Re: Beginner SMT techniques (hot air)
by philba on Tue Dec 27, 2011 8:01 pm
It demends on what chip packages you are using. SOICs and SSOP, SOTs, 805 or 1206 are easy to do with a soldering iron and a somewhat small tip. I've done quite a few boards that way.
The sparkfun guys were getting decent results using an electric frying pan a while ago. No joke. You put solder paste on the pads (by hand - hard - or via a stencil), position the components fairly close to where they should be and gently place it on the hot pan. The solder melts and the chips all snap into place. Let it cool and you're done. This requires a board with a solder mask (all professionally made PCBs have it) because it keeps the molten solder on the pads and the surface tension causes the chips legs to align over the pads. Kind of like magic. You should find tutorials at SFE's web site.
There are a number of people using toaster ovens as reflow ovens. search and ye shall find. I used to think this was the way to go but the reflow skillet approach is so charmingly simple and cheap. I recall they were buying the skillets from Target for like $15.
Frankly, I think using hot air is a bad idea. Especially from a heat gun. You'll be lucky if you don't scorch your PCB or blast the components all over the room.
Unless the board is reasonably large (dozens of ICs, many Rs and Cs) or you are doing a lot of boards, hand soldering with an iron is your best bet. Little set up time and once you get the knack, it goes pretty fast. In fact, I can assemble an SMT board in about half the time it takes to do the equivalent TH board. With SMT, you tin one pad per device, melt the tinned pad, slide the component in place and quickly solder the rest of the leads. The leads are small and have very little thermal mass so they heat up really fast. I can do a lead in about a second. No flipping the board, waiting for each lead to heat up, trimming the excess leads.