Beginner's Guide to Solder Paste

Note: By "beginner's guide", I mean a guide written by a beginner. (I made it at TechShop SF, during my first weeks!) I have some technique tips to share, but for more in-depth questions, Google is your friend.

Solder paste allows you to populate a board with many tiny components, without straining your eyes and fingers. Using minuscule components saves space, and you can dramatically cut down the space between them when you don't have to solder every connection by hand.

Using a jig promotes accuracy and efficiency. We've used a couple of pieces of cardboard, secured to form a rigid corner.

In this example, we're using a custom PCB (printed circuit board) from a manufacturing company. This piece will produce four boards, each of which will power an 8BitLit lamp. Each silver pad will be connected to a component lead.

Our stencil is laser-cut from a thin sheet of plastic. Be wary when cutting it: if the plastic puckers, solder paste will gather under the ridges and make it difficult to produce good results.

In addition to the boards, jig, and stencil, you'll need lead-free solder paste and a putty knife. Later, you'll use tweezers to place your components, and you can cook the board in a toaster oven.

Despite the fact that solder paste - at least, this kind - is lead-free and cleans up easily, you still don't want to get it on your food. Wash your hands!

I like to use this method, which requires a flexible knife:
• Lay down a line of solder paste next to the stencil area.
• Put some solder paste along the edge of your putty knife, so it will move smoothly across the stencil.
• Pull the stencil tight across your board(s).
• Start with your knife at a medium angle, about 45º, and decrease this angle as you pull across. By the end, you should be pressing solder paste down from the side of the knife into the stencil.

*NB: Though the image shows a single quadrant done, I actually recommend doing a full half at a time if you can. The less you pause in your sweep across, the smoother your pull will be, and the less touching up will be necessary afterward.

Since paste tends to creep up the knife blade, this approach has two benefits:
• It saves solder paste, by pressing what you already have down into the stencil;
• It gives neater results than a straight pull, because you don't have to stop midway through the pattern to re-apply paste.

Since surface-mount components can be exceedingly small, they tend to come in paper or plastic rolls, with plastic on top to hold them in. Use a pair of tweezers to position them.

You may end up with some solder paste distributed over the board's surface. This isn't usually a problem (unless you have a lot), since it will be "sucked" onto the solder pads as it melts, or bead up on the neutral surface of the PCB.

Below, you can see an uncooked board, with matte grey solder paste showing under each component lead. The second picture shows a fully cooked one, where the paste has melted and solidified onto the pads and leads.

Only turn off the heat when all the connections are shiny silver, and leave the board to cool before moving it - the paste stays molten for a little while, and you don't want the components to shift around.

In this case, we were making four boards at a time. We used a hand shear to cut them apart. The results are glorious!

As I mentioned earlier, each of these bad boys is going into a touch-sensitive lamp, which we're building at TechShop SF (techshop.ws) - a useful place to build your own projects. Thanks to us, the toaster oven smells terrible now; fortunately, it isn't intended to cook food. Burning PCBs WILL make your space smell awful, so check the cooking directions on your solder paste!