Applying solder paste through a stencil can be a finicky task. If you are having trouble getting a clean result, there are numerous variables that could be effecting your attempts. Dry solder paste or cold solder paste are often easily identifiable issues; however, an often overlooked but very common and easy to miss issue is the amount of space between the circuit board and the stencil itself. Though it may look at first as though they are flush, when you apply pressure with your solder scraper, the stencil may bow up due to gaps around the PCB.
You may have seen or used a set of L shaped brackets to hold a circuit board in place when applying your stencils. These pieces are useful for creating a reusable setup for pasting multiple identical boards; however, they also serve a very important function aside from this. These pieces reduce the amount of empty space around the PCB, thus creating a wider flat area to apply the paste. This eliminates the bowing affect greatly, allowing for better contact from the board to the stencil.
Without L Jig
With L Jig
At Leeman Geophysical we design and manufacture a plethora of PCBs with a wide range of shapes and sizes. Using L brackets we were able to get consistently good stencil runs, but we often found ourselves working with big, small, or oddly shaped circuit boards that didn’t want to work with the brackets we had. We did what anyone would do and designed our own set of tools! Using the laser cutter and a sheet of 1/16″ acrylic, we created brackets in varying sizes to create a flat plane for any size and shape of PCB.
With this simple addition too our solder pasting tool kit, we have eliminated this issue for every PCB we produce! If you would like a set of these they available for purchase HERE on our website. If you have access to a laser cutter, the files are available for free below.
Without L Brackets