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Hand soldering an MSOP-10 package

8 Oct 2016
Progress: Complete

If you look at my projects from only a couple of years ago you can see how terrible at soldering I was. There are a couple of things that have led to my improvement: understanding the physics of solder and flux (oxidation, surface tension, thermodynamics, whatever - just having a 'feel' for how solder behaves) and also, last week, I bought a microscope. Yep, my World's Smallest MIDI Synthesizer(s) were done with the naked eye. I looked at them under the microscope the other day, and just thought, boy, I can go so much smaller now!

MSOP is smaller than TSSOP which is smaller than SOIC which is smaller than DIP. MSOP-10 has a 0.5mm pitch.

Basically, you don't have to be good enough to solder every joint perfectly. You only have to be good enough to recover from your mistakes. Here is some advice:

Materials -- Protoboard and Kapton Tape are essentials. I'm using leaded solder in the video (Sn62/Pb36/Ag2) but it doesn't really make much difference, I've soldered TSSOP using the same technique with unleaded solder. The enamel wire was pulled out of an electric motor, my callipers tell me it's 0.25mm thick. The enamel burns off at about 380°C.

Get some good tweezers, mine are labelled 5-SA and cost about £8. Having a microscope does make things easier to see, mine is a fairly cheap stereo microscope with 10x magnification and (importantly) 9 inches working distance between the objective and the focal plane. Unsurprisingly, having a DSLR with an enormous macro lens right where you want your head to be does not make things any easier.

The hotter your solder is, the faster it oxidises. As this happens it becomes harder to use, stickier and less shiny. Using flux is supposed to reverse this process. Most solder comes with a thin layer of flux on it, you can see that brown residue that appears. You can use this to your advantage, poking fresh solder into a joint is like a targeted delivery of flux.

A mistake I used to make is to wipe the soldering iron on the sponge just before you put it back on the stand. Incorrect! This means there is almost no solder on the tip as it sits there waiting, and the thin layer of solder on it will oxidise completely which forms an annoying insulating coating on the tip. Most soldering irons you find in the wild will have been abused like this and take tons of flux to clean up to a wettable surface. If you leave a lump of solder on the tip as it's sitting there, then only wipe it off immediately before you use it, this problem disappears.

To burn off the enamel coating on the wires, you have to turn up the temperature to 400ish. But I always turn it back down to 300 after, since it oxidises so much quicker at the higher temp.

To make a good joint, the solder has to wet both surfaces. In order to wet properly, the surface has to be as hot as the melting point of the solder. This is why the normal soldering technique has you holding the iron on the joint for a few seconds before touching it with solder. The alternative technique to this is to tin both surfaces beforehand. If both surfaces have been wetted with solder, all you need to do to join them is melt them together -- you don't need to bring the actual copper up to the same temperature. This is the principle used in the video above.

The very first thing I do to the chip is tin all its legs, and at first the solder blobs them all together which looks like a disaster. However, because the soldering iron tip is free of oxides immediately after being wiped, and the solder on the legs hasn't oxidised yet, simply touching the blob causes some of it to jump back onto the iron. It's handy to rotate the iron so that multiple surfaces of it can wick the solder away.

Oxides block heat flow. If you touch a solder joint with a clean, tinned soldering iron it will melt it almost immediately, even at temperatures just barely above the melting point. If your tip is oxidised, even very high temperatures will struggle to melt the joint. One technique to get better conduction is to press quite hard on the joint, if that's an option.

After each join in the video, I wiggle the wire a bit to see where it bends, and if it's pivoting on the joint then it hasn't joined properly and needs to be redone.

One thing I didn't show in the video is that one side of the chip, after tinning it, I wiped it with isopropyl alcohol to clean away the flux residue. It's not clear if this helped or not, I mean, the first joint on the cleaned side wicked across between two of the legs. So maybe leaving the flux residue there helps. But in general, once the flux has been used up, it just sits there getting in the way and it's best to clean it off.

In total it took me just under half an hour to solder the MSOP-10 package. Not bad. Next up, BGA!

(Although I feel like I'm making progress in my quest to follow in ELM ChaN's footsteps, it kinda puts things in perspective when you remember that he was able to do this back in 1997...)