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Old 03-13-2023, 06:22 AM   #1
dicky96
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Default How to desolder power devices on PCB covered with silccone adhesive?

Hi guys
I'm working on some whatspower P222C power supplies. I have short circuits in either or both of the PFC mosfets and/or output transistors. I need to remove at least one heatsink to diagnose this but they are very hard to get to (the device screws) and there is this white sh!t everywhere on the PCB (which is double sided tracks at least)

Anyone can suggest the best way of working on this sort of thing?

see pics
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Old 03-13-2023, 04:16 PM   #2
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Uh, its looking bad... no space to work...
probably not the best way, but you can try with small diametar screwdriver (flat head 2mm maybe) and dig a continuous row near components you are want to lift. Be careful to not push into pcb too hard or can damaged some traces or smd components.

BTW I like your video
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Old 03-13-2023, 06:27 PM   #3
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

dental picks and maybe some hot air if needed .
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Old 03-13-2023, 06:34 PM   #4
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

What I have done in the past is to use a small side wire cutters to cut it away from the heat sink and bigger parts that you can identify and then use a small screw driver and carefully remove the rest of what will come off easily then go to to a small point pick

I have not tried a heat gun on this stuff so I can not comment on how well this works
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Old 03-13-2023, 06:47 PM   #5
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

i would remove the large caps to create room,
then unscrew the fets using a screwdriver bit in a mini-ratchet,
or just grab and turn the heads with longnose pliers

that had to be made in israel or ukraine - just saying!
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Old 03-13-2023, 09:18 PM   #6
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

That white sh!t (some times in other color) is a very common Silicone Adhesive often sold in cylindrical tubes containing 290mL to 340mL (or grams).
Common Europe (pt and es) brands: Soudal, Quilosa, Ceys, Axton, UHU, Wurth, ... You must have seen those around.

Don't feel intimidate by it neither the lack of space to use a screw driver!
Unsolder all transistors screwed to the heatsink, then use an universal plier and pull it out (the heatsink along with the screwed transistors).
Just make sure when pulling that you press down the PCB, closer to where the silicone meets the pcb, to avoid breaking the pcb (use a piece of wood for example).

Alternatively, if you're to afraid, follow sam_sam_sam method (btw, on this stuff a heat gun won't help in any way).
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Old 03-13-2023, 11:56 PM   #7
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Screwdrivers will do da.age if they slip. I use the sharp end of a plastic spider to remove the white stuff.

At least it would be able to be made in those countries stj if it came from Russia it would just be a musty potato.
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Old 03-14-2023, 01:22 AM   #8
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

lol
russia has to supply the u.s. with rocket engines to get into space.
did i say "has to"? not any more
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Old 03-14-2023, 06:22 AM   #9
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Check this pliers with blue grip maybe help, it has fine tip like tweezers but a very strong to grab a chunk of coating. Piece by piece.
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Old 03-14-2023, 12:01 PM   #10
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Quote:
Originally Posted by stj View Post
i would remove the large caps to create room,
then unscrew the fets using a screwdriver bit in a mini-ratchet,
or just grab and turn the heads with longnose pliers
+1
This is the method I use on PSUs that are cramped - remove large components that are easy to remove (such as caps) and then have the free space to remove other stuff.

The white silicone, I poke with various sharp tools (typically a small flat head screw driver) and pry small pieces at a time so as not to damage any SMD components on the board (if there are any on that side.) Pliers, cutters, and tweezers often come into play too. In other words, I tackle it with whatever manual hand tools that I got. I don't think heat from a heat gun would help any. Nor mineral spirits.
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Old 03-24-2023, 06:14 AM   #11
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Thanks for all the suggestions. I managed to work on this by removing the two large capacitors and the two large chokes between the heatsinks. I could then unbolt the transistors, put a piece of cord through the hole on the transistor, heat the three pins at the same time and pull them like teeth

Rich
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Old 03-25-2023, 02:57 PM   #12
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Default Re: How to desolder power devices on PCB covered with silccone adhesive?

Quote:
Originally Posted by dicky96 View Post
(...) I could then unbolt the transistors, put a piece of cord through the hole on the transistor, heat the three pins at the same time and pull them like teeth
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