Repairing my dump mistake - soldering bodge wires to tiny 0.1mm uVias? - Page 1
Forum section: electronics / repair

author: W3AXL
author_rank: Regular Contributor
timestamp: June 27, 2023, 02:55:35 pm
content: Well I've gone and done it now. Secure your test setups everyone! This is what happens when you accidentally knock your JTAG programmer off the desk with the FPC still attached to the board:As you can see by the lifted TXD/RXD pads from a previous hacking attempt, this is a very fragile PCB.So my question is - any chance those tiny little uvias on pins 1 & 4 are solderable? This is the only place the JTAG pins are exposed on the whole board. I've already tried to get them to wet with a heaping of flux and too large of a soldering tip, but they seem to not want to take any solder. The footprint is a 0.5mm FPC connector, so each of those pads is 0.3mm wide meaning those vias are 0.1mm wide at most.I'm also worried when reworking the rest of the pads they'll suffer a similar fate. They're already weakened by the connector being ripped off so I have little faith they'll stay on the PCB with any more heat.How would you approach this?

author: RoGeorge
author_rank: Super Contributor
timestamp: June 27, 2023, 03:19:29 pm
content: Identify all the ICs with JTAG from that PCB, then lookup their datasheets to identify the JTAG pins of each chip. JTAG pins on an IC are usually fixed (can not be remapped in software). Once you know how many chips have JTAG, then with a DMM on continuity, identify their order in the JTAG daisy-chain. They are usually connected like this:https://www.fpga4fun.com/JTAG1.htmlOnce you know which IC is the first in the JTAG chain, you can solder wires directly to the IC pins, instead of the missing pads from the former JTAG connector. Often there is only one chip in the JTAG chain.In theory, might be possible to get small pieces of PCB traces (from another donor board) and repair the one you need, like this:https://youtu.be/JtPmpqRWU2Qbut in practice I never tried that. On the same channel there are a few more videos about PCB traces and pads repair/replacement, but I don't remember any example with a via under the damaged pad.I would try to solder wires directly to the JTAG pins of the main chip.

author: W3AXL
author_rank: Regular Contributor
timestamp: June 27, 2023, 03:27:16 pm
content: Unfortunately, there's only one chip in the chain, and it's a BGA package. This is basically a worst case scenario.I did find a video from NorthridgeFix doing a very similar repair, which gives me some hope.I've got some 38AWG enamel wire and some precision tips on the way so we'll give this a shot. Worst case I should be able to get the 6 JTAG signals broken out of this mess.

author: coppercone2
author_rank: Super Contributor
timestamp: June 27, 2023, 09:59:46 pm
content: lol a trick is to get a long wire and tack it on then cut it after its soldered on rather then trying to position a tiny wire

author: thm_w
author_rank: Super Contributor
timestamp: June 27, 2023, 10:15:43 pm
content: Precision sharp conical tip should help, but no guarantee.I wonder if conductive epoxy would also work, but the problem is there is no quick way to test that its made a connection until you wire everything else up.If you actually do get it functional, cover it in epoxy so it cant move.

author: W3AXL
author_rank: Regular Contributor
timestamp: June 28, 2023, 02:18:34 am
content: Quote from: thm_w on June 27, 2023, 10:15:43 pmOh that's for sure. My main mistake was using the slightly incorrect FPC connector footprint - the metal ground tabs weren't fully secured and didn't provide much of any mechanical support when the connector got tugged. Almost all the force went into the pads, with predicable results.The perfect solution would be some kind of conductive epoxy that is solder-able. I could just fill the pad voids in and boom, problem solved. But like you said there wouldn't be any good way to check continuity.

author: Greybeard
author_rank: Frequent Contributor
timestamp: June 28, 2023, 08:57:38 am
content: What about conductive silver laquer?

author: wraper
author_rank: Unknown
timestamp: June 28, 2023, 09:18:28 am
content: Those vias are solderable. However unless they come from BGA chip, you should find from where they come and solder wires from there. It looks like there is ESD protection chip on the left side of the photo. Quite likely traces come from there.

author: SeanB
author_rank: Super Contributor
timestamp: June 28, 2023, 10:31:41 am
content: 38SWG wire, with the end tinned (hot soldering iron tip and lots of solder applied to burn the laquer layer off), and use a fine tip to heat the board, then dip the tinned wire into the hole. This should make a good contact, and then take the wire ends, and use a small piece of scrap board, preferably a break out with the connector on it, trimmed to the right size, and solder the other ends to it, then clean the lot, and use a blob of epoxy to hold the lot down. Done that before to attach wires to a clock PCB, where i wanted to get the cheap slide switches outside, and to a better switch, and apply power as well. Still working 30 years later on.

author: W3AXL
author_rank: Regular Contributor
timestamp: June 28, 2023, 06:25:18 pm
content: Quote from: wraper on June 28, 2023, 09:18:28 amUnfortunately as I mentioned earlier they come directly from the BGA OMAP chip's JTAG pins. I guess except for the ground & 1.8V lines, those can be sourced elsewhere. But the important ones no.