What heat-resistant compound for repairing pads?

[DavidAlfa]

Usually I don't glue them, just align/repair the broken pad, the solder will hold it fine.
However, with problematic repairs, you might need to lift the part again, then everything will come apart.

So, just out of curiosity, is there something that glues the pad neatly (avoiding bulky, uggly blobs) and stands some hot air rework without messing everything up?
If possible, leaving minimal traces, as if that part was never repaired. Just like like a sticker would.
I've tried several options, most glues will disintegrate while heating, UV compounds are weak, and they won't harden below the pad.
Epoxy compounds are too thick to work with such small parts, won't stand the temperature, and it's a mess to setup/mix/wait for fixing a simple 0.5mm pad.
Superglue burns/evaporates instantly.

Is there some miracle product I might have missed out?

[abdulbadii]

costlier is conductive ink marker e.g. https://www.amazon.com/Circuit-Scribe-Non-Toxic-Conductive-Silver/dp/B00OZATJ3A

[BrokenYugo]

Maybe a dab of high temp neutral cure rtv silicone?

Or what about that red stuff you see holding SMD parts sometimes?

[vk6zgo]

Way back in antiquity, when I did my first "High Reliability Hand Soldering" course with Telecom Australia, each student kit had a tube of a special glue with those very characteristics.
Unfortunately, after that, I never saw the stuff again, anywhere!

[thm_w]

Maybe a dab of high temp neutral cure rtv silicone?

Or what about that red stuff you see holding SMD parts sometimes?

RTV is good in that it has the highest temperature rating: https://www.grainger.com/product/PERMATEX-High-Temp-RTV-Silicone-Sealant-2GXY9
But not super solid.

Red stuff: https://www.farnell.com/datasheets/301458.pdf
"REWORKING
Once cured, BBIEN dispensing epibond adhesives can be removed from a printed circuit board by heating the board to approximately 80°C (176°F to 185oF). When the temperature of the surface mount component reaches 80°C (176°F), the adhesive will be soft enough to permit component removal using a "lift and twist" method."

Also needs to be kept in the fridge.

[Whales]

High temp, conductive and paintable:  https://www.intelliparticle.com.au/  (they have several products IIRC, but the lower end ones work like acrylic paint?)

I bought a small pot off them, but never got around to doing tests.  It's still on my shelf.

[bdunham7]

I've used clear nail polish, but never tried a rework.  Red RTV silicone would take the temperature, but I think adhesion would be iffy.  I've thought about getting an epoxy for this application and I think this might work pretty well:

https://www.masterbond.com/tds/ep17ht-lo

[T3sl4co1l]

How is epoxy "too thick" to work with?  All it takes is a dab from the tip of a needle.

Slow to cure?  Yes, such is the price of quality rework.

Mind you still need the right kind.  Hardware store rapid-cure turns to goo at a fraction of soldering temps.  Get the high temp, or "two ton", or whatever specialty industrial blend is appropriate.  Curing can be accelerated with heat (maybe even required), and with such a thin application there's no danger of self-heating so that's nice.

Tim

[BrokenYugo]

Revisiting epoxy, ordinary JB Weld claims 260 Celsius continuous and 315 for 10 minutes. Probably good enough, not sure you can buy the stuff in Spain though.

[T3sl4co1l]

Mind, you wouldn't want to use JB Weld here due to its fillers, it'll just barely not sit flat.  Or maybe it's little enough not to notice, dunno.  But the resin seems to be just the trick.  It's definitely something different, it has a more phenolic smell than the usual funky-skunky hardware store types.

(It also says "steel filled", but I haven't seen it be conductive in any modest width application.  Possibly/probably, for very thin gaps comparable to particle diameter, it's prone to shorting?  Would be interesting to use it to make a PCB purely from hardware store materials -- just for the S&Gs I mean.)

