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Beginner SMT techniques (hot air)

by zenwebb on Thu Dec 15, 2011 2:29 pm

Sometime in the next month or two, I will be ready to try my hand at assembling a really simple board I'm working on that uses SMD parts. I have a few years of experience with PTH soldering, and have soldered SMD chips before with a soldering iron, but never a whole board of parts. I'm not ready to go the whole nine yards and buy a professional hot air rework station, or to get / build a reflow oven. I'm hoping I don't even need to get into stencils or PnP stuff too much either. At the end of this post, I'll post the board design I'm thinking specifically about, if it helps.

Recently, MAKE Magazine posted a link to this heat gun from Harbor Freight: http://www.harborfreight.com/1500-watt- ... 96289.html

  • Will this be a good thing for me to use to solder small 0603 parts (LEDs, resistors and caps) to a small board and maybe even ICs?
  • Can I simply dab a small amount solder paste onto each pad / part, stick them together and then run the hot air over them until they turn shiny?

Here is the current unrouted board design (a work in progress), just so you can get an idea of parts count, placement and board dimensions. The board is just a hair under 2x2" (5cmx5cm).
The large rectangle on the left is an RN-42 Bluetooth module from SparkFun.
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Re: Beginner SMT techniques (hot air)

by neutron spin on Tue Dec 27, 2011 6:54 pm

A hot air station even the cheap ones will work if you are careful. That heat gun most likely will blow the smaller components off the board since you can't control the air flow. Even with the hot air station you must be careful not to overheat the components or that module. I have in a pinch used hot air guns but I do not recommend it. Solder paste must be applied manually but the nozzles are fine enough to work. Purchase some liquid flux and visit YouTube and watch a few videos of folks doing weird things :D with surface mount components...
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Re: Beginner SMT techniques (hot air)

by philba on Tue Dec 27, 2011 7:01 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.
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Re: Beginner SMT techniques (hot air)

by amigabill on Thu Jan 26, 2012 3:47 pm

I've recently got a hot air station. I hope to save up through teh year and get one of the cheaper preheaters (Aoyue or the low-end air bath from someone) and stand/holder for the air gun. But even the cheapest stand/holder I've found, the Aoyue one, is rather expensive for what it looks like. Someone on another forum suggested a retort stand (lab equipment) which can be much cheaper. Some say to hack a Dremel stand, but the one I have for my dremel doesn't seem to have much depth to get to the center of a board without hitting the stand's pole.

Has anyone tried a desktop microphone stand? The flexy gooseneck style seems it may be too flexy and not hold the gun stable in place. The "scissor" style movable booms look convenient, but I'm not sure they'll be strong enough to stay in place well. A metal rod boom may be good for stability, but I'm not sure about convenience, as someone mentioning the retort stands said he sometimes had issues trying to move the gun out of the way to pick the chip before it cools off. I'm not sure of a metal rod book stand would be better than that or not.

Any thoughts?
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Re: Beginner SMT techniques (hot air)

by smilingcat on Sun Jan 29, 2012 8:43 pm

hi Zenwebb,

Just looking at your PCB layout. Your RN-42 Bluetooth footprint seems to be missing some features. Looks like you have soldermask but copper? or through hole?

And you might want to add more bypass caps. Never hurts to have too many. You don't necessarily have to populate all of them. But if you find that you need it but no site to put the caps, that'll be tough.
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