Desoldering Guide

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Original written by Womble

When I first got the desolder station I was not overly impressed, but after a month or two with it I can honestly say its utterly awesome, probably is the 3rd most used tool in my armoury after scope and soldering iron. It takes a little practice to get good at it but once you have it nailed you can get components to litterally fall off the board with no damage to the chip or to the board itself. To practise I went through my scrap drawer recovering CPUs and RAM chips - got a dozen or so 68000s, 768 pins on the 68000s alone.

I have a couple of tips for its use tho.

  1. Use pliers to straighten any bent pins on the chip before you try to desolder, dont try to use the desolder nozzle to do the straightenning, it doesnt work well and you end up with a half cleared pin which wastes more time than fixing the pins first.
  2. Re-tin the joint you are about to clear, this is utterly essential, especially with very old low lying joints. With a soldering iron get a blob of solder on to the joint, doesnt have to be neat, but heat it with the soldering iron until you can see the new solder mix with the old. This achieves two things, firstly it gets some flux into the joint, without flux the old solder is going nowhere, and you will roast the board trying to get it out. Secondly it adds metal which gives more surface area to transmit the heat through, sometimes a bigger blob is better than a small one.
  3. Secondly you need to be aware whats around the solder joint you are trying to remove, any tracks nearby are at risk as the desolder nozzle is bloody hot, you need to waggle the pin when you have the solder hot as you hit the vacuum pump button. Try and do your waggling into blank space rather than over any tracks. If you have got enough solder onto the pin in step 1 you should be able to be pretty quick tho.
  4. If you can watch the pin from the component side of the board while you apply the desolder sucker you can see the solder wet the leg of the pin as it melts. When you see it melt you need to push the pin in towards the middle of the chip and press the button, as the solder slurps out move the nozzle to bend the pin towards the inner side of the chip while still holding the button down, then allow the pin to settle back in the middle of the hole and remove the desoldering iron. If you do this at the right speed you will get a pin that is totally free from solder, do it too fast or too slow and you end up with a pin stuck to the egde of the plate through hole, about 1.5 seconds for the whole process works for me. You may have to repeat the process by adding new solder and going again, its useful to ping the chip leg with your finger nail, if you get a dead sounding click its still trapped, if you get a metalic ping sound the pin is free. Just repeat on all non pingable pins and the chip will be loose.
  5. You should be able to see daylight through each of the PCB holes by this stage, if you can still see metal then repeat the process, add new solder and try again. Struggling on with a half cleared hole will get you nowhere, and you will apply far more heat to the board than if you just start again.
  6. The only problem pins are likely to be the 5V and ground pins, these connect to large amounts of copper in the board and you may find that you cant get the solder hot enough due to the copper heatsink effect. This is not a huge issue once you have all other pins free, just apply some solder to the leg on the parts side of the board, hold your soldering iron against it and us a screwdriver to lever the chip up. The solder will be like treacle but the chip should slowly slide out, repeat for the other ground pin and your chip is free. Then put a good blob of new solder on the PCB hole and put the desolder nozzle on it, leave it for as long as it takes to melt the solder then press fire - hole cleared.
  7. Once all legs are free the chip will just fall out, you may have to lever it out if the spring in the legs is clamping it into the holes still, but it should come out very easily. If it doesnt, go back and check for stuck pins, if you do have to give up on the chip then so be it, chips can be replaced, damage to the PCB can be a nightmare to fix.

Also bear in mind that some boards are easier to desolder parts from than others, depends on the quality of the original solder and how large the plate through holes are, the larger the better obviously. Was working on a board last night, in the course of troubleshooting a board I took out a Z80 (40 pins), a 68000 (64 pins), two 65256 RAM chips (28 pins) and a 6116 (24 pins), ie 184 pins without any damage to the board or the chips, no chip took me longer than a few minutes to get off, and my desolder station is a cheapy one.

I was loath to pay the $300 it cost me when I was first looking for a desolder station, but in terms of sheer sanity saved its worth every penny.