Why rework pen? Well, I figured everyone else had a rework pencil, so I had to be a little different!
After doing some research on hot-air rework soldering pencils for building and reworking SMT PCB's, I decided that it simply was going to cost me too much. Plus, I'd miss out on half the fun of figuring out how to make something new and exciting! So, I hit the web and within half and hour or so, I'd read through the only two pages that I could find on the subject and decided to build my own. Those two pages are listed below.
SMT Hot Air Pencil
Under $20 Air-Pencil Soldering Iron
I decided that my design was going to vary slightly from the two designs that I saw on the web, but the concept was going to be the same. After a failed attempt at using an existing soldering iron with the tip drilled out and stuffed with steel-wool, I decided that I was just going to need more heat. Here is what I came up with.
To start with, I needed some kind of tube that would be electrically non-conductive and also hold up to the extreme temperature that I was going to be dealing with. A 2" long by 3/4" diameter section of Alumina tubing would be idea for this, but I just couldn't find anything like that in my junk. What I did find though was an old ceramic power resistor. This was a little crude, but would work fine for this prototype.
So, I removed the ends of the resistor and what I had left was a ceramic tube. I then proceeded to wind a length of Nichrome wire around a screw driver shaft to form the heating coil. This coil was placed inside the ceramic tube with the ends of the coil sticking out either side.
The next step was to find a piece of tubing that would fit into the end of the ceramic tube and provide a path for the air to flow across the coil. The tube also holds a thick copper wire that connects to one end of the coil. I cut a slit in the tubing so the wire could exit the tube and I taped it up good so the air wouldn't leak around the wire.
The rest is better served with a few pics, so here they are:
Here you can see how I crimped the solid copper wire around the Nichrome heating element wire. This provides a good heat-proof electrical connection. The vacuum tube fits snugly into the end of the ceramic resistor.
My power supply is a transformer from an old UPS computer backup power supply. I wired it in reversed and it provides plenty of current to run the heating coil without heating up the transformer. The aquarium pump was $6 from Wal-Mart
Here you see the Nichrome wire starting to get red after only 2 or 3 seconds with the power turned on.
This thing cranks out some serious heat and will easily burn a PCB if you aren't careful. The nice thing is that you can adjust the heat by just backing the pen away from the PCB a bit until the correct temperature is achieved.
Full heat is achieved after about 20 seconds with the power on.
Soldering these D-PAK SMT transistors to a 1 oz PCB took 15-25 seconds each