Hot Air Gun for Reflowing a Board?

Question

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.

Answer 1

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.

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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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

I have used a "hardware store"-grade heat gun for smt reflow dozens of times as of now. It works really well, no problems so far.

Here are a few crucial hints as to how this is done repeatably:

• Clamp one edge of the board (not both!) into a vise, make sure the board is perfectly horizontal so the components don't drift away while pre-heating.
• Heat the board from below starting with a long distance and creeping ever closer.
• Do not keep the heat gun in one place, always sway around a bit. You do not need to get super close to the board as the heat gun is very powerful.

I have used this technique with boards that were super tiny (12x12mm) as well as with boards that had hundreds of fine pitch components on them (100x70mm).

You cannot really overheat your components this way as the heat transfer is dampened by the FR4 material between the bottom of the PCB where you apply the heat and the top where your components are sitting. I also haven't experienced any discoloration of the bottom pcb yet.

Example of how the board looks after reflow: