[sdiy] Desoldering techniques for chips that you want to keep?

[ASSI]

On Montag, 30. Januar 2006 14:11, steve jones wrote:
> I'm looking for info on how to desolder and remove chips from a
> circuit board without damaging either.

This is a pretty tough requirement and may not work with some types of 
board material or parts. The only way to get consistent results for 
many boards or chips is a repair wave solder station. If the machine is 
intended for repair work, then it should also have a set of tools to 
safely remove the parts that are to be desoldered without bending their 
pins. The boards should be evenly preheated to about 120°C on a hot 
plate to reduce thermal stress. Depending how many boards you have, it 
may pay to look around for a shop or institution that will let you use 
their machine.

Single ICs can be safely desoldered from most boards with a dull syringe 
(the pointy end ground down). Keep a few sizes so that the syringe fits 
into the hole and the pin into the syringe; get the solder liquid and 
slide the syringe over the pin, then until the solder solidifies. Don't 
do a second pin on the same IC until it has cooled down. The moral 
equivalent of this method for SMD IC is a thin blade of spring steel. 
With some practice you can desolder a PLCC package in four continous 
swoops (with the requisite cooling pause inbetween), one for each side.


The syringe method does not work too well with resistors or power 
transistors where the wire fits very snugly into the hole - if you put 
the board safety first, you need to cut off these and then push the 
remaining wire through the hole with a hardened steel pin of the 
correct size. Make sure the wire and solder does not re-attach to the 
board somewhere.

Last but not least, if you do not need to save 100% of the parts and the 
boards, but have many of them: get yourself a temperature controlled 
hot-air gun and practice to blow & shake the parts off the board. 
Especially for those big digital boards found in old computers this 
works surprisingly well. Preheat the board as with the wave solder 
machine.


Achim.
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