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USBmicro's Under $20 Hot Air Soldering
This app note involves using very inexpensive hot air soldering equipment. Using the methods mentioned in this app note, surface mount soldering becomes easier for small shops and hobbiests.
This is how I created the (probably) worlds-first air-pencil soldering iron for under $20. (Early 2001)
On the PICLIST the discussion of surface-mount soldering has often, uh, surfaced. People have talked about using a toaster oven as a reflow oven. With a little effort, this method works. You can pick up a new oven for $50. A used oven even less. This is a nice low-budget solution.
But this is about a different surface-mount soldering method. I have used a hot-air soldering tool at work that melts solder with softly moving air at about 250 degrees Celsius. This cool tool will set you back a cool $1000 or so.
So how can you make one for almost pennies? Well it turned out to be very simple. And inexpensive.
The "Under $20 (USD) Air-Pencil Soldering Iron" is made from a desoldering iron and an aquarium air pump. I purchased a RadioShack® 45-watt desoldering iron (64-2060) for $9.99 (USD) and removed the vacuum bulb. I then attached the air pump (about $8.00) tube to the location that the bulb had occupied. I needed to melt the end of the tube slightly to get it to fit over the end of the metal tube.
At this point if you plug in the iron and pump, the iron will heat and warm air will eventually come out. This air just isn't hot enough to melt solder. But here is the trick to get it to work. Remove the hollow iron tip and stuff a small piece of steel wool into the open end. This will slightly restrict the air flow. But the steel wool will heat to the temperature of the iron and transfer the heat to the stream of air. The steel wool provides more hot surface area for the air stream to interact with. Put the tip back on.
Below are two pictures from a video capture of this setup.
This creation is dedicated to my 4-year-old son's fish Sam and Julia, who passed away but donated their pump to science... and to Roman Black who prompted me to prove that this was possible.
I now use a hot air gun made by MPJA (www.mpja.com 15159-TE MPJA 306 Hot Air Gun). The hot air gun provides control over the temperature of the exit air, as well as the rate of air flow. The temp ranges from 80F to 1050F and the air flow is adjustable up to 15cfm. Certainly not "under $20", but well worth the price of $79USD.
The picture below is from a quick test of the air gun. Works rather well, even if in my haste I fudged the left two pins with my thumb. This is a 128-pin package. (I should have cleaned the flux from board before the photo, but as I said it was meant to be a quick test.)
I cleaned the board, added liquid flux, positioned the part, then ran a thin bead of paste solder along the pins. I ran the hot air gun along the pins, left to right, pacing the rate that I moved the gun to match the rate that the solder was melting and wicking up to the pin⁄pad contacts. In 3 seconds these 38 pins were soldered.
I find the performance of the gun to be on-par with the commercial, very expensive hot-air soldering tools. Many different nozzles are available for the MPJA gun to match the SMD device type.
With any soldering, contact or hot-air, I can't stress enough that the parts should be cleaned as much as possible prior to soldering.
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