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I'm curious how well a hot air gun would work for reflowing an entire board. My circuit board has about 250 components (including 0402 passives, and a couple of 0.5mm pitch TQFPs) and it's a bit of a pain to assemble it using a soldering iron.

Here's what I was thinking might work:

  1. Apply leaded solder paste via a stencil onto the PCB.
  2. Place components using a vacuum pick-up tool. Use a stereo microscope to place the more fine pitched parts.
  3. Once all the components have been placed, place the PCB onto a pre-heater and raise the temperature to, say, 100˚C.
  4. Start the hot air gun and slowly sweep it over the board as the paste reflow. I could utilize a fixture like this to keep the gun perpendicular to the board and just move the gun in the x-y plane.

It would take some time to sweep the gun over the board and ensure all of the paste has reflowed and during this time the pre-heater would still be on. Could this possibly damage the board? What about all the parts? Are there any other pitfalls that I might encounter or would this method work well?

I know there are better methods to reflow a board, like a reflow oven, but I'm specifically interested in how this method would work.

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  • \$\begingroup\$ This is viable if you use a cheap hot air rework station instead of a "heat gun" and do it carefully while watching for components that get displaced, blow away entirely, or "tombstone". Fortunately the hot air tool is what lets you specifically correct those issues, as well. A 25 gauge dispensing needle on an reversible aquarium pump is about right for 0402's. \$\endgroup\$ – Chris Stratton Jun 28 at 18:37
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Could this possibly damage the board?

Of course.

What about all the parts?

Yes. It could certainly damage them too, and most probably would.

Are there any other pitfalls that I might encounter or would this method work well?

The problem with this method is that it is inherently poorly controlled. It is potentially able to work and an exceptionally skilled and experienced and well trained operator may be able to achieve somewhat acceptable results some of the time. But most of us would just end up with a work of art or a pile of smoldering slag.

Probablility ~= 1.: Reflow soldering is an exercise in controlled death. Components and board are heated up hot enough and long enough that they are well on the way to destruction. Manufacturers design parts to meet the stresses of this process with an acceptable margin of safety. If you read up on the reflow process in detail, as you MUST have already done to make this question more than idle time wasting, you will have found that temperature profiles - rates of temperature change, holding times and cooling times and temperatures are all tightly specified. If you can manage the sort of control that this implies over the surface of a PCB containing 250 or so components including fine pitch TQFPs then you are wasting your time in your present role and probably want to enroll as a micro-surgeon or Formula One driver or similar :-). ie it's far too demanding a task for this to have any certainty of working.

Probability ~= 0: Not everyone is Wouter - he is an extremely experienced and capable engineer. All that said, it is "just possible" [tm] that a consistent approach, well aligned jig, temperature controlled air source etc may be able to do the job quite well. Finding out could be expensive. Or not. Given the very great success achieved by the toaster-oven-PCB-assembly community and the large amount of on-web information available on this method and the relatively low cost of doing it, I'd expect your TQFP's to thank you profusely for taking that route.


Related:

Spark Fun show you how to do Toaster Oven PCBing - lots of details - MANY photos

Some amateur results

Lots and lots and lots of PCB-toaster-oven ideas

Open Hardware PCB toaster over project

And more ...

Even a small BGA - an instructable

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  • \$\begingroup\$ Thanks for the praise, I am not sure whether I am worth it or just taking big risks :) Next time I do a bunch of these boards I'll take the thermocouple from you old toaster oven and measure the profile I create with the air gun. Might turn out to be an embarrasment to myself... \$\endgroup\$ – Wouter van Ooijen Sep 28 '12 at 13:51
  • \$\begingroup\$ @WoutervanOoijen - I've managed to form what I think is a reasonable impression of your capabilities over many years - from far away. While others may be able to do as well as you have, or better, it's likely that a typical beginner will produce disasters. Those who are not discouraged by my dire warnings may just succeed :-). \$\endgroup\$ – Russell McMahon Sep 28 '12 at 14:26
  • \$\begingroup\$ Saad talks about 0402's (which I never tried!), I hope for his sake that he is not a beginner! \$\endgroup\$ – Wouter van Ooijen Sep 28 '12 at 14:41
  • \$\begingroup\$ @WoutervanOoijen I am a beginner :( I can solder them by hand but if I do say, 100 of them, there's bound to be a mistake or two. \$\endgroup\$ – Saad Sep 28 '12 at 14:58
  • \$\begingroup\$ Fancy that - 6 years on the phantom downvoter strikes. "The answer is not useful". Just maybe it was a soul who upvoted long ago and they have now been cast into outer darkness - which causes upvotes to be revoked. If it's an actual downvote then they may wish to brush up on their soldering. \$\endgroup\$ – Russell McMahon Dec 9 '18 at 12:00
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For a double-sided board I do the most populated side with a simple reflow oven (T-962, typical Chinese medium-quality low-price poduct with an awful manual and user interface. before I got this one I used a toaster oven with home-brew temperature control). The other side I do with a paint stripper host-air gun. It works quite well, but

