Most surface mount soldering sites concentrate on using a fine tip soldering iron for mounting components. This is a fine technique for placing components onto a board, but what about when you need to remove one? All too often people resort to clipping off the components. I was lucky enough to be shown how to mount and remove surface mount packages by an experienced surface mount technician. His tool of choice was a pyropen. Since then I have had good success in placing and removing several different types of packages, from surface mount capacitors to 64 pin quad flat packs. I hope that you find the following guide useful. Please let me know how you get on, especially if you can recommend improvements.
The first thing you need is a pyropen! I use a Weller pyropen which I bought second hand as the new ones cost over £100. They come with a hot air nozzle which works fine for the techniques shown here. I invested in a thinner nozzle, the WHC-50 for more precise work. However the regular nozzle will give you excellent results. There are several cheaper brands of pyropen available which are probably just as effective. You need something to work on that you don't mind getting scorched - I use a scrap piece of wood. Some kind of stand that angles the hot air in a safe direction is a good idea. You can see the one that I made from a can. Recently I started using a Weller Pyropen Jr which comes with an excellent small nozzle. It's ideal for getting into small spaces on a circuit board. I've quickly become a big fan.
First off the PCB pads need to be tinned. If you've sprung for a board with a solder mask or are reusing an old board then you're laughing. Otherwise you will have to tin the pads. Clean the bare pads first with some abrasive pad and contact cleaner. The best method I've found so far for tinning is to blob on some solder using a regular iron, then blot off the surplus using a piece of soder wick. Don't worry if you bridge the tracks when you are putting the solder on as the excess will blot off. To state the obvious, the soder wick gets hot - so don't go holding the end with your fingers. The solder melts and the excess is soaked into the soder wick. You may need to re-tin an old PCB if you are replacing a component. You'll note that I've already masked off the surrounding components that are in danger of being cooked. Bluetack works well for this; also good for sticking the whole board down while you work.
Dab some flux onto the pads. This will help the melting process and help to ensure a good solder joint. I use a liquid flux, but jelly flux also works.
Carefully position the component. Some tweezers or scalpels are the tools of choice. Even if you are slightly out on the placement, the surface tension of the solder as it melts will pull the component into the correct position.
Fire up the pyropen. I use maximum heat. The ignition chamber on the Weller should be shut once the pyropen is lit. Carefully heat along each edge of the chip. You will see the solder glisten as it melts. The chip should stay in position without needing to be held down as long as you angle the hot air stream downwards. Quit a lot of fumes come up - I sometimes hook up an old CPU fan to blow them away. In this case you might need to hold the chip down with a scalpel tip.
You can get a good idea of whether the pins have good solder joints by gently running a scalpel blade or small screwdriver along them. Any loose ones will flex out of position and you will quickly learn that a good joint has a certain 'ping' to it!
The main pins to check are the outside corners - these are the most likely to have not got stuck down. Put a scalpel blade inbetween the corner pins and their neighbours and give it a little twist to check that the pin is firmly soldered.
Finally remove the bluetack and clean off the flux. I use a 'No Clean Flux Remover Pen' to dab on some flux cleaner, then scrub it into all of the nooks and crannies using an old toothbrush. Then I rinse off the residue with contact cleaner. I have read that failure to remove the flux will cause your board to disintegrate in years to come, when you are not around to defend it. Check all joints with a continuity meter - ring it out from the pins to other pins, not from the pads, otherwise you are just checking that the board was made right.
That's it! Practice on some old boards first and let me know of any helpful tricks you pick up.
Mask off any surrounding components that are in danger of being cooked while you operate. Bluetack works well for this; also good for sticking the whole board down while you work. Bits of tin foil work well for shielding too. Then it is time to fire up the pyropen and set to work. I usually set my Weller pyropen full on for maximum heat while working. You will be surprised how much punishment these chips can take. They are desgined to be cooked in a surface mount oven at 200 odd degrees centigrade for a whiles, so you can get them quite hot without worrying about internal damage. Try and direct the heat onto the pins though! First off work your way around the entire component to warm up all of the pads. Keep on working around the component. The solder will glisten as it melts.
With a nudge from your tweezers the entire component will suddenly come free. Don't try to peel one edge up at a time - it is easy to tear the solder pads off the board this way. I've seen some folk preheating the entire board with halogen lamps or hot air guns when there have been large ground planes to suck away your pyropen's heat. I've not tried that myself, but necessity is the mother of invention.
Farnell supply all of the tools and consumables detailed on this site.
Omega Research make some excellent converter boards for soldering surface mount chips onto. The boards can then be mounted onto stripboard, breadboard or PCBs. The pads are pretinned so no solder is needed. You need to add your own pins - standard 0.1 inch spacing. Each pad has several mountings for the different pakages available.