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nige the hippy
November 13th, 2006, 04:24 AM
Well day before yesterday I discovered a technique for desoldering that's miles better than anything I've tried before.

I normally remove chips by snipping the chip off it's legs then pulling them out a leg at a time with a soldering iron + a magnetised screwdriver from boards I want to keep, as it avoids pulling the plating through out, and this I did.

..but then, instead of using a solder sucker to clear the holes, I blew them out with short blasts of compressed air, from the blow gun on my compressor. Can be a bit spattery on the back of the board if you're not careful, but it cleared the holes a treat & with no re-tries & absolutely no damage. The dross on the back of the board mostly brushed clear, and then what was left melted back when I soldered the new chips in.

I'll have to try it with chips I want to keep next, see if it clears effectively round the leg.

oh... goggles essential!

chuckcmagee
November 13th, 2006, 08:22 AM
Too bad I don't own a compressor. Those "air cans" you buy to blow dust off of keyboards etc could get expensive if you had more than one chip to do. The $11 Radio Shack "desoldering tool" works fairly well. I found out the hard way you have to let it recover (get hot again) after doing 1 side of one chip. I was using it to steal the 8K ram chips off of some boards. In this case, I wanted to reuse the chips in another computer. I think I averaged about 60% success rate. Took me awhile to figure out that I needed to let the tip get hot again after some use.

atari2600a
November 13th, 2006, 10:26 AM
I'm fine with my Radio Shack desoldering Iron. I think getting a compressor & dragging it to my desk would be kinda inconveniant.

Chuck, while the iron's heated, hold it so the tip is south, & sqweeze the bulb for like 2 minutes straight. There's ALWAYS some solder hidden in there...

DKDeckmann
November 25th, 2006, 05:47 AM
Anyone tried making a rectangular tip that would fit between the ICs rows, say an inch long. Something massive enough that it would remain above solder melting temperature, allowing all 14/16 pins to be heated in unison, then the chip plucked off... I expect such a tactic would leave the holes filled so you'd need to cleanup if the board were to be re-used...

chuckcmagee
November 25th, 2006, 06:07 AM
Hmm, such a setup might end up frying the chip. I remember in the olden days, "they" were always warning you about heat transfer to the inside of the chip.

nige the hippy
November 25th, 2006, 06:29 AM
I made my own tips like that for desoldering surface mount chips, usually a bit of very cludgy metalwork with copper or brass sheet / tube bent to a square at the business end so that I could heat all the legs at once, and pushed in on all sides to fit snugly round a standard soldering iron bit. worked a treat, and did a lot less damage to the board than trying to desolder and lift each leg. you tin everything up quite a lot, and apply quite a bit of jelly flux too to help it all melt at once. a quick tap once the chip was off dislodged the chip & most of the solder, then run over the pads end to end with flux & a hot iron to even out the solder, ready to fit the next chip.

It was once possible to buy desoldering tips to fit on 16 pin chips etc but I haven't seen them recently.

Incidentally, my mate used to have a titanium SLR camera shutter blade that he used to slide under heated legs on quad flat pack chips, so it lifted the leg slightly & wouldnt stick to the solder, and he did a side at a time that way. it also worked pretty well.

doing the process as swiftly and efficiently is the essence, minimum heat for the minimum time.

The professional kit uses hot nitrogen gas squirted round all the legs.

DKDeckmann
November 26th, 2006, 12:14 PM
Maybe, but I don't see this being any worse than wave soldering or like they do now and put the entire board in an oven and heat board, parts, and all to solder melting temperature...


Hmm, such a setup might end up frying the chip. I remember in the olden days, "they" were always warning you about heat transfer to the inside of the chip.

nige the hippy
November 26th, 2006, 12:35 PM
Wave soldering is extremely quick but you have to preheat to avoid stresses, hot air ovens are a little slower.
You do have to be really careful to get a good heating profile, we used to run an instrumented board, and a data-logger (in a little insulated jacket) through the machines, to ensure that the chips didn't get heated too quickly, or reach excessive temperatures in the middle, sometimes it was a difficult juggling act.

As I say, the least messing around leads to less stress & generally less heating.

DKDeckmann
November 29th, 2006, 02:04 PM
Agreed. I figure a massive 16 pin sized iron tip that can get all 16 leads upto temperature in 2 seconds is going to give a lot less time for the heat to be conducted into the chip than a five minute bake in an oven...


Wave soldering is extremely quick but you have to preheat to avoid stresses, hot air ovens are a little slower.

wagon
December 26th, 2006, 01:53 PM
One way of getting parts off boards THAT ARE STUFFED is to use a propane torch - very quickly heat up the solder side of the board and give it all a quick tap - the part should just drop out! But it stinks and burns the board and you will need goggles. I have used this many times for recovering parts out of all sorts of pcbs. It does not heat the component too much if you are quick and careful.

Kendall.

The Lightning Stalker
December 26th, 2006, 07:26 PM
I've got a PACE MBT soldering station that has a vacuum pump. Sorta pricey but sorta worth it.

Tony Pendrey
February 7th, 2007, 05:52 AM
The propane torch method is what I have used for years. I read about it back in the late 1970's when it was often neccessary to plunder old mainframe circuit boards for odd logic chips etc.

The important thing is to have some kind of tension on the chip so that it comes out as soon as the solder has melted.

With practise you can do this non-destructively but I wouldn't recommend using this method on an important pcb.

To date, I haven't destroyed a chip, but these have always been it quite robust packages.

Cheers from the sunny UK