What is a PCB Stencil?

A PCB Stencil is a sheet of stainless steel with laser-cut openings used to place solder paste on a PCB board for surface mount component placement. The PCB stencil is used to deposit solder paste on designated places on a bare PCB board so that components can be placed and perfectly aligned on the board.

Its main function is to accurately deposit the right amount of solder paste on SMT pads so that the solder joint between the pad and the component is perfect in terms of the electrical connection and mechanical strength.

Using a PCB Stencil to apply solder paste makes the PCB Assembly process faster. It also makes it easier to apply solder paste accurately.

PCB Stencils are manufactured by creating openings through lasers on a stainless steel foil based sheet at the positions where Surface Mount components need to be placed on the board. The stencil is then placed on the top of the PCB and aligned perfectly using positioning points called fiducial marks. After alignment, the solder paste is applied with a metal squeegee blade. As a result of this, the solder paste gets applied to the PCB board where the surface mount components need to be placed. So when the stencil is removed, components can be placed on the board.

While developing or ordering a PCB stencil, there are some important points to keep in mind:

Thickness of the stainless steel sheet: Accurate thickness of the stencil sheet ensures the release of the desired amount of solder paste through the opening. 

The thickness of the sheet and aperture size are important factors that affect the amount of solder paste to be deposited on the board since extra solder deposition can cause solder bridging. On the other hand, less solder deposition can also create weak solder joints, which affect the functionality of the final PCB board.

Formula to calculate the approximate thickness of the PCB stencil is stated below:

Aperture size of the opening created: To avoid solder defects like bridging and solder beads, the stencil opening is designed slightly smaller than the PCB pad size. After solder deposition, when the stencil is removed from the board, there is the possibility that the solder paste may get transfer to the PCB or stick to the aperture walls of the stencil. The area of the PCB pad should always be greater than two-thirds of the area of the inside of the aperture wall of the stencil to avoid this problem.

Stencil Material: The stencil material also affects its ability to deliver the solder paste from the opening/aperture to the PCB pad. Stainless steel is the material that is usually used for stencils.

Stencil Alignment: To get a perfect print of solder paste on the PCB pads, registration marks called ‘fiducial marks’ are added to both PCB and stencil. These fiducial marks ensure good alignment between the PCB and stencil.

PCB stencils can be classified based on their aperture type and mounting style: Laser-cut stencils, Chemical-etch stencils, Framed PCB Stencils, Frameless PCB Stencils and Hybrid PCB Stencils. Click here to learn more about the different types of PCB Stencils.

Usually, the PCB Stencil will be created or developed by the company that is fabricating the PCB Board. However, often you need more stencils or need to replace the stencils. In this case, PCB Directory has created an online directory of PCB Stencil Manufacturers. Click here to see a list of PCB Stencil Manufacturers.

You can also get quotes on your PCB Stencil requirement from multiple companies by filling out a single form. Click here to try out the PCB Stencil Quotating tool.