Here's an interesting article, on How To Make, DIY solder stencils from Aluminum Cans...

Don

DIY metal stencils – a step through

Posted on January 23, 2013

This is a step through to make your own metal stencils for PCB solder paste application. In the last post I described my trial and error details of picking the materials. I am using beverage can aluminum for the stencil material and transparency film for toner transfer. In the meantime I discovered that many other materials can be used, especially for the toner transfer medium, with various degrees of success. Read the notes at the end for details. This etching method will not produce the highest quality you can get. Laser cut stencils are higher quality but a lot more $$$. For my purposes such a home-made stencil serves me very well, and saves me a TON of time (I can now apply all the solder paste in one wipe instead of using a syringe to hand apply it on all the pads). Solder paste is very forgiving and even the most imperfect stencils I produced this way give good results after reflow. The whole point is to try to apply an approximately even amount of solder paste to the pads, such that during reflow, all your pins will look the same and won't form bridges. Overall I think the cost vs outcome/quality ratio is very good. Once you get the hang of this and develop your own routine and choice of materials, in less than 30 minutes, you can produce a high quality home-made metal stencil (almost comparable to a stainless steel laser cut).