I’ve slowly dragged myself into twenty-first century printed circuit board fabrication. That implies the need to use surface mount components. After some hit and miss attempts, I’ve finally developed a technique that requires no special equipment; just a plain old soldering iron.
My soldering iron isn’t even a thermostatically controlled one; it’s just a cheap hardware store model. The only thing special about it, is that it is has a fairly small 1.8 mm wide chisel tip.
Because these parts are so small, it is important to be able to see your work clearly. I use an inexpensive visor type magnifier.
My method for mounting these tiny surface mount chips will be explained in the following series of photos using a tiny SOT-23 chip which measures only 2 mm by 3 mm. The SOT-23 package is probably the worst case scenario for the hobbyist, because of its tiny size. Other packages with more pins are much easier to handle, because they are physically bigger. So, if you can work with a SOT-23, then you should be able to handle just about anything else that comes along.
I apologize in advance for the mediocre quality of the photos. The tiny size of the parts makes it difficult to zoom in on them and still keep things in focus.
Before getting into the actual soldering method, I’ll give some advance warning about handling these tiny parts. They are extremely tiny, and if they should ever go flying out of your grasp, you will never see them again. Until you’ve handled these things, it’s hard to understand just how likely this is to happen. Take my word for it: It is very likely. Even removing the chip from the tape carrier in which it is supplied, can be a bit daunting. If your fingers slip as you peel apart the tape, the chip is likely to go flying off into outer space. For this reason, I highly recommend that you purchase a few spares, and limit your first foray into surface mount parts to something very inexpensive. I’ll say it again: It’s nearly impossible to understand how likely it is to lose the part due to fumbling until you’ve experienced it firsthand. So, as silly as it may sound, it may be a good idea to get a clear plastic bag big enough to get both hands into, and open up the tape carrier in there. You’ll soon understand why. I’ve seen suggestions that you should handle these devices with tweezers. However, if the tweezers ever slip while holding the device, you’ll impart more kinetic energy to the part than you could possibly do by holding it with your fingers. Best bet is to wrap some sticky tape—sticky side out—around the end of a small bamboo skewer, and use that to pick up parts.
Now, with that warning out of the way, let us begin.
First, place the chip on a work surface. Take a piece of tape (preferably clear) about 3 inches long, and holding one end in each hand carefully bring it down onto the chip so that one edge of the tape aligns with one edge of the chip.
Press the ends of the tape down lightly onto the work surface to keep everything in position. Using a fingernail or a toothpick, press the tape firmly down onto the chip itself, in order to make sure that it is firmly attached.
Meanwhile, prepare the circuit board where the chip is to be mounted. Using a small dab of solder, tin the solder pads where the chip will go. Use the soldering iron to draw away any excess solder. You want the bare minimum of solder on the pads.
Next, apply some liquid or paste solder flux (electronic flux, not plumbing flux!!!) over the tinned pads. This will help ensure that the solder will flow properly when the chip is soldered (actually sweated) in place.
Now, secure the circuit board to the work surface, using tape or some other temporary adhesive.
Grasp the tape that is holding the chip, one end in each hand, and carefully align its legs with the solder pads on the circuit board. Holding it by the ends of the tape gives much finer control over the positioning, even if your hands aren’t very steady. When it appears to be aligned, press the ends of the tape down onto the work surface.
Have a very close look with a strong magnifier, to double check the alignment. If it’s not quite right, then lift the tape and reposition the chip.
It’s now ready for soldering. At this point, wipe all solder from the tip of the soldering iron. There is already enough solder on the pads, and you don’t want to apply any more, as this will increase the risk of a solder bridge. Use a small tool such as a jeweller’s screwdriver or bamboo skewer to apply gentle downward pressure on the body of the chip, to keep it in position while soldering. Now just touch the tip of the iron to each pin for about a second or two. The soldering iron tip is probably large enough that it will cover more than one pin at a time. This is okay, because as long as the tip has been cleaned of solder, there is no risk of a solder bridge.
At this point, use a strong magnifier, and plenty of light to examine the solder joint. A good solder joint will be quite obvious. If it doesn’t appear to be satisfactory, apply the soldering iron again.
With one side of the chip now soldered in place, there is no risk of it moving out of position. You can remove the tape, and then solder the pins on the other side. The next photo shows the chip after being soldered in place.
All that remains is to clean off the leftover flux with an alcohol swab.
From the photos, especially with the high magnification, you can see that my circuit board is not professionally made. I used the toner transfer process to make them, and there is a certain amount of undercutting visible. This does not prevent a successful operation though. However, I do recommend that if you design your own circuit board, you should do your best to use the exact solder pad dimensions and spacing specified in the chip manufacturer’s data sheet.