Hot air/hot plate/toaster oven soldering with solder paste are generally much easier than soldering by hand for circuits with more than a few SMD components. And a soldering stencil to accurately place consistent amounts of solder is much easier than laying down trails of solder with a syringe -- and there's much less board cleanup of solder bridges to do when a more controlled amount of paste is applied.
Unfortunately for those of us who prefer to etch a few proto boards at home when possible to test out a basic design & build quick development boards, stencils generally cost $35 or more and take a few days to get back. This is a way, using the same tools as etching circuit boards, to build quick proto solder stencils. The quality probably won't live up to the stainless steel or mylar ones you'd buy, but you might be surprised.
Note that the same method can also be used to chemically mill other designs -- decorative pieces for jewelry boxes, shadow designs for projecting with a Luxeon, etc -- possibilities are endless.
This method as posted won't eliminate the cleanup work, and I'm sure there are refinements to this method that will make it all work that much easier/better. I look forward to comments from others on ways to improve the method.
Apologies for lack of pictures in the later part; to get the pictures here I did a quick run-through but didn't do the actual etch/solder paste application. Poor quality pictures are due to close shots with a cell phone camera.
Step 1: Before We Begin
The method here is based on how I do circuit board etching -- which is likely very different from how you might do it. I've been etching circuit boards for a few years now and how I do it has changed quite a bit over that time as I find new tools & approaches. Almost any standard way of etching boards should work fine for doing this too; whether you do iron on, light sensitize boards, etc; just a bit of thought and you should be able to adapt this to work with your methods.
Step 2: Step 1: What You Will Need
A design (in this case I'm assuming you've laid out your board with Eagle)
A transfer method that you'd use for transferring to a circuit board (I use Press-n-Peel Blue)
FeCl or other etchant
Plenty of scotch tape
Very thin copper sheet (I found thin copper strips, about the thickness of a business card & about 4 inches across, 14 ft long, at my local hardware store in the garden section -- apparently it's used to keep snails out of gardens)
Step 3: Creating the Stencil Design Transfer
There may well be a better (more accurate) way to do this with eagle. I do my layout as normal; in the CAM section I select only the stop mask (which basically gives you the areas where the solder mask would not cover). This is a little looser than where the pads should be; unfortunately the "pads" layer does not include SMD pads. However the results still seem pretty decent; on high pitched parts, though, there will still be some minor bridging & necessary cleanup as a result.
As the output format, select "PS_INVERTED" -- we want the inverse of the stop mask.
If the stencil is for the top of the board, select "tStop" and check the "mirror" box.
If for the bottom, select "bStop" and make sure "mirror" is NOT selected.
(honestly, since we're etching all the way through, "mirror" or not doesn't really matter. It does of course when etching circuit boards though).
The rest of this step I'll assume you're using Press-n-Peel Blue as your transfer method; adapt as necessary.
Print out the design on to regular paper. Look it over, make sure it looks how you'd expect. When you're satisfied, cut a piece of PnP Blue just a bit larger than the image on the paper and tape it over the image, glossy side down. Make sure you've got some space all the way around, as alignment on laser printers is generally not so perfect and images can shift a few mm from one printing to the next. You should only need one piece of tape, on the side of the PnP blue towards the top of the sheet of paper, so that it's held in place as the paper gets pulled through the printer, but otherwise the PnP blue is free to move a little and flatten well against the drum/fuser in the printer.
(note: I've used this method on a LJ4000 and a LJ4 with no ill effects, but I take no responsibility if this eats your printer).
Hand-feed the paper back into the printer (or load it into a paper tray), making sure you know where the printer is going to print and everything's right. If you need to, write an "x" on the bottom of a plain sheet of paper and print the image again, making sure everything comes out how you'd expect it to, before passing the pnp+scotch tape+paper back into the printer.
Step 4: Prepare the Copper and Transfer the Image
Select a piece of copper (brass will likely work as well, plus give some more rigidity, though I'm not sure how this might slow things down); it should be the same size or a little larger than the stencil you'll be making. You may or may not want to leave extra along one edge for gripping & moving the finished stencil around.
