I guess the prices are high being a niche product. Technically hot tweezers are simply 2 soldering irons combined into 1. I am sure they can be diy’ed but Im too lazy knowing an off the shelf product exists.

(22-Jul-2022, 10:22 AM)kafie1980 Wrote: I guess the prices are high being a niche product. Technically hot tweezers are simply 2 soldering irons combined into 1. I am sure they can be diy’ed but Im too lazy knowing an off the shelf product exists.

I’m cheap and love me some diy.
You know those chopstick trainer things that hold your chopsticks together at the far end so all you have to do is squeeze them together to grab something? I’m thinking two small lightweight fine tip soldering irons, and modifying one of those chopstick trainers to fit the handles, 3d printed out of something flexible.

I’ve got a decent amount of troubleshooting and electronic repair experience, but, it’s primarily older through hole or point to point- ie: tube amps, solid state amps, etc.
The troubleshooting is the same- but these tiny tiny things take a completely different skill set to actually repair once I’ve narrowed down the problem. The bits are just so damn small- the top markings (if there are any) are so truncated that even if I can find them in a code book or database, it’s really a toss up on whether or not what I think it is is what it is.
I’m assuming that will all make more sense in time as I gain more experience- I remember having similar frustrations with larger through hole components until I became familiar with everything.

Anyhow, back to my adventures.

iFly4rotors kindly sent me some of his dead electronics to play with (thanks, dude!). Some of these bits are going to take me a bit of time before I’m comfortable with my skillz to tackle them, but I decided to try repairing one of the boards that doesn’t require removing and replacing a tiny IC.

This is an iFlight Beast AIO FC. It had an unfortunate incident where a wire got caught on something and pulled off the video in pad to the OSD chip. I’ve ripped off and burned off multiple pads in the past- so that’s something I’m familiar with.

But, it appears that the traces that go to this pad are internal. Damn multilayer boards.
I looked at it through my phone camera with my cheap clip-on macro lens- at first I thought I was lucky. I could see a little bit of copper.

I should trace that back. I know from this thread about repairing this board that video in is pin 22.
https://intofpv.com/t-remap-cam-input-to-uart-pad-on-fc

Hrmmm. Now, I’ve had a few drinks, so I could be wrong here… but I’m getting nothing to pin 22, and 0.9 ohms to pin 23. Yeah, pin 22 is pulled to ground- but it’s pulled to ground with a 75 ohm resistor. So, I’m thinking that little exposed bit of copper is actually a bit of the ground plane in the board.

Okay, cool. I’ll go ahead and isolate (insulate?) that little bit of exposed copper with some solder mask before I glue on a new pad.

Applied some solder mask, turned on my uv light to cure it while I smoked a cigarette… still tacky. Grrr.
Put new AA batteries in my uv light, smoked another cigarette… cool, it’s cured.

Put a very tiny little bit of 5 minute epoxy over the solder mask, cut a very tiny little bit of copper foil and pressed it down on top of the epoxy.

Think I might have poorly mixed or measured my epoxy, because it doesn’t seem to be sticking well.
Got impatient and applied a bit more solder mask to the sides of the new pad and hit it with the uv light.

Cool, pad seems to be well affixed. Tried to check for continuity between corners of the pad… nothing. @$&!. I didn’t remove the top plastic from the copper foil prior to gluing it down.
Scraped away the plastic with an exacto blade. Got continuity from corner to corner. Checked for continuity to ground and anything nearby- no continuity. Perfect! Drowned it in flux, new pad took solder fine.

Okay, I’ve got a new, completely isolated pad. Now I need to attach it to the right thing.

After some prodding and poking with some very fine multimeter probes, I found the 0.1uf capacitor connected to pin 22. The picture with the probe is pointing to the far side of the capacitor from the osd chip, which is where I need to connect to.

Holy crap that smd cap is tiny. There is no way in hell I could attach anything to it with a fine tip soldering iron. The smallest magnet wire I have is 32 gauge. I looked at some of my burned out motors to see if any of them had smaller gauge wire, but no.

Okay, I can do this. I tinned the tip of my 32g magnet wire, drowned the board in flux, set my hot air temp at 350 and 25% airflow (because I really don’t want to blow anything off the board, and that seems to happen when I go above 30% airflow).

Preheated the board, got the cap that I want to attach to all nice and melty, and… yeah, not enough solder to attach the magnet wire.

Okay, second try. I applied a wee bit of solder paste to the end of the magnet wire. Applied more flux. More flux! Carefully placed the magnet wire against the far side of the cap, held my breath, and removed heat…

I added a little more solder paste than I would have liked- but, it is solidly attached and it’s not bridged to anything near it that it shouldn’t be touching.

Awesome. Tacked it to the new pad, probed it to ground… and I’m reading 76.4 ohms to ground. Holy crap, did I actually just fix a tiny thing?!?

Won’t know for sure until I run a video signal through it- it’s late, so that will have to wait until later in the week. I’m leaving that excess magnet wire attached as a lead to attach a video signal to test- if all goes well then I’ll clip that excess off and attach the magnet wire/jumper wire to the board with some more solder mask.

I’m definitely going to have to double check that I’m not shorting to that fet- it looks awfully close.

What epoxy did you use? I assume the epoxy temperature rating was high enough to withstand soldering temperatures. Or probably the solder mask helped provide additional anchoring to the pad.

What thickness copper foil? I have used guitar tape in the past but that would curl up once heated and the best I found was salvaging copper pads from old pcbs and using just solder mask to hold it in place.

