Desoldering a RoHS PCB. Need advice from a rework master on best way to proceed.

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Roflberry Pwncakes®

Joined May 18, 2021
7
So I upgraded my PC's power supply from a 350W to a 600W cause I got a GTX 1660 Super and a 2nd 2TB HD so I can run RAID striped... but I digress. Anyhow, that 350W wasn't doing any good just sitting there so I thought "hey, there's probably some great transformers, bridge rectifiers, and high capacity/high voltage caps in there that will come in handy for something evil and super sciencey." So I got the solder sucker out and set my station to 260º, which is what the RoHS guide says that Sn99Cu1 solder reflows at. Aaaaaaand nothing happened. It didn't even reach plasticity. So I kept bumping the temp up on my station. Finally, around 380º it became plastic, and finally became liquid at 460º and at those temps oxidation is like coronavirus on soldering tips. It's not like I have a board washer at home, and the board washer I used at work got shipped to Malaysia along with my job, so yes Virginia, I was trying to do rework on a dirty board from a smoker's house (not gonna mention who the smoker was cause I don't want a lecture on how bad tobacco smoke is on precision electronics). I was literally having to sponge, tin, copper wool, and then tin again and sponge again after every suck attempt to keep my tips from getting the oxidation chills. It wouldn't be an issue if I knew my station could take operating at the high temps for long periods... I mean, it's a hobby level soldering station, not a wire feed welder, I don't think it was made to operate at over 350º. When I turn the dial till the LED counter stops, it says 986º, but I reckon it's like a car, the needle goes up to 180, but something will blow before you even get close. But back to the desoldering... I had an idea to 'reverse solder wave' it by putting the board up in my oven (kitchen, not reflow!) upside down on makeshift corner posts right under the broiler and I let it have it. So I did that and removed the oven door and waited patiently on a chair with a pair of pliers and vicegrips to pull the parts down and free once the solder became liquid... and it never happened. I waited until the PCB began visibly warping and I thought the meniscus? Not sure that's the right word. Whatever word is used to describe the insulating outer packaging of components, that word. I was worried that would start melting off the parts I was trying to harvest. Do they coat RoHS solder with some kind of polymer, kind of like a super thin conformal coating to stop oxidation? Cause that would explain why my tips kept getting build-up on them like I stabbed my iron into styrofoam. There's got to be an easy way to desolder this board, but I don't know what it would be. I'm, or I was I should say, a guru at paste printing and wave soldering but that was before the EU forced the world to adopt RoHS and we could still use things like Pb in manufacturing. So if there are any masters out there wise in the ways of a lead-free world, I could really use some advice on how to work with this crappy modern tin-copper alloy.
 

Externet

Joined Nov 29, 2005
1,667
Hi.
It is not only about temperature; it is also about amount of heat. Bigger mass solder joints and components take much more heat (watts) to reach melting point. To make your life harder, transmisivity of heat by shape and size of iron tip, its amount of tinning and cleanliness, heat sinking effect and surface of contact plus skills also count.

A lit matchhead may be at 2000 degrees F, and a house on fire may be burning also at 2000 degrees F but the amount of heat is waaaay different.
 
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