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Solder paste is a critical component of surface mount device assembly (SMT) that fills tiny gaps between components. However, this thick sticky substance is a complex mix of metal alloys, fluxes and solvents that is highly sensitive to a variety of factors that affect its in-process performance.
The first thing to know is that just like food or other products in your home, solder paste has a shelf life and, if not stored correctly, it can spoil! This essentially means that over time the paste will lose its viscosity, which can cause printing, spotting and soldering problems.
In addition, solder paste is water-based and requires moisture in order to chemically react with the metals on your printed circuit board. This is important because if the paste does not get enough moisture, it will dry out and lose its ability to activate the flux, which in turn will not be able to create a good connection between the components on your PCB.
Once you have the paste in the correct viscosity and condition, it must be deposited on the PCB in a precise manner that is compatible with the specific printing equipment in your facility. This will include understanding the speed and separation distance required from the stencil and squeegees to the underlying circuit board, as well as the size of the bead needed for each build. Moreover, you must also be able to determine the volume of paste that will be used during a full production run, which can vary from one board to the next depending on pad area and thickness, bead diameter and other factors.