Can a solder blob vs. a 0 resistor in the antenna path affect signal strength?

[akasaka]

I've ordered a pack of ESP32s for a project I am building. However being the one I am, I've not paid enough attention while ordering and ended up receiving a batch of those with an IPEX connector fitted and the onboard antenna disabled.

Luckily, or so I thought, the ESP32-WROVER has a tiny (0102 size) 0 ohm resistor fitted on the board, which chooses between the PCB antenna and the IPEX connector. Thus instead of buying antennas and fitting them in an already tight space, I decided to try and swap that over.



Granted, I don't have a hotplate or a heat gun, so what I did is have a go with a soldering iron (after burning out the 2.4GHz LNA output on one — a grounded one, too).

Tons of flux and messing around later, I've managed to pull the 0 ohm one off and put in a blob of solder where it belongs. The blob has continuity to the PCB antenna (and the ground — which I presume is part of the PCB antenna, so is normal)



However, after installing it into a test device, I get very bad RSSI readings — on the level of -68 to -71dB. Just an unplugged IPEX connector got me -75dB, and a factory built ESP32 with the jumper in the right spot is getting -52dB in the same location. (That is, just across the room from the transmitter!)

Washing the flux to no avail, and redoing the blob many times or attempting to sneak in a tiny strand of magnet wire in there didn't help either.

Is there some specific trick to soldering in the antenna path that I'm missing?

[uer166]

That seems like a simple broken/open trace to me. Re-do it and ensure continuity. (The F antenna has a test point for that).

At 2.4GHz the difference between a jumper and a blob of that size would be hard to measure. Your photos also show a crap load of flux leftover which will have a minor effect.

[xrunner]

Is there some specific trick to soldering in the antenna path that I'm missing?

Nah I don't think so. Do what uer166 suggested ... clean off and try again.

[coppercone2]

I did this with a raspi mod, you just need to solder it well. 0201 bridge is somewhat difficult, some people might need magnification for that

[akasaka]

Tried that again, it seemed fine, went to clean the flux off and one swipe of the cotton swab yeeted both pads of the LNA output and the antenna.

F it, I just went and ordered a new set of ESP32s with antennas this time.    The other ones will stick around until I get a better thin tip or something. The one I currently have just doesn't seem to be capable enough to make a blob, with flux it sticks to the pads and not inbetween, and without flux it just does that other thing that leads to a -70dB.

[coppercone2]

i think your temperature might be a bit high

usually when pads come off, unless its really old, your running hot

and rosin flux, lol I tried SMT soldering with no clean flux, what the fux is that

I recommend you verify your iron with a clean tip before you attempt fine work again

[akasaka]

The temps are set at the usual 300ºC so not sure there. Plus the pads went after the thing was cold when I was cleaning the flux.

Rosin is sadly the only thing we have here in town, I wanted to catch a tube or two of chipquik while at Digit on a trip the other day but airline regulations would probably not let that fly (pun intended).

The tip itself was barely used and I made sure to clean it with the usual dance of sponge-refresher-sponge-solder-sponge before work to make it shiny. I think it's just so thin that it barely transfers any heat and definitely doesn't pick up any solder properly, maybe some other tin percentage would work.

[coppercone2]

ROSIN is good I just worried you were using no clean

If solder is not sticking then it needs replacing, its sadly not so durable when you are doing very precise things. It should wet perfectly, and you really should test it.

I basically think that if you over heat the pad, the epoxy becomes damaged. Thermal denaturing.

I think I used the smallest chisel tip that I could buy for the 0201 job, along with very thin solder, alot of flux, and speed. That raspi has a problem with a big part being a heat sink, so my initial plan to use thermal tweezers aint work too well.

I recommend just spending $30 on a new micro tip, leave it in a film canister for special hard jobs, figure out if you can clean the tip you have now later, its too hard a job to try to ressurect things And get fresh solder, flux and rosin. I really thought it was special job that people realllllllllllllllllllllllyyyyyy underestimate.


im not gonna lie and say I know how to fix all the tips, sometimes you can make them better with abrasives and stuff, but I see the number people pull on tips at work, that is troll station alpha

[akasaka]

Sorry for the confusion, yes it is the no-clean variety, rosin with alcohol basically.

The solder is sticking — just such tiny amounts that they don't make any sense and no help in the work. The tip is like 1/3mm in diameter, shaped like a needle.

The solder and flux I bought just a week ago, so should be fresh enough. I've built a bunch of PCBs with them already without much issue at all.

And you don't want to do any abrasive stuff on the nickel-plated tips which most of the modern "endless" ones are — one chip in the coating and the tip just won't wet anymore, plating it at home is a pain so it's bin material after that.

[Ian.M]

Scaling off the images, the offending pads are actually only 0.5mm square, and you are dealing with 0402 (01005 metric) zero ohm jumpers.  0402 is basically close to flysh!t - if you drop one, even right on the board its gone forever!   Therefore hoping to move and reuse the offending jumper is unreasonable, and unless you've got a strip of the same jumpers and a *VERY* steady hand replacing like for like is also not an option.   

Your best option would be to carefully wick off the pads, take a single strand of tinned copper wire no more than 1/2 the pad width thick, and tape it down with Kapton or aluminum foil tape with it laying across the centerline of the pads and its end at the far edge of the far pad, then solder it to the pads, ideally with the minimum extra solder needed to get a good fillet.  The wire can then be nicked with a scalpel where it emerges from the near pad, the tape removed and the excess wire gently bent to and fro till it snaps off at the nick.

However you aren't going to have any success with a fine conical tip in poor condition. See https://www.eevblog.com/forum/beginners/how-are-therese-conical-pointy-soldering-iron-tips-supposed-to-be-actually-used/

Personally I'd use the corner of a 1.2mm chisel tip, or maybe its end, bridging both pads.

[Doctorandus_P]

I'm not very good with RF stuff, but I guess the tuning / matching circuit is different for the two output versions.

[Randy222]

Define "affects signal".
Rf is sensitive to all sorts of things, gnd plane, whiskers, trace dimensions, solder blobs, micro lint.

Best to clean it up, then use toothbrush and rubbing alcohol to clean the area good.

[RadioNerd]

solder blob or 0402 0-ohm resistor will not make a relevant difference at 2.4 GHz.
Reliably bridging two pads just  with solder is surprisingly tricky however. I would therefore always recommend to use a 0ohm resistor in such situations.

I would also strongly recommend to preheat the whole module while doing any soldering work. this will allow you to use a lower soldering iron temperature, limit the thermal stress, lifting pads etc. and simply make your life a lot easier

[akasaka]

Good advice everyone, thanks so much. It will come in handy for later, but for now this batch is in the "maybe someday" drawer and a new pack of ESPs is on its way.

Doctorandus_P
Doesn't seem to be the case that the tuning is different, the schematic on Espressif's website states specifically this jumper is the only thing that needs to be switched over.
I did run the firmware procedure to erase the calibration data though, but that had no effect.

[eb4fbz]

What about ESD damage? Is your soldering iron ESD safe?