The rest of this step is pretty much the same as you'd use for transferring a design to a circuit board. Only difference is instead of transferring to the copper clad board, you transfer to the copper piece.
Do the standard scotch-brite scrubbing of the copper (wet scotch brite pad, scrub until the entire area that will be covered by the transfer is bright & obviously scrubbed -- even if the copper comes all pretty and shiny, scrub it anyhow; often with pretty copper there's a thin coating of something protecting it, which will also keep the transfer from working well). Since scotch brite pads are fairly expensive, I usually just cut a small rectangle from a full pad & use that, rather than a whole pad each time.
Let the copper piece dry completely.
Remove the PnP blue film from the printer paper & pull off the scotch tape. Trim as necessary to get the PnP film to fit on the copper piece. Use scotch tape to attach it, making sure it doesn't bubble up in the middle. I suggest taping on at least two alternate sides to keep it from possibly slipping around during the transfer. Try to make sure to blow or brush off any dust that might find its way between the film and the copper.
Tape the combination of PnP film & copper to the back of a single-sided circuit board (or one which has no copper on either side). You won't want to attach it to the copper side of the board, as the extra copper on the board will draw heat away when doing the transfer. again make sure it's all completely flat.
Use an iron or modified laminator to do the transfer. Using the laminator method, I find I usually need to pass things through 6-7 times for a good transfer.
Step 5: Prepare for the Etch
Assuming the design transferred well & completely (if not, just like any other etch, do it until you get it right/sharpie pen minor blemishes), you're almost ready to etch. However you've got the whole back side of the board that needs to be covered to keep the copper from completely disappearing.
I use 2 layers of scotch tape; it's a lot of scotch tape and takes a few minutes, but it works well to keep the etchant off the back, and it's possible to remove without completely destroying the thin copper (packing tape for instance would probably be hard enough to remove that you'd end up with a mangled & completely bent board. Duct tape doesn't work so good when put it etchant & heated.)
If you left a tab to hold on to the stencil, put tape over this on the front side too to keep it from disappearing. Make sure that the transfer is continuous between the tab & the transfer (you don't want any thin lines uncovered that will end up seperating your stencil from the tab).
Step 6: Etch
I etch with FeCl heated to about 90 degrees (Celsius). Since the copper's a bit thick compared to the copper on circuit boards, it'll take a bit to complete & will be a bit harder on your etchant. Check periodically to see how things are going & determine when the etching is complete.
(edited) note: I've recently read that it's recommended not to go over 55C for FeCl -- as you'll end up with fumes which can damage the resist (and probably other things in the vincinity).
Step 7: Clean/remove Tape
Clean the board well, making sure to remove all traces of FeCl. Some folks recommend using a small amount of diluted ammonia on the board to neutralize any small bits of FeCl left. Remove the scotch tape carefully from the back (this may take a bit of patience). I clean the front with acetone or scrub again with scotch brite pad (not sure what impact toner ink getting scraped into the solder paste later might have if this isn't done).
What you're left with is a piece of copper with sharp details cut into it.
The image is a bad cell-phone picture taken of the last stencil i made. (Note that the TQFP package in the middle has no seperation between neighboring pins; that is to be expected, though not necessarily optimal).
As mentioned before, you can make pretty much any design; it doesn't have to be a stencil... and from some basic tests, brass should also etch with FeCl, though likely a bit more slowly.
Step 8: Using the Stencil
Check the sparkfun tutorial for how to use a stencil.
Note: for my spatula I used a piece cut from what I think was marketed as a "Japanese putty knife", found at an Asian market (sf people, it's the one across from Thrift Town in the mission). They're like $.99 for a pack of 3. I used a mini brake to cut a small rectangle that is easy to manipulate working on small boards.
Also you don't have to have a big glop of solder paste; I just squirt a blob from a small syringe of paste (bought mine from Chipquik) with the needle removed, mash it around a bit with the spatula to spread it out, and then the spatula across the holes, making sure they all get filled.
Step 9: Let Me Know What Improvements You Come Up With
And I'm also curious to see what other kinds of things people use this method to chemically mill.