Assembly Process and PCB Stencils

    Conventionally, board assembly process was performed manually placing and soldering the components by hand. However, with increasing complexity of circuit boards and extremely small sizes of surface mount packages, board assembly is moving towards application of solder paste using PCB stencil and machine-based component placement.

     During the board assembly process, solder paste only needs to go WHERE a component must be in electrically conductive contact with a metal landing pad on the board itself. Hand soldering method only makes sense where design is simple, and the number of components is less. Naturally, hand soldering techniques are also prone to human error which results in inconsistent results. On the other hand, PCB stencils resolve these issues and are an effective and efficient method of applying solder paste. A stencil, as the name suggests, is a tool to make repetitive impressions. A PCB stencil acts as a guided template for the application of solder paste only on the desired areas of the board. Stencil is placed on the circuit board and aligned in the correct position. A runner is moved across the stencil screen which squeezes appropriate amount of solder paste through the holes onto the circuit board. Every stencil is customized according to the design of the PCB so that the solder paste gets applied only on the solder pads. For surface mount technology (SMT), the stencils are called SMT stencils. An SMT stencil consists of outer aluminum frame, polyester mesh and stencil foil. Stencil foil is tensioned and fixed to the aluminum frame by polyester mesh.

Manufacturing Processes

    Typically, stencils are made of stainless steel or nickel and can be categorized based on the manufacturing process (process by which aperture is formed on the stencil foil):

Electroforming:

    This is an additive process in which stencil foil is created by electroforming nickel. This process is also known as E-FAB. Although this process results in higher initial cost, however price is independent from the number of apertures. Finally, higher processing time is considered a drawback of this manufacturing process.

Chemical Etching:

    This is s subtractive process in which apertures are achieved through chemical etching. This is relatively a low-cost process; however, it results in inferior results due to rough aperture sidewalls.

Laser Cutting:

    This is also a subtractive process in which removal of the stencil foil material is achieved by laser cutting. Naturally, laser cutting results in finer results and helps in achieving greater accuracy. Laser cutting also produces more consistent results than chemical etching process. Since the process is not dependent on the chemical conditions so there is essentially no requirement for protection from moisture.

    Depending on the method chosen for the application of solder paste, stencils can be further divided into following types:

     1- Framed Stencils

    These laser-cut solder paste stencils are permanently mounted in the frame where the border mesh provides the necessary tension to the stencil foil. These are designed for bulk machinery production.

     2- Frameless Stencils

    These stencils are not required to permanently mounted on the outer frame. These low-cost stencils are recommended for prototype PCB assembly or short runs.

     3- Prototype Stencils

    The Prototype stencils are designed for manual printing application. It removes the element of human error from the manual printing process.