Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are stencil, solder paste, and a printer.
The stencil printing function is achieved through a single material namely solder paste which consists of solder metal and flux. Paste also acts as an adhesive during component placement and solder reflow. The tackiness of the paste enables the components to stay in place. A good solder joint is one where the solder paste has melted well and flowed and wetted the lead or termination on the component and the pad on the board.
In order to achieve this kind of a solder joint, the component needs to be in the right place, the right volume of solder paste needs to be applied, the paste needs to wet well on the board and component, and there needs to be a residue that is either safe to leave on the board or one that can easily be cleaned.
The solder volume is a function of the stencil, the printing process and equipment, solder powder, and rheology or the physical properties of the paste. Good solder wetting is a function of the flux.
The process begins with loading the board into the printer. The internal vision system aligns the stencil to the board, after which the squeegee prints the solder paste. The stencil and board are then separated and unloaded. The bottom of the stencil is wiped about every ten prints to remove excess solder paste remaining on the stencil.
A typical printing operation has a speed of around 15 to 45 seconds per board. Print head speed is typically 1 to 8 inches per second. The printing process must be carefully controlled. Misalignment of motion from the reference results in several defects, hence the board must be secured correctly before the process begins. A snugger and vacuum holders are used to secure the X and Y axes of the board. Vacuum holders must be carefully used, as they may affect the pin-in-paste printing process if not secured properly.
The longest process is the printing operation, followed by the separation process. Post print inspection is crucial and is usually performed with special 2D vision systems on the printer or separate 3D systems.