Epoxy- Bob Smith Industries. I’ve got no clue what it’s max temp is, but it was primarily applied just to tack the foil down and keep it in place when I applied solder mask to the edges to properly affix it. Ideally I would have used adhesive copper foil and skipped the epoxy bit, but I couldn’t find my roll of foil tape.
Since I mixed my epoxy wrong and it didn’t properly cure, the replacement pad moved around a bit as I applied solder mask- but it had enough grip to hold it down.

The copper foil- I honestly have no clue what thickness it is. I cut a bit off of some ReLife R007GA solder lugs. The thickness of the adhesive copper foil that I’ve used in the past is 1.4 mil, but even that can curl up on you if you don’t apply solder mask to the edges.

OK good to know.

I have been looking for the magical high temperature epoxy because I hate it when the surgically inserted pad moves on me and I was hoping to do it without solder mask just for the aesthetics or cleaner look.

Looks like most consumer grade “high temp” epoxies top out at 500 f.

If you do a search for “ultra high temp epoxy” you’ll find some options like Aremco 526n that can handle 600 f, but it looks like all of those ultra high temp epoxies are targeted at industrial use, I don’t think they’re available in very small quantities.

If you keep your temp below 500 f, I think this would do the trick as long as you made sure to overlap the edges of the pad and let it fully cure.
https://smile.amazon.com/Permatex-84102-...15&sr=8-10
Edit- maybe not. Apparently water causes this epoxy to fail.

I actually did a search for such a thing from the common AliExpress sellers for mobile repair stuff and none of the Epoxy stuff they sell is actually for solder pads. Usually to glue chips or other components on the pcb etc.

I see a lot of people getting away with just solder mask but the reality is that most of those pcbs are enclosed and not part of moving flying objects that are designed to be crashed.

I’m not real happy with that big ol’ solder blob coming off of that cap, but, I cleaned it up a bit and I’m confident that it is not shorting out and touching anything it shouldn’t. Ideally I’d go back in and remove some of that excess solder, but I’m not currently confident enough in my skills to say that yes, I can go in and remove some of that excess solder without screwing anything else up.

I did a quick and dirty wiring up to test things- there is good news and bad news.

The good news: I did fix the thing. I’ve got video through the osd chip. Winning!

The bad news: this solder mask is absolute crap. My carefully and lovingly placed pad came off the board. I’ve spread solder mask thinner and over wider areas in the past without it coming up with some heat.

So, I need some higher quality solder mask to reattach that replacement pad and to attach and insulate my magnet wire- but once I’ve got and applied that, this board is ready to fly.

Re-doing that solder blob on the cap will basically mean re-soldering the jumper wire to the cap. The minute you touch a hot iron tip it will such all that solder off.

I have used 0.1mm enameled jumper wire which works well for small components as it instantly heats up and sticks to the component/pads.

Did you scrape off the ends from the magnet wire to expose the copper when you tried soldering it first time?

I didn’t scrape off the ends, but I did hit the end and other point that I wanted to solder with a higher temp soldering iron after applying flux to pre-tin those bits. The higher temp burned off the enamel just fine, but I added a bit too much solder paste when I did the initial soldering of the magnet wire to the cap.

Would you suggest properly scraping the enamel off in the future?

At this point I need to redo my replacement pad, so I’ve got no problem also going back and reconnecting a new jumper wire.

The enamel does burn but leaves behind some residue which is why my preference is to scrape off the ends. This ensures I have a clean copper surface to solder to and the surface of the jumper wire gets coated with solder instantly when I touch it will the iron.

Thanks!

I will do this when I redo that wire, as well as in the future.

I’m not too happy with any of the solder mask that I have.

I’ve got 3 random brands, and even after sitting under a ridiculously overpowered UV lamp for an hour or sitting out in the sun, it’s not the hard thing that I was looking for.

You know all of those YouTube videos that show someone using some solder mask, hitting it with a uv lamp, curling up some magnet wire and applying some more solder mask, hardening it with a uv lamp, and then scraping it down to bare wire?

Yeah, it’s not that easy.

My pcb repairs in the past have primarily been through hole boards- you’ve got a lot more room to scrape and solder and glue on those vs these tiny little things.

Alright, screw the solder mask.

I decided to experiment with my cheap UV glue to see what its max temp was. I’ve got 3 bottles, labeled 180, 270, and 360.

I used some scrap prototype board to try to figure out max temps of each.
For each bottle I did:
A bare pad, a pre-tinned pad, and a blob for a temperature test.
The attached photo is a bit of a mess, because I hit each 
pad with increasing temperature until it failed.

I’m using Kester 63/37 and MG 8341 flux paste.

All blobs of UV glue got baked for 31 minutes under a 20w UV source.

The bottle labeled 180 couldn’t handle anywhere near soldering temps before it got all melty and goopy.
The bottle labeled 270 handled up to 300c for up to 5 seconds before it failed and got all goopy.
The bottle labeled 360 handled up to 330c for up to 5 seconds before it failed and got all goopy.

Okay! 330c for 5 seconds? More than appropriate for most of my repairs.

This could work.

Once dry it’s a bit flexible- kind of like a hard silicone. I can sand/scrape it down with a file to expose bare metal, but if I try to take a blade to it to scrape it down it doesn’t work, or it starts to wiggle.

That tells me that this glue could be appropriate for a repair with magnet wire where I can allow it a tiny bit of slack to move, but won’t work for a repair where there is no room for any movement.

I really wish I could find that magic super high temp epoxy that Kafie has been searching for.