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What are SMT Stencils?

SMT stencils, also known as solder paste stencils, are custom made stencils used during the printed circuit board assembly (PCBA) process when soldering SMT components. SMT stencils are usually made of stainless steel and cut with an ultra-precision infrared laser. SMT stencils contain apertures that match the SMT pads on the bare boards. Using a squeegee blade the solder paste is pushed through the apertures and deposited on the SMT pads. This procedure can be done manually by just holding the stencil during the printing process or automatically by using a stencil printer. Once the solder paste is printed on the bare boards the SMT components could be placed manually with a pair of tweezers or automatically using a pick and place machine. Finally, the boards are placed in a reflow oven to complete the SMT soldering process.

The Gerber files, particularly the paste files contain the XY location, shape and dimension of each aperture. Prior to manufacturing a CAD engineer usually edits the paste files and makes aperture modifications as needed. CAD application can be used to perform a geometric analysis and obtain the optimal stencil thickness.

TYPES OF SMT STENCILS

Stencils can be manufactured with or without frames:

• Framed Stencils
A sheet of stainless steel also known as foil is permanently mounted in an aluminum frame using a mesh edge to firmly stretch the stencil foil on the frame. Framed stencils are designed for high volume printing using an automatic stencil printer.

•Frameless Stencils
The foil is not permanent mounted to a frame however, some frameless stencils are designed to work with stencil tensioning systems also known as reusable stencil frames. Frameless stencils are significant less expensive than framed stencils and reduce storage space requirements. They are designed for low to medium volume printing.

• Prototype Stencils
They are a type of frameless stencils specifically designed to manually print solder paste onto prototype PCBs. They work best when the boards are small (should be no larger than 12”x10″).

• Step / Multi-level Stencils
Step stencils are used in electronic assemblies that contain a mix of large and very small (fine pitch) SMT components. The stencil thickness is used to control the amount of solder paste on the board. The foil is thicker (e.g. 0.006”) in areas that contain large SMT components and is thinner (e.g. 0.004”) in areas that contain small SMT components.

• Electroformed Stencils
This type of stencil is nickel-based, and it offers the best paste release characteristics available. Electroformed stencils have a longer stencil life than regular stainless-steel stencils because nickel is harder and has a lower coefficient of friction compared to stainless steel. The trapezoidal sidewalls of these stencils allow for better paste release.

PARTS OF SMT STENCILS

•Apertures
They are holes on the stencil used to deposit/print solder paste on printed circuit boards.

•Fiducials
They are marks on the stencil used to align the printed circuit board with the stencil. The fiducials can be a hole etched on the foil all the way through or they can be half etched on the board side or on the squeegee side.

•Frame
Frames are mainly used to stretch the foil and keep it from bending. Cast aluminum frames come in the following standard sizes: 8”x8”, 10”x10, 12”x12”, 15”x15”, 12”17”, 23”x23”. Extruded aluminum frames come in the following standard sizes: 23’x23” and 29”x29”.

• Foil
It is a sheet of stainless steel usually mounted on a frame containing the apertures and fiducials.

• Thickness
The thickness of the foil determines how much paste will be deposited on the PCB. The thicker the foil the more paste.

•Design
Aperture modification is an important part in stencil design. Most apertures are designed to be smaller than the pads on the board. Apertures can be designed with different shapes to prevent or reduce assembly problems such as solder balls or tombstoning.

STENCILS UNLIMITED’S PROCESS?

The customer provides the Gerber files during the order process.

If the files for the solder paste layers are not provided our CAD team creates them using the copper (top and/or bottom) and drill files.

  • Aperture modification is done based on standard rules and/or according to customer’s requirements.
  • The outline of the stencil may need to be adjusted. Frameless stencils may require special mounting holes.
  • A check plot (in PDF format) is sent to the customer for confirmation.
  • After confirmation, the design file is sent to the laser team for manufacturing.
  • After manufacturing the stencil goes to QC for vision inspection.
  • Finally the stencil is packed and shipped. A tracking number is automatically emailed to the customer.

For more information please visit our SMT Stencils page.

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