bent scalpel thing for conformal coating removal?

[coppercone2]

I have some resistors that are 'varnished' on a pcb that when I try to remove, it messes with the text. I got lucky after I removed one resistor because I managed to use a chisel to scrape the remains off the bottom so I can read the resistance value.

I need something like a curved scalpel that I can go from the side of a resistor after its desoldered to cut the conformal coating without destroying the resistor. I want to get a blade underneath the resistor from the side so I can delaminate the conformal coating at the PCB, not sheer the resistor. I thought to maybe try to sharpen a piece of spring shim stock to act as a curved knife but I dunno.

The best I can come up with is to try to heat a exacto blade and curve it, but I am wondering if I can buy a tool kit. I think there are some clay sculpting tools that are KIND of what I need but they are too thick.

THF and Xylene work on the PCB but I have a feeling they will effect resistor text. They are the ohmite power wound resistors that kind of look like they are dipped in epoxy or something. Kinda jelly bean like, but its from a 2000 era design, so its not archaic (miller welder PCB). They solder the resistors value down and cover with goo.

[mastershake]

i use a dental scraper they work pretty good they have flat style ones and round tip ones.

[Neomys Sapiens]

Tools for Linocut ( Linolschnitt) arts might be a good try.

[thm_w]

Tons of useful shapes here: https://www.aliexpress.com/item/32933912116.html

[coppercone2]

but I want the blade curved in both driections. Those kits are only shaped correctly in 1 dimension.

Maybe I will try to heat a exacto blade and bend it into a shape, if that works then the 'weird' attachment kit might work.

[Neomys Sapiens]

I did read your description again. How would you think about the Edsyn desoldering blade or maybe a suitable size of shim stock? (Used feeler gauge blades can be substituted for the latter)

[coppercone2]

well I defiantly can get some stuff off using my pace lap-flo tool, but the problem is usually the close proximity to other components,I have a tough time getting in at the right angle to use it near capacitors n stuff.

I will try to sharpen a piece of shim stock when this breaks again.. brace yourself

[JDubU]

I'm having trouble picturing exactly what you are trying to do, but you can pinch a thin, double edged razor blade to cause it to bow downward in the middle. The angle of the curve is easily controlled by the pressure of the pinch.  Doctors use a version of this technique to thinly and precisely slice the surface of skin for biopsies.

[coppercone2]

hell the way I am thinking about it is like wedging a scythe under a log with some kind of tool that lets you be directly on top

The flexible stuff would work, if the bend radius is correct, I am not sure if it is, but it might be worth ordering because its cheaper then the talk is at this point. worth a shot

[JDubU]

Do a Google search for "Shave Biopsy" and select "Images" to see the technique.

[jpanhalt]

As suggested earlier, I don't see why the TS doesn't just use a real dental pick/scraper (not some of the Amazon fakes).  My kind dentist gave me a few.  They are professionally designed and tempered for such scraping.

"Shave biopsy?"  I'm too tired to wade through all the Google garbage.  Is that a new name for Mohs biopsy/surgery?  A choice for images of the technique didn't appear.

[JDubU]

Shave Biopsy:

https://www.mayoclinic.org/tests-procedures/skin-biopsy/multimedia/shave-biopsy/img-20007312

[jpanhalt]

That's what I thought you meant.  It is not meant to scrape away under something before unsoldering it.

[coppercone2]

So if I cut stainless shim, how do i sharpen something so flat? I have a feeling I will end up getting a mangled edge. I thought to tie it to my finger and try to 'pet' a wet stone with it to get a blade (like a fingernail extension).

[JohnG]

+1 dental scraper

Not the pointy ones, but there are ones will a short, flat edge. It's meant for scraping with good control (dental plaque must be a lot like solder mask), and works great for scraping stuff off a PCB (solder mask, conformal coating, even scraping down excess solder on SMT pads. It works fast and with good control which is important to avoid collateral damage. I have never gone back to an Exacto for this task.

On my dental scraper, it has one flat edge and one rounded. The edge looks small, but it works quickly.

John

[jpanhalt]

So if I cut stainless shim, how do i sharpen something so flat? I have a feeling I will end up getting a mangled edge. I thought to tie it to my finger and try to 'pet' a wet stone with it to get a blade (like a fingernail extension).

Why bother sharpening a "stainless shim" when virtually all modern razor blades have really sharp stainless shim built into the blade cartridge?  Even more important, why does it need to be stainless or even sharp?  Why not use a dental pick/scraper as suggested long ago?  Please answer.  Or is your purpose simply to prolong this thread into a discussion of Mohs surgery, and its alternatives.  Glad to oblige if that is what you want.

[thm_w]

but I want the blade curved in both driections. Those kits are only shaped correctly in 1 dimension.