  • it takes some self-control to approximate the normal reflow temperature curve: I must keep the air gun at ~ 20 cm for maybe a minute (my arm does not like this, and neither does my brain), then move it to ~ 10 cm and move it around for the actual reflow.

  • the air flow is quite strong, some components 'drift' around the board. I don't know whether this will be more of a problem or less with smaller components (they are lighter, but also smaller and have relatively more contact area with the paste and (later) solder).

  • My stencils and eyesight are good down to 0805, I did not try below that

  • some people are worried about ions in the hot air flow that could cause static charges that can damage components. I think that has not been a problem for me, but I am not sure I would have recognized symptoms of this. I live in the Netherlands, so humidity is never low.

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  • \$\begingroup\$ How well does the T-962 work for you? Would you recommend it or do you still end up with shorted pins etc.? \$\endgroup\$ – Saad Sep 28 '12 at 14:12
  • \$\begingroup\$ I'd say it works better than my toaster oven, but a lot less good than I had hoped for. I think the very limited air flow might be the problem. The quality of the stencil I use means that I get shorted pins on an FT232 anyway, so I can't blame that on the oven. With larger distances short are very rare. \$\endgroup\$ – Wouter van Ooijen Sep 28 '12 at 14:40
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An alternative is a electric skillet. I like this method because I can get a good view of the parts with a magnifier as they reflow. I can lower the heat the second the largest part reflows. It's also easy to monitor the temperature with a infrared temp gun for the initial soaking. I have had good results with this method.

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That's a lot of components to handle this way (and a lot of components to handle even with the toaster oven technique).

Have you reworked 0402's with a hot air nozzle before? If you haven't, you should try it, and see if you still want to consider this approach. You might be fine with it, or you might rule it out on your own right then and there. You'll need a very small nozzle on your vacuum pickup, or you'll suck the 0402's right in.

If you want to still try it, I'd go in the order of most critical to least critical. I can picture myself doing something silly like going left-to-right and ruining the whole board on SMT component #248.

I think the mount for the hot air blower won't give you enough fine control for what you're trying to do. I don't envision even heating, even with a preheater, and I'm betting that many components will need plenty of individual attention.

I think that the time you spend to do this will be much longer than the time you spend putting together a toaster oven rig and reflowing. With a board big enough to handle 250 components, though, you may have a lot of rework to do anyway. The $50 you spend on a toaster oven (or even the added $125 on a temp controller and half day to assemble, if you go that route) is well worth it to preserve your sanity.

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  • \$\begingroup\$ You mention a toaster oven, but would a (more $$) dedicated reflow oven do a better job - something like an LPKF and not necessarily Chinese. \$\endgroup\$ – Saad Sep 28 '12 at 14:59
  • \$\begingroup\$ I've just used a toaster oven. I haven't had experience with anything between that and an assembly house handing me a finished product. I haven't heard rip-roaring reviews about the inexpensive Chinese products, and I haven't seen much at all in the in-between space of better quality. \$\endgroup\$ – Scott Seidman Sep 28 '12 at 15:07
  • \$\begingroup\$ I've been peeking at the LPKF ovens, and they do look nice! \$\endgroup\$ – Scott Seidman Sep 29 '12 at 12:51
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I've used a similar method on smaller prototype boards (minus the PCB Preheat), it worked fine for a quick prototype in my case, but it's very hard to avoid damaging components and still get everything hot enough for the surface tension to take effect (that's completely ignoring a reflow profile).

With that many parts on the board (and certainly with tiny 0402s) it's probably better to use an electric skillet (sand in the bottom would help diffuse heat), or if you have lots of large components that have high heat capacity perhaps a toaster oven.

For a couple boards you can typically get away with a thermometer and some manual temperature control.

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