Neven's Electronics Page

Soldering And Desoldering of SMD Components

Many people are convinced that working with SMD is very hard and even impossible without expensive special equipment, while it is in fact, with a little practice, no more difficult than working with standard through-hole components. Some things are really impossible to do without the right equipment (like soldering/desoldering of BGA components), but there are many instances where it is even easier to work with SMD than with through-hole. Of course, if you have problems soldering a simple through-hole resistor then working with SMD is and will be difficult until you master the technique, but if you have good general soldering skill then it will not be difficult to learn working with SMD. Steady hands and good eyes will help, though. Some minimal equipment will still be needed, like a good soldering iron (you can't really expect to work on SMD with a bulky, oversized, all purpose soldering iron bought in a supermarket for a price not worth mentioning).

Now, let's see what tools would you need:

1. Soldering iron

This should be a higher quality temperature controlled soldering iron, with a few types of fine soldering bits (1mm and such, smaller for SMD). Weller and Ersa would be a good choice, but some cheaper will do.

2. Desoldering pump

Get a big one with a big barrel. Smaller pumps can be used, but they require more skill in use and won't suck in all the melted solder from holes, and their tips have to be closer to the PCB in order to work and this will burn their tips faster than those on bigger ones which usually can be held a millimeter or two away and do the job better than the small ones.

3. Desoldering wick

Again, use high quality wick, because some lower quality wicks don't have enough flux and won't suck in the melted solder very good Get smaller ones, like 1.5mm and 2mm wide. Also, they should be nicely flat and thin because thick braids will need much more heat and then you will have problems with the braid sticking to the pad because of too low temperature or risk damaging components because of too much heat.

4. Tweezers, dental picks, needles

A pair of precise tweezers is needed, especially for SMD. Dental picks are needed also mostly for SMD, and some needles for SMD ICs in really dense packages. You can also make your dental picks by forming a needle to a form you find the most suitable for particular job and gluing it to some handle.

5. Solder wire

Solder wire should be resin cored and thin. I find 0.5mm solder wire suitable for most purposes. It should be with silver alloy as an ordinary solder will dissolve pads on components such as resistors or ceramic capacitors, but it is expensive and sometimes hard to find. It's not such a big issue anyway, as the ordinary solder will not momentarily dissolve the pads, rendering the component useless, it takes many repeated soldering and desoldering in order to sufficiently degrade the pads, so you can safely use an ordinary solder wire.

Soldering

Keeping the component in place is the main problem when soldering SMD components. Also, preparing the pads is almost as important as the soldering. To prepare the pads you should tin them with a fine, thin film of solder. If the solder film is too thick it will not be possible to add more solder during soldering because there will already be to much of it. It will also make it harder to correctly position the component as the component will tend to slide off the raised mount of solder. Too much solder on pads when soldering fine pithed ICs will also make shorts between pins.

To correctly thin the pads you should flood them with solder, then pick up the excess with desoldering wick. But you will have to get the feeling how much solder should be left on pads. With a little practice you will be able to control how much solder will be left on pads after using the desoldering wick. The right thickness of solder film can be judged by the color. You should make it as thin as possible, but it's color should be silvery (the color of solder). If you can see the golden color of the copper beneath, then its too thin (or nonexistent). Sometimes it is enough to just use the wick to clean all the solder and then slide the soldering iron tip a few times over the pads, which will leave just enough solder on the pads.

To solder a component like a resistor or a transistor, pick up the component with tweezers and place it on the prepared pads, put a little solder on soldering iron's tip and while holding the component in place with tweezers, touch one pad with soldering iron. This should hold the component in place. Then solder the other pad of the component just like you would do a standard through-hole component, and then go back and fix the first pad because it's probably not soldered good enough due to solder being added to the iron tip instead of to the joint. Also beware, because of the components' small temperature mass it is very easy to melt the solder on the already soldered pad while you are soldering the second pad, and displace the component or even pick it up with soldering iron.

When soldering an IC, you should position the component on the pads so that all pins and pads line up. Then, holding the component in place, solder two opposite pins of the IC, to keep the IC in place while you are soldering the other pins. Now, there are two ways to solder an IC. You can solder pins one by one, and use a desoldering wick to correct eventual shorts between pins. But this is very time consuming, and with ICs with smaller pitch you'll end up using more time to fix the solder bridges than to actually solder the pins. Another method, and much faster, is to flood all the pins with solder and then pick up any excess solder with desoldering wick. Again, this requires some practice until you will be able to leave just enough solder after sucking it up to keep the pin soldered, but not too much as to leave bridges. This is best accomplished by moving the wick with soldering iron tip over the pins quickly, and not resting the wick on the pins and waiting, as that would almost certainly pick up too much of the solder, leaving the joints dry. This is a really quick way to solder ICs, as you can solder a hundred pin IC in less than a minute!

