[sdiy] How to replace an SMT part, a tutorial - desoldering surface mount components

cheater cheater cheater00social at gmail.com
Sat Jun 25 18:50:30 CEST 2022

I've recently written to someone about how to replace an SMT op amp,
and I thought it would be useful here, so here it is. It's obviously
useful for other SMT parts as well.

You'll find that desoldering an SMT op amp is way easier than a
through hole one! Here's what you do - you might be able to do it
using a normal temperature controlled iron, but I can't /guarantee/
results - best to get a T12 soldering iron, they are way better. First
try the process on a broken piece of SMT equipment, like a piece of
electronics junk you can get from the recycling...

1. Take some normal, leaded, solder, like 1mm thick.

2. Take AmTech NCC 599 TF Tacky Flux. Get the original from a
distributor listed on AmTech's website. It's the most commonly ripped
off flux ever, and the flux is the most crucial part of this whole

3. The flux comes in a syringe, without a plunger. Go to the pharmacy
and buy a normal syringe with a plunger of the same size. Take out the
plunger and insert it in the flux syringe. You'll notice it goes very
slowly because it traps air - that's why you make a hole in the
forward circular wall of the plunger.

4. Put a bunch of flux on all the pins of the op amp. It will melt
when you apply the soldering iron.

5. Melt some solder on all the pins. Make a huge blob that covers all
the pins of one side, and heat it up. Do the same with the other side.
Take the op amp off with the tweezers.

6. Clean off the solder on the board by picking it up with the iron
tip and then gently tapping it on the edge of a metal container to
drop the solder off the tip.

7. Make sure there's a little solder on all the traces. Not so they're
bridged, but a bunch is good, it's fine if it's more than you'll need.

8. Optional step. Put a wetted soldering iron to the undersides of the
op amp pins if the pins are difficult to solder due to oxides or age.

9. Put more flux on the pads of the pcb.

10. Put the op amp on and hold it down with an Orange Stick. You can
buy them on amazon or in places that sell manicure supplies. They're
thin wooden sticks that are used for removing skins from the nails at
a manicure shop. You can use anything else, but this is what I use.

11. heat up one side of the op amp until the pads melt and the pins
assume their position. Do the same with the other side, then come back
to the first side again. The op amp needs to be in the right spot.

12. There is going to be too much solder on the board.

13. Make sure there's enough flux on the board around the pins. Add
more if necessary.

14. If there's any bridging, lift off the solder like before.

15. Take some soldering wick (the kind that has flux in it), you can
buy it locally in small rolls that look like UFO saucers.

16. Unroll a little wick off the spool. It is compressed into a thin
"wire". Spread it out using your fingers like a chinese finger trap.
When it's spread out it can accept way more solder.

17. Clean your soldering iron tip on some brass wool so there isn't
any solder left on it.

18. Apply the wick to the solder joints and the soldering iron to the
wick on top of it. The wick should be between the tip and the joint.
Use the wick to suck up excessive solder, but don't overdo it.
Eventually you'll get a feel for how much solder you need. This step
is important to have a result that isn't only technically functional,
but also looks good and won't cause problems in the future - gloopy
solder joints turn into cold solder joints more easily than solder
joints with good looking fillets. A fillet is the solder between the
board and the component.

19. Take the board and clean it off using 96% or better alcohol. You
can get it in various volumes at the pharmacy, tell them you need it
for soldering flux removal at the pharmacy. 90% alcohol bought at the
general store is not good enough. The easiest way to do this is to
take some cotton buds and clean the pads off, repeatedly, until
there's nothing left. If you've burnt the flux it'll be more
difficult. A better way is to put it in a container and soak it. An
even better way is to put it in a ziploc bag filled with alcohol, then
put it in an ultrasonic cleaner bath filled with water. You do this so
you don't waste so much alcohol. Make sure the zip in the ziploc bag
is resting outside the edge of the ultrasonic bath, and there are no
animals in the same room as the ultrasonic generator. As a bonus you
can clean your glasses and various other things - a US bath is a great
thing to own. You'll only need a small one for guitar pedals.

20. After cleaning everything off and making sure there's no sticky or
slimy residue (with your fingers or with a piece of paper tissue),
clean the board off under running, high pressure, tap water. Some
people insist you should use distilled water here, they are wrong. The
water you're cleaning with here will be contaminated with whatever is
on the board already, including flux residues that remain even after
the alcohol clean.

21. Soak the thing in more alcohol again to displace the water, then
let it dry. Do it twice. This alcohol is a separate bunch of alcohol
that you keep re-using over and over, keep it in a separate bottle.
Normal store-bought alcohol is fine, you don't need the expensive 96%
stuff. However, the 96% stuff is nicer, because it doesn't have
bitterants and other additives that store bought alcohol has, so your
PCBs don't end up stinking.

22. Lubricate the potentiometers and switches using contact lube (not
contact cleaner). Move them back and forth a few times to spread the
lube around. Of course, if you suspect there was any corrosion when
you got the board, use contact cleaner first, then do the alcohol
soak, then do the contact lube. You need to make sure the time between
first rinsing the board with water and then dipping it in alcohol and
finally adding lubricant is minimal, to prevent oxidation.

23. If you can't make out the small parts, make sure you have a LOT of
light. Halogen light bulbs in a small desk light are OK, but the best
way to do things is to solder on a window sill with cast over light
coming in - cast over sunlight has properties that make our eyes
resolve details better than usually. If soldering extremely small
parts get a microscope like the kind Louis Rossmann uses or a Mantis
or instead just be severely short sighted.

24. You're done!

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