Science/Engineering/Technical forums

A SMT Hot-air gun can be constructed from a piece of small dia. ceramic tube with a small coil of Ni-Chrome inside and connected to a low voltage PSU and fed from an air-pump - useful for rework. The coil glows red-hot with a gentle flow. Better to fix rigid and bring the work under it.

We made one of those here, modifying a normal hot air gun by making some special tips for it that would for instance direct air in a square pattern around a quad flat pack. In a pinch, with care, you could solder as well as remove packages that way, but you tend to overheat the insides, which is why industry doesn't use that method, or IR any more, but does vapor phase.
We made one of those too, using fluoinert, and it worked pretty good, very fast, and no failures from overheated chips. However, for short runs that wasn't worth using, as it's hard to get all the little parts placed well enough and not knock them around on the way to the vapor phase oven. Here the pros will use a glue spotter, or prebake a little right after placement to "set" the solder paste and make the board more resilient to handling. Our main problem with that was things like tomb-stoning on the little R's and other discretes. It was just too much work without some pretty specialized other tools for glue and solder placement.

So for short runs, we went to the Metcal soldering system, which works fantastic (Jerry and I have mentioned these here). You can wipe down a line of pins and all are soldered perfectly in about one second, so it's not slow. For the little discretes, you hold them down with a toothpick in the other hand so they don't stand up. My metcal has a second, tweezer handpiece for removing simple things as needed (perhaps to try a different value). But in the main, this stuff is so cheap that if you have an occaisional failure, you just toss it rather than rework it -- after removing a big flatpack, is it really worth an hour to clean and restraighten all the pins for re-use? Not usually if it's a 3 dollar chip, or even a 20 dollar one. All of our short run (which is what we do) production has been the Metcal system, it just works better with less frustration. I do it under a stereo microscope which makes it much easier to get right.

I guess if I ever needed real quantity, I'd go to a job shop. But as someone said, you can make money on ones, and tens, hundreds kill you, and then with thousands it's starting to be good again.
No one wants hundreds, either to make or buy. That's at the bad point vs setup costs and so on, where there are none in small lots, and in really large ones, they don't matter much.

If anybody saw the DDS boards I built several years back you will notice the lead pitch on the ICs
is very small. I can't imagine what they are like today but I'm probably not up to it. So, I realized
that I needed a hot-air gun and came up with what's in the photo below. The unusual block in the
picture is a fish-tank pump. There are two things missing from the photo and they are a pet-supply
store fishtank valve to adjust the temperature and air flow (inverse functions with one control) as
it is placed in the plastic air line. The other is a syringe containing micro solder paste. The
techniques are similar to those that you may have read above so just follow these and you'll be OK.
The soldering iron and tip were bought at Radio Shack. The fish stuff can come from Wallmart
but I can't remember where I bought the solder paste. May have been DigiKey: