PM on September 22, 2006

Best answer: I use a copper-mesh conductive-self-adhesive tape to extend out the pads, and then solder to the copper tape.

Unfortunately, I got it at a surplus place, which sold out and have never seen it since. They said it was made for NASA or something. But some day, I'm going to run out, so if anyone knows where to get some, post! :)

(Unfortunately, the self-adhesive copper tape you see everywhere isn't the same stuff - the adhesive isn't conductive)
posted by -harlequin- at 4:31 PM on September 22, 2006

Response by poster: Practice is really good advice. I have some spare printed flex circuits copper ribbon and such. The main problem is that I'm out in the boonies and I'm totally in MacGuyver-land, as my soldering iron sucks, I can't find my good iron or solder/wick, and, well, hand soldering onto miniscule traces on melty plastic substrate with a crappy, dirty iron just isn't kosher.

I have a feeling I should hold off on the soldering and try drying the OEM digitizer pane instead. I've tried leaving it atop an electric heater, placing it in front of a toasty heater for hours, leaving it in the sun, etc. (It's ok, I have the digitizer pane parted from the LCD pane.)

None have worked. The problem is that the layer where the water is trapped between the glass and the flexible plastic of the touch-screen is that it is such a thin space it's held there by extreme tension and capillary action. I've tried squeegeeing it out, too. It's literally less than a raindrop's worth of water in there.

(That's how it got wet. One rain drop on the exact bottom left corner of the bezel and capillary action sucked it right in.)

There is an airgap where the water can escape.

I'm considering toasting it lightly in an oven.
posted by loquacious at 4:40 PM on September 22, 2006

Response by poster: -harlequin-:

I used to use the old Wrigley's foil-on-paper gum wrappers for temporary printed circuit trace repairs, and it worked great for printed circuits on fiberboard - you just cut or peel a strip the width of your pads or trace and burnish it down at the contacts, but eh, A) I don't think they make those wrappers anymore and B) It's probably not suitable for a flexible circuit.
posted by loquacious at 4:43 PM on September 22, 2006

Response by poster: Best answers for everyone, 'cause indeed they are each in their own. And because I can't stand leaving it unmarked, or marking just one, when the answers are all helpful.

copper-mesh conductive-self-adhesive

I've heard wild rumors of this fabled material, but I have a couple of friends at JPL. If I had me a few strips of that I could just make myself a flexible ribbon cable extension.
posted by loquacious at 5:46 PM on September 22, 2006

Those flexible circuits are often some sort of silver alloy, or silver plated - normal solder won't stick to terribly well, if at all.

If you're careful, you can use silver-loaded solder paste - dab a bit on with a toothpick, then heat it with a clean iron freshly tinned in the same stuff.
posted by Pinback at 7:06 PM on September 22, 2006

DIY reflow soldering is actually really easy, Chuckles. I've done it with a varity of heat sources, including a paraffin lamp. A little flux, a little soldermask, and everything just slurps itself into place.
posted by hattifattener at 11:41 PM on September 22, 2006

Those flexible circuits are often some sort of silver alloy, or silver plated - normal solder won't stick to terribly well, if at all.

Worse, the plastic substrate tends to melt. I haven't yet found the magic trick for soldering them, and I've given up trying. Conductive epoxy is the right answer -- but if you need a very low resistance connection, you'll need the silver based conductive epoxy, which is expensive and a pain in the ass to mix -- it is very intolerant of improper proportions, and you want to mix as little as possible.

The trick to rework if you've glued the part down with cyanoacrylic is twofold -- CA is weak in shear, and acetone will dissolve it.

DIY reflow soldering is actually really easy.

Initial soldering, yes. Rework, less so -- the problem is getting enough heat down on *all* the pads (hard on a DIP, very hard on a TQPF, basically impossible on a BGA.) without getting so much heat down that you deliaminate the board. Mutilayer boards also tend to have a ground plane, which is also a heat sink (ever tried to desolder big capacitors from a motherboard? Don't with anything less than a temperature controlled 40W iron.)
posted by eriko at 8:28 AM on September 23, 2006