Desoldering

To desolder small components such as resistors, diodes, capacitors, you have to add solder to both sides (pads) of the component so that it forms a bigger temperature mass. Then heat the pads alternately, quickly moving the soldering iron from one pad to the other repeatedly, while pushing the component to the side with a small screwdriver until it gives in and you push the component away from pads. Often the component is glued to the board before soldering, but this glue is not resistant to temperature so it will release as you heat the component, if it resists just heat the component a bit more and it will give in. Even better method is to add so much solder to form a blob of solder all over the component: Heat one pad and add much solder, then heat the other pad and also add much solder, then heat from above the component trying to form a bridge with solder (add more solder until the bridge forms) and then just lift the soldering iron up and, because of surface tension of the flowing solder, the whole blob of solder, with the component in it, will be lifted up and stick to the soldering iron tip. Now just quickly brush away the component from the soldering iron tip (you have to do it quickly otherwise you'll destroy the component with heat). Clean the solder off of component with desoldering wick and also clean the pads with wick. This requires some skill, especially if surrounding components are close because you could make a mess with all that solder, but with practice it will become very quick and easy way to remove the components.

To remove three pin components such as transistors, add a little solder on the pad on the side of the component with just one pad, then add much solder on the pads on the side of the component with two pads, bridging the two pads and heat it well, then quickly move the iron to the single pad on the opposite side and push the component away from pads with a screwdriver. Again, the goal here is to form a big temperature mass of solder which, once melted, will remain in melted condition long enough to allow you to melt the solder on the remaining pad and lift the component while the solder on all pads is still in liquid state. Take care that you do all this as fast as possible otherwise you'll destroy the components by overheating them. Five seconds should be enough to remove a component.

SMD ICs can be divided in a few groups considering the way they can be desoldered. One group is the ICs with J leads. These are the ICs that have leads formed like letter J, going from the side of IC and bended all the way down to the underside of an IC. Bad news. It is difficult to desolder these ICs without special equipment (hot air gun). You can try to desolder these by cleaning as much solder as possible from the pads. First add some fresh solder to all the pins (flow it, don't try to do one by one pin because it doesn't matter if they become shorted) This will help by making the solder more fluid and easier to suck in by desodering wick. Now take a desoldering wick and try to clean as much solder as you can. Do it in two passes. First clean all the pins off of the excess solder that you've just added, and then do a second pass with a clean wick trying to suck in as much solder as possible. If you put the wick side wise to the pins (not the front side as usual), it will do a better job. Pads should be really clean of solder. This would leave the pins holding with so small hairs of solder to the pads that it should be possible to break the IC off the PCB with a moderate force and, with some luck, and if done properly, it won't lift the pads and traces off the board.

The second group of ICs would be ball-grid array ICs (BGA), the ones with pins like halves of a ball protruding all over the underside of an IC. Even more bad news. No way you'll desolder these with simple tools.

But luckily these two groups are not all that common, and there is much more PCBs populated with the ICs that can be desoldered using simple tools. One kind of these are the ICs in SOIC packages, the ones which have leads on two sides. These ICs can be glued or not glued to the PCB. If they aren't glued then it's an easy job. Add excess solder to all the pins on one side of the IC, making them all connected together by a blob of solder, heat the solder with soldering iron until all the solder is melted and fluid (you can move the soldering iron from first to the last pin and back a few times in one smooth move to make sure all the solder is melted) and then lift that side of the IC off the board (use a small screwdriver wedged between IC and the PCB). Remove excess solder by soldering wick, freeing all the pins on that side of any remaining solder, and then do the same thing to the other side of the IC. Now you can clean the bridges between pins with desoldering wick. The best method is not to put the wick over the leads and then heat the wick, but to put the wick on the table (well, not directly because you'll ruin the table), put the iron tip on the wick to heat it up and then touch the front (cut) side of the wick with IC pins. If the IC is glued to the PCB, you'll have to lift the pins one by one with a needle. First use desoldering wick on the leads of the IC to suck up as much solder as you can. Then take a needle and stick it between two pins, heat up a pin with soldering iron and bend it up, away from the pad, with a needle. Do this for the rest of the pins on that side of the IC. On some pins there could be solder left which will still form a bridge between the pin and the pad, even though the pin is lifted away from the pad. Use desoldering wick on these again to break the bridge. Then, when you're finished, you will be left with all the pins free, and just the glue holding the IC to the board. Break the glue by taking the IC with tweezers and twist it until the glue gives in, or use a small flat screwdriver, stick it under the IC and lift the IC up. Take care not to cut the traces on the PCB.

The ICs in quad flat packages, the ones with leads on all four sides of the IC are done the same way. First do two opposite sides of the IC by raising pins one by one and then the other two opposite sides you can do by flooding the pins with solder and lifting one side of the IC and then do the same to the opposite side.

There is another method. Take a thin wire and thread it between the IC casing and the IC leads on one side of the IC, tie one side of the wire to a suitable stronger component on the PCB, take the other side of the wire and pull it at a slight angle away from the IC while heating the leads at the same time, so that the wire will go through the melted solder between the leads and the pads, raising the leads off the board. This might be much faster, but the risk of lifting the traces off the PCB is quite big and I wouldn't really recommend that method.



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