The primary purpose of using stencils in surface mount assembly is to allow a smooth transfer of solder paste onto the bare circuit board. This process requires the stainless steel foil to be laser cut, creating an opening for every component on the board. Once the stencil is appropriately aligned over the board, the solder paste is deposited over the apertures/openings. Then, the stainless steel foil is removed from the board, ready for surface mount assembly. When compared to manual soldering, it is more consistent and time-saving.
The amount of solder paste being deposited is crucial because too much of solder paste deposit can lead to solder balling, tomb-stoning and bridging. Whereas, too little solder pate can lead to inadequate solder joints. Since both the conditions can disrupt the electrical functionality of the board, it’s important to make sure that the right amount of solder paste is being deposited.
There are different kinds of stencil used for varying purposes. Knowing that every business and industry has different needs, some of the most common ones include:
Framed SMT Stencils
Also known as glue-in stencils, they are laser cut stencil foils that are mounted in a stencil frame on a permanent basis. These stencils are specifically designed for high volume of screen printing on PCBs. Also, they are a great recommendation for production runs. Offering solder paste volume control, these stencils have smooth aperture walls.
Frameless SMT Stencils
These laser cut stencils are used in collaboration with stencil tensioning systems, also called reusable stencil frames that do not require to be permanently glued in a frame. They are relatively inexpensive than the framed ones and have reduced storage space needs. They are typically designed for screen printing on PCBs and are also well-suited for short runs and PCB prototypes.
Prototype SMT Stencils
These are customized laser cut SMT stencils that are made for varying PCB needs and Gerber files. Prototype SMT stencils come with a squeegee blade and board holders. They were primarily designed for manual printing purposes.
Hand/rework stencils are basically laser cut stencils that are used to print an individual component over the printed circuit board. These stencils are mainly used for PCB rework, but can also be used for prototype PCB assembly of plain boards. Hand/rework stencils consist of a flap that is used to hold the stencil between finger and thumb.