Friday, December 09th
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Stencils in the world of manufacturing electronics and printed circuit board assembly (PCBA) are not so different from the stencils we were all made familiar with in grade school. . They are designed for the exact same purpose: to ease the process of applying a medium only to specific areas for your intended project while protecting the unexposed areas.

Letter Stencil

Letter Stencil

 

What is a laser cut SMT stencil?

SMT laser cut stencils are used to aid the application of solder paste to surface mount component pads in the field of printed circuit board assembly. In other words, SMT stencils allow you to place solder paste only where you want it, in the quickest and easiest possible manner. Simply place the stencil onto your board and then apply the solder paste over the top. When the stencil is removed, you are left with your solder paste only where you need it. This process is particularly useful for those who intend to place their components by hand.

 

Why do you need it?

In electronics manufacturing, a stencil is used for the following reasons:

1. When using SMT (Surface Mount Technology) in the design of the printed circuit board (PCB).

2. When using automated component placement equipment (unless it is a hand-loaded prototype).

This equipment replaces the process of hand soldering each component.  Once components are placed onto the solder paste covering the  printed circuit board, the PCB is then heated or ‘baked’ to solder the components to the board.

The stencil is used in conjunction with a stencil printer like the one demonstrated in this video.

 

What are some of the different kinds of stencils?

There are several variations of stencils available: Desktop Stencils, Framed Stencils, Frameless Stencils, and Rework Stencils:

Desktop Stencil

The Desktop Stencil is a simple stencil that is commonly used for assembly of prototype boards by hand.

Desktop Stencil

Desktop Stencil

 

 

 

 

 

 

 

 

 

 

Framed Stencil

The Framed Stencil has two main categories: Fixed Frame and Universal Frame Stencils.

  • Fixed frame stencils range in size from 15″x15″ up to 23″x23″ and use either a solid (extruded) or hollow aluminum frame to hold the stencil.  In a fixed frame, the stencil itself is permanently attached to the frame.
  • Universal frame stencils, on the other hand, allow changes to be made to the stencil as needed.  This can help manage overall costs because, when it comes time to reorder, you only need to purchase a frameless stencil without having to replace the complete stencil with a frame.

 

Framed Stencil (Extruded)

Framed Stencil (Extruded)

 

 

 

 

 

 

 

 

 

 

Frameless Stencils

Frameless Stencils are most commonly ordered by those who already have a frame that they will attach their stencil to manually.

Rework Stencils

The Rework Stencil is a small, frameless stencil that is typically used for just one component. It is designed to be small so that it can be used to replace that one problem component by hand on a printed circuit board that is already equipped with working components.

 

What are some of the standard characteristics of Stencils?

  • Most SMT Stencils are made of Stainless Steel due to its durability. Mylar is often used as well.
  • Stencils typically range in thickness between 0.005″ to 0.007″ thick.
  • The pad size (hole or aperture) on the stencil compared to the actual pad size on the PCB is usually scaled down by about 10% or 0.002″ from each side of the opening.  This is usually done during the designing of the PCB and is generated as one of the Gerber files.
  • The most common size of a framed stencil is 20″ x 20″.
  • The standard for stencil design from IPC is detailed here: IPC 7525.

 

For those more inclined to DIY projects, here is a video that explains how to make SMT stencils with an aluminum soda can.

 

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