Knowing just how much solder paste to dispense onto a stencil for a production run can often seem like a guessing game. The ultimate goal is for the paste to completely fill the aperture and release onto the board with 95% transfer efficiency. If the board is not sitting flush again the stencil, then you may see a greater than 100% transfer efficiency. This can lead to a final paste height that is higher than anticipated. Too little solder paste, and you risk not covering the board entirely, leading to an array of problems.
The amount of paste that you dispense onto the stencil will vary depending on the size of your production run. For a smaller number of boards, ten or under, a minimal amount of paste will be needed to achieve the smooth, rolling action that you need. For larger production cycles, you will want to apply the maximum amount of solder paste possible without loading the squeegee too high, creating flooding and spilling over the sides. A good rule of thumb is 0.75” diameter down the length of the squeegee as a minimum and a 1.5” diameter as a maximum.
Operators should always be watching for paste leaking out of the sides of the squeegee, which can result in skip defects. Paste remaining idle during the printing process can dry out and harden faster. To prevent this, operators should be trained to open the printer regularly, every 30 minutes is a good rule of thumb, and reposition the paste to the middle of the squeegee.
It is good practice to print one dummy board before each production run. This first run often uses an excessive amount of flux and can pick up any dust or contamination that can then be loaded into the apertures of the board. Running this test board also ensures that the paste is rolling and that the squeegee is set up properly, with no streaks left behind.
Using a quality solder paste paired with the right squeegee can make a big different in your product run.