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I need to transfer a working eight-pin 1/4" (4 mm) chip from the top PCB to the bottom PCB that has a non-working chip. Is there a special soldering tool for such micro soldering? Has anyone tried desoldering/soldering something this small?
Any special technique for this or can it only be done by machine?
This is easy to do.
Tools:
Soldering iron with fine tip.
Fine solder (0.5mm)
Tweezers
Solder wick
Alcohol and a small brush
Magnifying glass
Note that I do not include flux in the list. It isn't needed.
Remove parts
Flood all pins in solder. That is, put solder on the four pins on each side such that all four pins are covered in a puddle of solder.
Have a pair of tweezers ready in your left hand. Hold the soldering iron in your right hand (reverse if you are left handed.)
Lay the soldering iron flat to the table - it should be nearly horizontal.
Use the length of the tip of the soldering iron to heat all the pins on one side.
Grab the part with the tweezers - stick the tip under the housing. Like you would hold the part flat between your finger tips. Keep the heat on the solder.
Move the soldering iron from one set of pins to the other - quickly.
Heat the other four pins so the solder melts.
Because of the big puddle of solder, all 8 pins should now have liquid solder over them.
Pick up the part with your tweezers.
Remove excess solder from the pins with solder wick (if needed.)
Remove the dead one first for practice, then do the good one.
Reinstall good part
Use solder wick to remove the solder from all 8 pads.
Put solder on one pad. A pad on the corner, not one in the middle. (Pin 1, 4, 5, or 8)
Use your tweezers to put the part in place. Line it up so all the pins are straight over the pads.
Apply the tip of the soldering iron to the pin over the pad you put solder on. Press down lightly so that the pin sinks into the solder and onto the pad.
Remove the iron. Let the joint cool. The part is now held in place by one pin.
Get a small blob of solder on the tip of the iron.
Use your tweezers to straighten the part so the pins are really lined up properly.
Apply the blob of solder to a pin and pad diagonally opposite the first on you soldered. (If you did pin 1 first, then do pin 5 now.)
The solder should connect pin and pad. Remove iron, let the joint cool. Remove tweezers. The part is now held down by two pins.
Use your iron and solder to solder the other six pins.
When the other six are done and cool, go back and resolder the first two (they were poorly soldered because proper technique for soldering couldn't be followed with them.)
Clean up any excess solder with solder wick.
Cleanup excess flux with alcohol and a brush.
Inspect with a magnifying glass, resolder if needed.
Test device.
Done
When soldering the pins, many folks will recommend "drag soldering" in which you use loads of flux and a lot of solder to "smear" solder on all pins by dragging the (heavily) solder coated tip down the the row of pins.
I just solder them one at a time. It's easy enough, and I always know that every pin was soldered - even before inspecting.
The main thing is you have to get all the pins hot at once, the typical way to do this for a relatively small chip like this is to make big blobs of solder over each side. Moving back and forth with an iron can get both blobs molten at once and then you can pick the chip off.
There certainly do exist special tools, I have access to a set of heated tweezers (specifically an ERSA "chip tool") with large tips that can remove a chip like this in seconds and were a godsend when I had to replace 64 such chips. Hot air is another option but it can easilly damage nearby parts.
For re-soldering a chip with a pin spacing this big I would just do it one pin at a time. The trickiest bit is getting the chip lined up and the first pin soldered. Some people like to use magnification, personally I have found it more of a hinderance than a help. Some people like to do drag soldering, but it's harder than it looks.
Don't worry about solder bridges, they can be cleaned up easilly with solder wick.
The easiest way is with a rework station(some refer to it simply as hot air gun, but a hot air gun may be too big for this kind of work), you can heat all pins at once with it.
If you don't have one, what I would do is add solder to each side of the chip until all 4 pins on that side are connected by the same bib blob of solder. This way, you only have two places to heat up, and since this does not look like a board made to dissipate heat, you can heat one side, then switch to the other and heat it up while the other is still liquid. So basically get some tweezers, and keep pulling the chip, heat one side until it liquefies completely and quickly switch to the other side, you should be able to heat enough before the other side cools and the chip should come off. You can then remove the excess solder with the chip outside by various means.
Remember to not take too much time making the blob or heating up, as it could harm the component. I could not find a picture of what I mean with blob of solder, but I thinkyou get the idea, just add solder until the 4 pins are connected.
edit: I see you mentioned soldering as well, I do think it is easier: Remove any excess solder from the chip and the board. For this kind of very spaced pins, a solder pump should suffice. If you don't have one, you can use wick, if you don't have wick, try a copper wire with flux next to it, the solder should cling to the hot copper wire. After cleaning, position the chip in the desire position using tweezers, and solder one of the pins. Then solder the most distant pin from the one you just soldered. This will keep the chip in place, so now just solder one pin at a time. Use flux in every part of the soldering process.
While I usually use the "big blob of solder" technique, another technique I don't see mentioned is to use something to create a "heat bridge".
Take some big copper wire (2.5mm2 or so) and bend it so that it lies across all pins on one side, have it make a loop over the IC to the other side, then fold so it touches all pins on the other side as well. Heat the copper wire locally to solder to each pin, then use it to heat all pins at the same time.
Maybe not the best option for your particular situation, but comes in handy from time to time....
Hot air rework station with a nozzle that directs the airflow across all pins at one time - heat about 30 seconds and lift it off.
Search 'nozzle' here. I own several sizes for the chips I use. Pins on just the parallel sides, pins on all 4 sides, different widths and body sizes. https://www.mpja.com/
Another method is to use chipquik to give you more working time with melted solder https://www.adafruit.com/product/2660
