Air pencils for SMT soldering?

Are air pencils really a lot easier for SMT assembly? Are there drawbacks, besides the cost? Recommendations for a good system?

I've seen impressive demos using QFP packages, but I've also heard frustrations about passives getting blown off the board. (I've also read that ceramic chip caps shouldn't be iron-soldered because of the thermal stress.)

Metcal offers a model, and Zephyrtronics does an impressive demo at shows - any comments on these?

Pre-heaters seem like a great idea - are they practical, or a gimmick?

Thanks, Richard

in article snipped-for-privacy@uni-berlin.de, Paul Burke at snipped-for-privacy@scazon.com wrote on 11/29/04 10:52 AM:

I have to agree that IMHO they aren't too good for SMT work. You might be able to take off an 8 pin DIP or a discrete part with relative ease, but getting a larger part evenly heated is a lot harder.

If you use paste with impregnated solder, you have to be careful because the pencil will blow little balls of molten solder all over the place.

I have a Hart hot air station (functions like 2 hair dryers blowing at each other) that I used for 98% of the SMT repair work I do. I either use a smaller nozzle or usually mask off adjacent areas and focus the heat where I want it. Makes it easy to position the part just so.

I've discovered that hot air pencils do have one good use: "precision" shrinking of heat-shrinkable tubing!

Jon

I haven't had access to a real desoldering station for a decade. Been using a paint stripper with some custom baffles and nozzles to do SMT. Second gun on the back for preheat if necessary. Small hotplate also works great for preheating the backside. Plenty of heat, but too much air. Blowed stuff around...

My stripper bit the dust and I had a SEVERE case of sticker shock when I went looking for a real hot air pencil for soldering. This is the cheapest thing I found that looked decent.

Anybody used one of these?

By accident, I discovered that a Weller Portasol Butane soldering iron with the catalytic hot air tip works wonderfully for small jobs. Air flow is almost zero and there's a lot of IR going on too.

Experimented on a laptop hard drive board. Was the best/easiest hand job I've ever done... on a circuit board.

mike

It's hard to prototype QFN and other packages that have no leads protruding (eg. some SMT inductors) without hot air.

Best regards, Spehro Pefhany

I've wondered about re-using the pads after their "co-metalization" solder is used. Does it clean off effectively, or might it taint a subsequent joint (and would it be an issue)? I expect it's a bit like Chip-Quik's product, and the ability to remove & reuse is great.

Yep, even if I stick with the iron, it looks like I'll need a pre-heater for anything with power planes. While I got good results before at 225 celsius, the large planes sink too much heat even at higher temps. I may try an even shorter, flatter tip to get better heat transfer at lower temps.

Normal tip or the two-pronged 0402 tip? (Those seem like they're really for removal, not assembly - are they?)

BTDT. I've been using 0402 for a while now with an iron with good results, but it takes a steady hand to hold them in place - the surface tension on the iron is enough to pick them up. Helluva time keeping flux off the tweezers too; it's just sticky enough to make things interesting.

Thanks! Richard

Good feedback, thanks. Can it be set low enough to do 0402's without holding them in place?

Anything special about the Zephyr unit vs. others, or just positive results moving to an air pencil in general? (I like that it's core to their product line vs. other vendors' treatment of their air pencils.)

Thanks!

?? Because the hot plate would touch the bottom-side components, or because they'd de-solder? (But solder shouldn't melt at the 150c pre-heat temp...)

I've seen some (e.g., Metcal's) that are a flat hot element that I guess you'd set the PCB onto, but an air-based unit would seem to make more sense with bottom-mounted parts.

A lot of our bypass caps end up on the bottom, so I'm curious if you ran into a particular problem.

Thanks!

Slick, if even just for bulk removal work. I wonder how low the CFM goes.

I've found them totally useless. Maybe it's just my technique, but for the small quantities I do (i.e. prototypes) the soldering iron has proved the best bet, with a wavetip for QFPs and TSSOPs.

Paul Burke

I agree. You are supposed to use air pencils with a hotplate to get everything pretty warm first, but since I have components both sides, this doesn't really work ...

De-soldering is another game entirely

Dave

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I like the Zephyrtronics pre-heater. The desoldering stuff that comes with it is very easy to use and leaves the PCB un damaged.

The hot-air-soldering pencil is the easiest way to solder the TQFP packages. You put on the paste place the chip, preheat the PCB and then go around the part with the hot air. If you use only modest amounts of paste, there is virtually never a solder bridge. Every pin gets soldered.

Soldering SMT film caps is a job that must be left to the experts. Ceramic capacitors can be soldered with an iron if you are careful but it is not recomended for thermal stress reasons. Use the coolest most thermally conductive tip you can and ideally pre-heat the PCB if you are using an iron.

On proto-types I often solder parts with a metcal. 0402 components are about as small as is practical for manual work. If you are working on stuff this small, you have to have non-magnetic tools. Even a slightly magnetized tool will pick up an 0402 part.

Not sure. I usually have some 2nd-hand activity.

Not familiar with the others, but the frame and under-heat setup of the is Zephyrtronics is very nice. We modified it a bit to take into account our smaller boards. Intended for prototype work of course.

And what comes out doesn't contain much oxygen so it doesn't oxidise the solder. It might even contain a bit of carbon monoxide to reduce it?

I've reused the pads with never a problem. You use those foam Q-tip like things to clean the area while it is still hot. Almost all of the metal comes off clean leaving just a tiny amount on the tined pads.

I use the cone shaped tip. The long thin ones don't conduct heat well enough. There is a temperature gradient on those small tips. Solder always flows to the hotter place which in this case is away from the work and up the tip.

I just finished replacing 3 resistors. I did the 3 with no trouble except for losing the part among the visual clutter of the PCB.

The very thin wirewrp wire can be soldered to 0402 pads. I've had to add some circuitry to my design and this is how I hooked it in.

Yes, I've only limited experience with QFN. What I did there (just for the prototype, the great thing about PCB Pool is that you KNOW you've got to pay for the phototools again, so a few specials just for the prototype don't matter) was to make a hole - for a larger QFN I'd use more than one- in through to the underside pad, large enough to get the tip of a fine iron right through. Solder the component on "as usual" (hard with such little pads on the side, but possible), then flood the undeside with solder. Worked pretty well.

Paul Burke