SMD Soldering with Gas

[desimon] had a wanted to use some accelerometer chips, but their 3x3mm 16-VFQFN packages made it pretty darn hard to solder by hand. While there are endless ways to approach this, we found this one peculiarity interesting from his use of a gas torch, though it is pretty much hot air reflow.

A PCB for the tiny devices is etched and tinned, the pads have a healthy but not overdone amount of solder applied to them. A liberal coat of flux, rough alignment of the chip and a few gentle passes from the torch and the hobby grade solder melts while the surface tension pulls everything into final alignment.

Having personally used a hot air gun a number of times (and also burning my hand about the same number of times) the localized heat of the torch does make more sense, and there is virtually no heat up time for it either, though it appears just as easy to scorch the board. It is a live flame so be careful!

24 thoughts on “SMD Soldering with Gas

  1. That’s a nice trick!

    Dave Jones from the EEVBlog has a different method for soldering these packages with the contacts at the bottom. It’s for slightly larger devices, but I’m sure it’s also possible for these with sufficient magnification.

  2. @tim, use a pair of tweezers and very gently press down on the part as you heat it up. Keep the side of your palm on a table if you do not have steady hands. If you still have trouble, tape a nail to a cylinder and place the nail atop of the component so it presses down slightly.

    As long as you keep the air flow down, once the solder melts, you shouldn’t have to worry too much about blowing the chip off. Just don’t get too close.

  3. Hot-air reflow should be fairly low flow of air just hot enough to melt solder and should not blow parts off the board. Most have temperature and air flow speed control.

  4. Indeed to stop the chips from flying away, either use a tweezer (not recommended with hot air gun, since you will burn your hands) or blow perpendicular on the PCB surface. Also with butane gas solder tools, the airflow is really minimal, so the problem isn’t as big.

    Indeed I could use more practice, the wires are soldered “preliminary” to test each of the 5 chips, so they weren’t supposed to be quality ;-).

  5. @tim: he’s using a butane torch not a heat gun.

    @jochem: wonderful! thanks for the link. i never thought of just turning the thing around. {a really nice video by the way)

    1. Solder alloys that contain lead, and tin aren’t necessarily “hobby grade”. Using the browse search feature doesn’t return the phrase hobby grade on the desimon web page, so I really don’t know why Kevin used the term is his post here. ?

  6. Based on a (successful — YAY) reflow attempt on a 360 motherboard, I think if you want to do this on a larger board and are afraid of cracking it, place the board on an electric cooktop griddle to spread even heat across the board first, then apply heat with the gun. After you’ve finished the reflow a few minutes later, reduce heat and cut the power to the griddle when fully done. Let everything cool (30 min+) without bumping it to avoid bad solder joints. Caveat: I’m not an expert and have only done this once — but I was successful for what it’s worth. Also, I used some machine screws with nuts to make feet for the board so it wasn’t sitting directly on the heated surface. I also set it inside a cookie sheet on top of the griddle so I didn’t scratch the teflon coating — I like that griddle and I also like making pancakes on it. :-)

  7. @kelvar:
    Cracking due to what? Rapid heating or trapped moisture?
    I know we always bake components for at least 24 hours in an oven at just over 100 C to drive out any moisture. That way, when you do come to solder it there’s no chance of any steam cracking the package.

  8. Nice method, I’m a bit scared of the open flame part (prefer a hot air station myself) but it’ll work. Just be thankful they don’t have those stupid ground pads on the bottom face of the chip – those are what really make *QFN packages hell of annoying.

    @BLuRry: What you described is basically what most shops that do 360 rework have – a custom jig to hold the motherboard, heat it uniformly from the bottom, and clamp it down to prevent flexing due to uneven heating (which could cause / worsen solder cracks on other components.)

  9. I wouldn’t do it this way. I guess that it is far to unreliable for sensitive components.
    If I was really desperately looking for a way to do this, I would instead use a regular hotplate. Heat it up, put the PCB on top, wait for the visual indication of melting solder and take it off. Much much safer.

  10. I would try doing it like the video in the second post, except that I wouldn’t glue it upside-down, but I would first solder the wires and then solder it like an ordinary DIP package.

  11. good video there..

    I did one by heating the board underneath with a hot air gun until the solder reflowed. Rework gel made it easier and it took a bit of research to learn that the surface tension pulls it in. Great when the tension just locks it into place! ;oD

    Doing it this way meant that it had to be the first component on the pcb mind ;o)

  12. My butane torch kit came with the fiery-endpiece as well as a gas soldering bit — but also a hot-air endpiece, which has an incandescent metal gauze much like those gas-powered camping lanterns.

    The flame is much too hot, scorchtastic, but the hot-air endpiece is much the same as a hot-air rework station and excellent to solder with.

    Nicely done with the QFNs, they’re tricky :)

  13. i wonder if one could improve on the dead-bug-mlp soldering with the right kind of proto board.

    the board:
    many mlp are about as thick as a pcb board, and the pads go to the edge. so, one could maybe create a small board with a rectangular hole the size of the package and traces leading to the edge of the hole. fitting the package into the hole would allow to “drag”-solder the whole thing. i’d call this adapter a “break-out” board ;)

    for stability, the chip has to be glued in, or one has to close the hole with a pcb as a lid on either side: solder it into the hole, than glue on the lid and turn it around, or add the lid to the other side, and solder the chip in the pit.

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