Maybe I will try to heat a exacto blade and bend it into a shape, if that works then the 'weird' attachment kit might work.

They show them being bent in the photos.
May or may not have to apply heat to get it to keep a new shape.

[coppercone2]

just don't post in the thread if you dont like it

their annoying oval shaped resistors (~3W) that have 'softened skin' (maybe from the potting compound reacting with them) and are wedged between tons of potted capacitors, inductors, heatsinked transistors, etc. I have a dental pick kit but none of that worked well for the situation. if I braze a thin sharpened shim to one of the picks it might work. THF and Xylene both dissolve the text and its too cramped for any heated tools (pace) to fit in there to de polymerize the goo.

I think I need something along the lines of a extruded sharp question mark added to a standard flat pick so I can cut through it like a ice cream scoop. If I sharpen a shim, I will braze it to the end of a pick to make a tool that looks like a extruded question mark so I can insert it from the top and then rotate it to force the shim to cut the compound under the resistor.

I want to be able to target single parts on a heavy copper heavy soldered heavy populated conformal coated PCB without having to mess the entire thing up.

Think of like a modern computer power supply thats made cheap, working on them SUCKS. You need to do a shit load of desoldering to be able to use standard tools on weird shit like power resistors tucked under weird shaped heat sinks

something like this maybe, with the blade being cut to be slightly smaller then the resistor body. The action would be similar to a 'pry bar'. The closest thing I have seen to what I want is those heat gun tips that allow you to work with heat shrink with a little curved shield so you block the hot air from getting behind the wire


So you would place the tool at a shallow angle and then rotate it past orthogonal and it should complete the entire cut. Just prying at it with a needle after de soldering (the hook dentist tool) breaks the resistors.


Why stainless shim? Because I have like 2 big ass rolls of stainless steel sheet that I should be able to form into the correct shape, but it would need to be sharpened. If it works I can make a few different sized tools for different barrel components (i imagine a axial potted electrolytic to be a nightmare to deal with).

I feel if the tool is properly sized you can work at nearly 90 degrees, so long you see the component from the top it should be easy to remove, no matter the PCB density.



I am actually wondering if maybe a right angle slicer (right angle bent razor blade segment) might be good enough to just insert from the side too, without the fancy curve.

All the suggestions in this thread are appropriate for a ideal PCB, but not suitable for the 'marinara trench' formed by heat sink + transformer + soldered on daughter boards. This is really common in switching power supplies.

There is a medical tool that is kind of like a chisel meant for piercing the skin above the fingernail to drain fingernail infections (swolen fingertip cure) that would work if it was longer (it is plastic and you can heat bend it into shape),  but I have only seen it in a medical office, and the blade is too short (only 2mm or so, I would need like 5mm), but IIRC its made flat so it 'slides' on top of the fingernail to offer a very clean and repeatable cut in order to pop open infected tissue in such a way to allow for drainage with minimal injury.

I also don't like working with razor blades, and I don't like tools that require tension to bend into the correct shape while in use, that feels like struggling to me. (desperado shit).. save the nail clippers for defusing the nuclear bomb lol

[gnavigator1007]

I'm wondering if an attachment for one of those cheap electric grinding pens might work for you. Probably depends on how much space you have to come at it from the side while the tool is upright

[coppercone2]

I kinda want one but it seems like wank since I have a dremel pen. I am not sure if it would help, i really need to make  sure the coating is broke under neath the parts so the belly is not ripped off. I had bad experiences with those mini screw drivers.

The thermal tool can do the same job as the dremel of getting through the side goop.

[gnavigator1007]

Any chance you could post an image of the resistors on the board in question? Want to see if I'm visualizing this correctly.

[coppercone2]

Well I realized its been too long since I gardened..

I just made it with a larger older exacto blade (probobly 90's), I heated it to red hot for a little bit on a torch, bent it with pliers then finished putting a slight curve on it with some round noise pliers.

When I was trying it before, I was trying it on the sharp triangle tip blade exactos (standard), and they kept breaking. However, this exacto blade had a 'natural' rounded spear shaped tip (semi scalpel), sharpened on only one side.. anyway it survived the hot bend and the tool is good enough to cut into cardboard. I think it will work on the goo. I guess it has to do with the taper of the point, if its too sharp then its tough to heat-bend it.. I need to clean it up on the dremel to get a nice polish on it.. makes me think of reentry vehicles for some reason

Only problem is that its left handed. It would do nice with a sharpen on the other side but I should have done that before the bending, I only had one weird exacto blade.

I am wondering if I should reharden it, or if I should leave it anneled. I am thinking the flex and dulling will not be a problem compared to bending with soft goo?

[FrankE]

A curette