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Printed circuit board (PCB) printing is an intricate process whose quality depends on various inputs and outputs. The quality of a print is measured by the consistency of solder paste volume and height, as well as printed solder paste alignment on the PCB pads. The quality of these parameters is determined by the design and material inputs, and is subject to a number of process specifications that are defined during the manufacturing. These process specifications are usually a set of limits that must be adhered to in order to obtain a consistent yield.
One of the most important factors in solder paste printing is determining the proper size of the squeegee blade. This is based on the stencil aperture width and is also influenced by the shape and outer dimensions of the stencil openings. The squeegee blade hardness is another factor that determines the quality of the print. Using a metal or soft polyurethane squeegee blade will ensure that the printed solder paste is transferred to the PCB surface without damaging it.
Other factors that influence the quality of the printing include the thixotropic index and metal content of the solder paste. The thixotropic index controls the thickness of the resulting solder after reflow, while the metal content influences the strength of the deposited solder.
Lastly, the temperature of the solder paste and its storage conditions are also critical to the printing quality. It is recommended to store the paste at room temperature and in a sealed container. It is also advisable to use only the paste that has been stored for less than four hours, as old paste tends to cause oxidation and other problems during printing and reflow.