Nuts and Volts - May 2014

in. I’m very cost-sensitive, so I’m willing to wait two weeks or more to get my boards back. You’ll need to decide what you can tolerate as far as waits and prices. Maybe have a couple different go-to places for when you might need something right away or for when you’re in no hurry. As always, do searches and compare.

Once you have chosen the board house you want to use, just follow their onsite directions and upload the folder containing the Gerber and NC Drill files. Make sure you read their file submission and design rules. For example, some might require that your minimum spacing is 6 mil. That’s actually fairly small. To give some perspective, the spacing between the pins on a Q44 chip (like the Propeller) is 16 mil. Ordering Your Stencil

Ordering a solder stencil is very easy, especially if you ordered your boards from a place where the stencil is included with your order. Then, you’re done already! However, if you still need to order a stencil, you have lots of places to choose from.

The first thing to decide is what material you want your stencil to be made out of: kapton or metal. Metal costs more than kapton, but will last for thousands of uses. Kapton stencils will last for hundreds of applications with proper care, and are perfect for prototyping due to their cost. A typical metal stencil will cost more than $125 plus shipping, while a kapton stencil will cost approximately $25 plus shipping. (Refer to Figure 8).

One nice thing you can do with 8. 5” x 11” Kapton sheets (if the board house allows it) is to fit two smaller stencils on one sheet to save money.

Ordering a stencil should be very straightforward. Just follow the instructions for uploading the Gerber file which is typically called “top paste” for single-sided boards. If you’re doing a two-sided board, don’t forget to also upload the “bottom paste.” Ordering Solder Paste and Storage

There are a number of different varieties of solder paste; some with lead and some that are lead-free. I normally try and order my solder paste from the same place I’m getting my stencil from so that I save on shipping. Just make sure you follow all of the safety instructions and local laws if you are using lead-based solder paste. I’ve only used lead-based solder paste myself, so I can’t comment personally as to which is easier, but I’ve been told that lead-free solder tends to be a bit more difficult to work with.

A number of solder paste manufacturers recommend that you store it in a cool, dry environment. This helps prolong shelf life by preventing the solder flux from evaporating. I picked up a cheap dorm room style fridge that I store my solder paste in. It is a “non-food” fridge with a sign on it marking it as such, along with the material safety datasheet (MSDS). Holding Your PCB with a Stencil Frame