Secrets to v fine pitch PCB trace repair? - Page 1
Forum section: electronics / repair

author: JoeBean
author_rank: Contributor
timestamp: October 14, 2015, 09:44:03 pm
content: Hi everyone,I've been busy over the last few months and haven't been around much - still haven't put up the review of the Ersa i-Con Vario 4 I picked up last year although as a brief comment I've been using it daily and it's been working perfectly, although sourcing parts from Ersa NA is an exercise in futility.At any rate, I actually have a soldering-related question that I thought someone more experienced might have suggestions for: I've been doing board-level repairs on phone/tablet/computer PCBs with a new side-business I started but I've been stymied trying to find a reliable solution to one problem: There's a common issue with some Apple phones where screws are swapped during reassembly and one screw that's <0.05mm longer than another (some sources say 1/2mm and some say 0.1mm, but it's actually just under 0.05mm) and it damages traces that are on the PCB underneath the screw. Here's a photo I found from Googling the issue for an iPhone 5S to give you an idea of what's happening:To give some perspective, those traces are about 0.075mm wide and the spacing between is about 0.1mm.Anyway, I've been repairing a number of boards that have this issue and I keep thinking there must be an easier way. My normal go-to repair procedure for PCB traces is to expose some remaining trace, tin it, and solder a jumper between. I've used that system with 40AWG magnet wire successfully but it's a huge pain and getting more difficult as I have some neurological problems that cause my hands to shake a bit. It's almost impossible to get the jumpers held in place fpr soldering (usually I'd tape them with kapton but that's impossible here). I've tried silver conductive paint but the smallest amount on the head of a pin will flow to bridge all the traces, thus necessitating removing some material between the traces after it dries. It works, and it's less of a pain than soldering, but I'm hoping there's something better.I was thinking about getting a track repair kit such as the CircuitMedic one (which seems ubiquitous as half or more of the sites selling the kits are selling rebranded CircuitMedic) or the Best epoxy kit, but it seems the smallest trace anyone makes is 0.005"/0.127mm wide, which would leave VERY little wiggle room.Does anyone have any alternative solutions/wild ideas that might work?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 14, 2015, 10:06:39 pm
content: Solder paste may be an easier option as it avoids the need to tin, and means you're placing a wire on a flat track, not one rounded by the solder after tinning.I assume you mean tinned copper wire, not magnet wire, as you don't want something with insulation on.Also, stretch the wire first, so it's completely straight - this also hardens it so it stays straight.

author: Rasz
author_rank: Super Contributor
timestamp: October 14, 2015, 11:30:43 pm
content: There you go, and good luck

author: JoeBean
author_rank: Contributor
timestamp: October 15, 2015, 01:19:17 am
content: Quote from: mikeselectricstuff on October 14, 2015, 10:06:39 pmSolder paste seemed like a good idea to me as well. But the problems I ran in to were a) hot air can't be used (even the slightest airflow will blow the wires off location, and because of the size you pretty much have to do all 7 at once, and b) if an iron is used it will wick up the solder paste before the traces/wire, assuming the tip is clean, and if the tip is wetted with solder already then that negates the point of using paste. I may very well be going about it the wrong way - I've usually only used paste with stencil, so I'm not very familiar with hand soldering it.The wire is referred to by the manufacturer as magnet wire - it isn't tinned, but it's not insulated either. They refer to their pre-insulated stuff as "insulated magnet wire". The guy on the phone said anything under 30 gauge they call magnet wire for some reason or other that made no sense to me. I haven't been able to source any pre-tinned or silver plated 40AWG. Probably not looking in the right place.Quote from: RaszYes, thats the procedure I tried to describe above. It works, but it's tedious and not very conducive to oscillating handsShe has quite the skill, excellent work even without the background activity.

author: veedub565
author_rank: Frequent Contributor
timestamp: October 16, 2015, 07:50:37 pm
content: That's some seriously fine soldering work, I'm amazed anybody even attempts it never mind succeeds. And I've spent 15yrs in the mobile phone industry.Back in the day we used to have a machine that could replace torn off pads etc, but that was a long time ago and the pitch/track thickness was much greater. Not sure anything exists now to do the same task as the boards aren't really intended to be repaired anymore.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 16, 2015, 08:13:26 pm
content: Quote from: JoeBean on October 15, 2015, 01:19:17 amWith a very fine point iron, wiped of any solder - if you can touch it on the wire, applying pressure so it goes down onto the track, that can work.QuoteMagnet wire usually refers to enamelled wire.QuoteStrip some of the finest stranded wire you can find.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: October 17, 2015, 11:07:23 pm
content: Quote from: JoeBean on October 15, 2015, 01:19:17 amHave you considered gluing the wires down first?Cut and pre-fit the wires then put them aside.Put a dab of (perhaps?) 5 minute epoxy in the middle area between the points where you're going to solder.Place the wires in position. The viscosity of the unset glue hopefully will hold them in place.Wait for the glue to set.Apply solder paste (or maybe do this step before applying the glue).Use hot air to solder all the wires at once.This is just a thought. I haven't actually tried it myself.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: October 17, 2015, 11:18:58 pm
content: Quote from: JoeBean on October 15, 2015, 01:19:17 amHow about from the shielding of a coaxial cable with a braided shield?

