Hi All,
I was given a Raspberry Pi 4B with what appears to be a failed PMIC (the MXL 7704). I'm interested in trying to access it via I2C, as discussed at https://forums.raspberrypi.com/viewtopic.php?t=314996. However, the idea of soldering fine wires to the two tiny, adjacent vias shown here (https://forum.lothar-team.pl//ext/dmzx/imageupload/img-files/59/2f3763d/1628663/5267edec474c17506ec3f29459da9793.png) is well beyond my skills.
I've removed the solder mask with a fiber glass pencil, but haven't had much luck soldering. I'm unable to create a fine enough joint on one such that it doesn't overlap both. On the plus side, I don't seem to have irrevocably damaged anything yet.
Is doing this sort of fine work something that can be learned? Is it a matter of having particular equipment? Or perhaps there is some special technique that makes it slightly less difficult?
Any tips, comments, etc, would be appreciated. Mostly I am looking to learn and build skills; being able to diagnose and/or repair the Pi would be gravy.
You don't have to tap into the signals at the via. Anywhere on the trace is OK. So what you can do is to take a fine scalpel and carefully expose an area of one trace in one part of the board, and then an inch away, expose an area of the other trace. Just use the scalpel tip to carefully scrape away the solder mask. Then tin each spot (this also lets you verify that you've removed the solder mask; it's very easy to leave an invisibly thin layer on the trace, too thin to appear green, but thick enough to do its job!), tin your wire, and then you can tack them on.
If you want to use the spots you've already exposed, then
add mask in the form of Kapton tape. Cover one via with Kapton, solder one wire onto the second via, then remove the Kapton from the first via, and cover the second one (wire and all) with Kapton and solder the first.
As others have said, flux is your friend. Using the methods above, you don't need to use a particularly thin tip.
Kapton tape is also a great way to secure the wires if you don't want to use hot glue.
If you do use hot glue, you can remove it afterwards using isopropyl alcohol, which sneaks under the glue and makes it pop off effortlessly.
Practice makes perfect, it's all in the technique.
I'd tin everything, leaving a little blob on each via, then add some flux and tack the tinned wires on, a drop of hot snot to secure and good to go. I'd probably do it with the bigass Hakko K tip I usually use too. You fix bridging with wick, or by picking some of the excess solder up with the iron and then bang it over the edge of your trash can to knock the blob off the iron. Again, flux helps, you really can't work without it.
Don't bang your iron on anything. Many irons use ceramic heater cores that could be damaged. If the drop of solder is big enough, you can just shake it off by hand. If not, I highly recommend a tip cleaner with a rubber wiper lip. My Ersa came with one, as do modern JBC stations, whose rubber lip is my favorite. Weller makes a great tip cleaner, the T0051512499, with a fantastic rubber lip. (Avoid the smaller, cheap WLACCBSH-02, which unfortunately leaks solder dust out its bottom.) JBC also makes a standalone tip cleaner, the CLM, but it's ludicrously expensive.
The other thing that's nice is Pace's "fiber cleaning tool", which is actually just an ordinary all-metal chalk holder clutching a bundle of woven fiberglass tubing (bound with a piece of heat shrink at one end). You just brush off excess solder, flux, etc.
A good amount of liquid flux is also a big help. Look up YT vids by Louis Rossmann on soldering cell phone repairs.
FYI, Rossmann floods his boards in gel flux, not liquid. Liquid is best for applications like this one that don't take a long time. Gel is much better for extended operations like SMD rework. Also, I wouldn't necessarily look to Rossmann for soldering advice, as he does some things wrong, which not every board will tolerate (like rubbing SMD pads with desoldering wick).
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