Tim

[Ian.M]

There are a few consumer epoxies with enough heat resistance to survive normal soldering temperatures.  Here's one candidate that claims to withstand up to 260 deg C: https://permatex.com/products/adhesives-and-sealants/epoxies/permatex-500%e2%81%b0-high-heat-epoxy-25-ml/

[wasyoungonce]

JBWeld is non conductive...tested it...cannot say if the voltage is high though...but think its ok.   I've used it to hold motor coils.

I have some Chemtronics CW2500 for pad repair, never used it yet.  You are supposed to grind out a Hollow" for the pad to allow for glue thickness.....clamp allow to cure.  Its very much like a good PCB resist and is supposed to be ok for very short application of heat.

I get the impression JBWeld is better.

[DavidAlfa]

The "Red stuff" is solder mask I already mentioned. I've already played with it, no good.
It needs UV to cure. X-rays might get through the pad, but UV won't.
Below the pad, it'll remain as a soup, the pad will only be held by the borders, will suddenly come off, and you'll see the nice masking template.
Rub with alcohol and get a nice empty square!
It's brittle when holding stuff, but it's almost impossible to remove without causing at least some scratches to the board.


Yep, high temperature epoxy was my next idea. But epoxy is bulky,  hard to apply in very low quantities.
The worse part is the 5-minute curing time. If fixing several pads or you run into any issue, you have a very tight time window to work with it.
If by any means it cures too soon... good luck fixing that!

I've found EP17HT-LO, which looked great: one-part epoxy (No messy mixing), heat curing and widthstands 600ºF/315ºC.
...but curing time is 350ºF /177ºC for 1-2 hours. Too much, specially for populated boards.
So yes, I guess epoxi is the way but have to find a good one. Not too fast, not too slow.
I wonder what's the epoxy used in the IC packages. That stuff hanbdles anything.
I've tried to burn them at 400ºC for 10 minutes, not only it did nothing, but the mcu still worked!

Long story short:
I have a board in which I wanted to test several ICs.
For those reworks I would normally use leaded solder, but... I ran out of solder.
Dumb me, I took an old lead-free alloy from the 2010s, melted terribly bad and was for emergency use only.
So yeah, stupid me for not wanting to wait, the removal was a nightmare and the board quality 4/10, so the pads started to fly away like dry leaves in the wind!
For 1-2 pads, meh, a simple wire soldered to the trace, but it's 30+ pads

[BrokenYugo]

With 30+ pads (presumably SMD or you would have just socketed it) damaged perhaps hack in a breakout board?

[DavidAlfa]

It was an existing board which I was trying to upgrade... No way, I searched, but there wasn't a damn thing for the weird 0.8mm pitch TSOP-54-II package!
I swapped 5 chips without any issue, but the damn ROHS solder left the board like Berlin in 1944 

[Kjelt]

You're issue is probably the remaining old glue layer that was between the pad and the fr4 carrierboard.
Glueing on old glue residue is not good.  Cleaning almost impossible on the thin copper trace.

[BrokenYugo]

I found a couple?

https://www.ebay.com/itm/153426970863?hash=item23b8f5c8ef:g:juQAAOSwDXxcl2g4

https://www.digikey.com/en/products/detail/chip-quik-inc./PA0210/5014815?utm_adgroup=Adapter%2C%20Breakout%20Boards&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Product_Prototyping%2C%20Fabrication%20Products_NEW&utm_term=&utm_content=Adapter%2C%20Breakout%20Boards&gclid=Cj0KCQiAk4aOBhCTARIsAFWFP9GRzaEe6S4Gub6XyBxicAJS1FsU3-PgJcizNAWNeQzrKCPg0o5bMRUaAgNkEALw_wcB

[DavidAlfa]

I remember searching and finding only $80+ sockets, too much for just testing once and personal use.
Now $15: www.aliexpress.com/item/32842544655.html

BrokenYugo, these boards would require lot of wiring, I don't think high speed SDRAM signals would like it
For now, I'll try the epoxi method, and use 138ºC (280ºF) bismuth alloy.