author: JoeBean
author_rank: Contributor
timestamp: October 18, 2015, 12:14:12 am
content: Quote from: MLXXXp on October 17, 2015, 11:07:23 pmThat's a very good idea. Even if I solder without using paste that might work. Normally with larger traces I'd hold them in position with kapton tape and solder away, but as you can imagine these traces are so super fine that tape just won't work. But epoxy might work. I know Permatex also makes high temp epoxy for engines so if the regular stuff has an issue that might work. I'll have to give that a try, thanks for the suggestion!Quote from: MLXXXpI have some RG6 that I tried but the wire is closer to 24 gauge and too large. I know some coax has super fine wire though to allow for flexibility when bending, so that might work. I also though about strands of soldering wick but haven't had a chance to check yet.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 18, 2015, 11:55:26 am
content: How about tearing off the TAB driver chips from broken LCDs? This is a 1 mil thick flex PCB with hundreds of super fine pitch (like 1/1 almost) straight traces on it. Sand off the masking, cut to size, align. Think of it as a miniature zebra strip.I don't know, never tried it. But there's always a TV in the garbage somewhere!

author: JoeBean
author_rank: Contributor
timestamp: October 18, 2015, 11:59:05 am
content: Quote from: Alex Eisenhut on October 18, 2015, 11:55:26 amThat's a thought. I have some old LCDs sitting around that I could try. Also lots of broken phone LCDs with fine pitch at the LCD connection point:

author: poorchava
author_rank: Super Contributor
timestamp: October 18, 2015, 12:31:40 pm
content: Perhaps an old manual wedge bonding machine would work? The kind for example semiconductor labs use for one-off chip bonding. I imagine that getting your hands on even a very antique machine like that might be very hard and costly...Sent from my HTC One M8s using Tapatalk.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: October 18, 2015, 09:24:14 pm
content: http://www.circuitmedic.com/guides/4-0.shtml

author: Rasz
author_rank: Super Contributor
timestamp: October 19, 2015, 03:31:58 am
content: Quote from: John_ITIC on October 18, 2015, 09:24:14 pmsure, if you charge $300 and hour fixing airplane grade equipment, otherwise just use fine wire pulled from random coil.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: October 27, 2015, 02:38:02 pm
content: A lot of cheapie walwart ac adapters, and 3.5mm audio cables, use super thin stranded wire shields... cut a length you can hold in your hand as a handle, use one strand (36 -40 ga?) as your trace repair wire. A microscope is nice to have, but a magnifying headset will also help.

author: steve207a
author_rank: Contributor
timestamp: October 31, 2015, 06:59:40 pm
content: Hifrom an experianced hand shaking solderer dont try to repair it at point of damage trace tracks further away if possible and make jumper wires at different points that way your not soldering at one place and bridgeing traces sometimes as you trace back you can find a larger track to make things easier but that depends on the circuit .The wires look more unsightly but the repair is stronger and better than trying to fix it at one placeregardssteve

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: November 01, 2015, 05:51:51 am
content: You could try whiscut wire from an old dvd laser pickup.Cut the wire down to size,place some liquid solder tape over the middle with a needle then place the wires on top with tweezers,line up the wires and wait for tape to dry then solder the wires.Put more liquid tape over to finish.Afew other options are copper foil tape or car windscreen/rear heater repair fluid.

author: KL27x
author_rank: Super Contributor
timestamp: November 03, 2015, 08:12:58 pm
content: Lots of better ideas, already. I like the ripping up old FPC connector idea. Or perhaps you can make your own FPC board, with a large void in the soldermask at the appropriate intervals over the vias. Get a HASL finish. It's sorta doubtful if you could make it reflow, properly with vias that small. You could make them a big larger than the traces and stagger them in two rows. I mean, if it's that hard to get a connection, perhaps you could even make the vias so large they technically touch the traces on either side, and yet it would only connect to the trace directly underneath when flowed? Obviously, you could only do this on the outer set of vias.Another idea, I have no idea how to spec this, is to make a double sided PFC board with traces that terminate in drill holes and exposed soldermask around the holes. But spec those holes to be plated through holes, so there's copper wrapping all the way around the end of the traces from top/bottom.But here's another idea:Might it help to jumper every other trace? Then cover that with a dab of epoxy and a tiny square of kapton. Then go back and jump the other wires over top of that, a little further back? The epoxy will keep those wires from moving (when you inadvertently reflow them making the other half of the connections) and the kapton will insulate the bus wires.Also, maybe it's possible to use a colored epoxy and a tiny brush (like a cat whisker?) to paint on a soldermask over every other trace that you don't want to bridge?Overall, I suspect you may have already figured out the best way to do this. Perhaps a microscope and a suitable brush would help in the application of the conductive ink. I believe I read that cat whiskers are the brush of choice for one of those guys that makes paintings on the heads of needles.

author: vector31337
author_rank: Newbie
timestamp: November 17, 2015, 11:58:36 am
content: Copper wire from the cell phone vibrator motor will work best.