I've used several different techniques for removing flat packs, all with varying degrees of success. The most obvious is to use a temperature controlled iron, solder-wick as much solder as you can off the pins, and then heat each pin individually, applying slight force from behind with a very fine dental-type pick until the pin pops free. In most cases you can do a couple of pins at a time, and if you're very careful, you won't lift a trace. It's not very pretty, and the chip is mangled when you're done, but it works in a pinch.
Another technique is to take a length of thin guitar string or piano wire and slide it behind and through the pins on one side of the chip. Anchor the string with an alligator clip or hemostats to the board, and then gently pull the free end away from the chip as close to the surface of the board as possible, while heating the pins where the force of the is applied. As the pins begin to come free, move the iron along with the string. It takes a little practice, and it won't work well if there are a lot of tall components around the chip, but in most cases it works great, and because the string is pulling each pin outward, instead of upward, the chances of lifting a trace is very slim. The pins around the chip are left relatively unscathed- I've pulled 84 pin chips from scrap units this way and reused them in emergency situations with no problems.
Getting more expensive, our shop has a special flat-pack desoldering tool that I found doesn't work very well. It has a variety of "tips", shaped like the outlines of popular IC packages. The tip fits over the IC and applies heat to all the pins by contact simultaneously. When the solder is hot enough, the technician pushes a button on the handle that applies suction to the center of the chip. The suction causes the IC to "stick" to the desoldering tip and then the tech lifts the iron and (hopefully) the chip away.
The problem is that the tips are so large that even heating is nearly impossible, and I've had a few cases where traces (several, not just one) got lifted when I thought all the pins were free. Lifting the iron with the chip in it just gives no "feel" for when the pins are free. It also takes a long time to warm up, and requires a lot of tinning to get the tip to work at all.
The latest gadget we are now using is a hot air desoldering station. This is simply the best, fastest, easiest way to remove a flat pack. It has a selection of tips with nozzles that direct hot air around the perimeter of standard chip types. The velocity and temperature of the air are adjustable. After about a 5-minute warm up, the technician holds the tip over but not touching the pins, and in about 5 or 10 seconds, with a slight lift from a dental pick, the chip comes easily free.
Jaws drop when I demonstrate this thing. All pins are heated at the same time, and the solder fully flows, so board damage is nearly impossible. The chip is in perfect shape (with tinned leads!) and can be easily reused. The only thing that can be a problem is the possibility of loosening and blowing away nearby small resistors and capacitors if the velocity of the air is set too high. Turning the velocity down on densely packed boards will minimize it, though. I simply can't say enough about this machine, and I highly recommend it if your shop does a lot of flat packs. The high cost will justify itself in the time saved with this method.
But if funds are tight or the quantity of flat packs is minimal, try the guitar string. I've done many that way with great success. .
Soder-Wick from Chamtronics, is amazing - it can clean a pad so it looks like nothing's ever been soldered to it.
Dan Creagan says:
Dip the braid in flux if you have old braid. You only have to use a very little bit of flux - I just dab my fingers in the flux and then run them up and down the braid to wipe them off. The braid just has a 'patina' that is preventing solder flow .. the flux will fix it.
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