Introduction To PCB Stencil
A PCB Stencil main purpose is to simply transfer the solder paste to a bare circuit board. A stainless steel foil is laser cut to create an opening for each surface mount device on the board. Once the PCB stencil is properly aligned with the top of the board, the solder paste is applied over the openings (in a single pass, a metal squeegee). When the stainless steel foil detaches from the board, it remains the solder paste, ready for the manufacture of the SMD. In contrast to manual soldering procedures, this process ensures consistency and saves time.
The deposited paste release to the board is manipulated by opening size of the aperture and the thickness of the foil made out of stainless steel. The excessive paste will cause problems for the connectivity and bridging whereas, less paste will compromise the strength of the soldering and joints. The thickness of foil varies with the devices for which it is being used. 0.020” pitch SOICs can work better with a stencil giving a thin solder paste whereas thick solder paste works best with 0.050” pitch SOICs. The thickness of the paste stencil for solder ranges from 0.001” to 0.003”. The normal thickness for most of the boards is generally between 0.004” and 0.007”.
Selective Sizes With Stencils Are Now Offered With NANO Coating
For nano coating, the contacting sides of the tiny components are applied with a water-resistant nano-coating that opposes solder and flux as well. The phenomenon results in the better release of paste for the tiniest components. The application is made in the factory so the component is ready to use at the time of purchase. The nano coating will be recommended to be used for fine pitches of QFN and IC, micro BGA’s, and the electronic components of 0201 type.
The 6 most common verities of stencils by size and type is mentioned below.
- PCB Prototype Stencil
Size: 10” x 12” max
Type: Squeegee
Add on: Board holder
Size: Over 10” x 12”
Type: Squeegee
Add on: Board holder
- Kit-Leaded Prototype Stencil (SMT)
Size: 10” x 12” max
Type: Solder leaded paste
Add on: alcohol wipes x 2, gloves x 1 pair, temperature marker
- Kit-Leaded Prototype Stencil Oversized (SMT)
Size: Over 10” x 12”
Type: Solder leaded paste
Add on: alcohol wipes x 2, gloves x 1 pair, temperature marker
- Prototype Kit-Lead Free Stencil (SMT)
Size: 10” x 12” max
Type: Solder lead-free paste
Add on: alcohol wipes x 2, gloves x 1 pair, temperature marker, grab and place tool,
- Prototype Kit-Lead Free Stencil Oversized (SMT)
Size: Over 10” x 12”
Type: Solder lead-free paste
Add on: alcohol wipes x 2, gloves x 1 pair, temperature marker, grab and place tool.
Where Can We Make PCB Stencil
Framed stencils manufactured by stencils unlimited are easy to obtain. Upload your Cream Top from the desired PCB layout software after exporting it. After automatic calculation for the best suitable thickness of your stencil, your designed item is at your door the next day.
If your board fab house can make stencils, they can also make stencils for you. A tile Gerber (Cream) is important for panel making. Don’t forget to ask them to have the GBC file tiled if your expected design is also tiled.
IPC 7525 has a detailed document about calculations of stencil thickness. The approximate value shall be 2.64 + 0.0831 * pitch-of-component.
PCB Stencil With Frames
Soldering a board all at once is important if you aim to be a PCB stencil manufacturer using SMT method. It is important to know that it is better to reflow instead of soldering back. Now it is certain that depositing paste on the pads with precision is required here. You can use your bare hands to deposit the paste through a syringe or you can make use of DIY stencil-like laser cut Kapton or Mylar sheet. But it is important to know that using a framed stencil is going to be helpful if you want to make hundreds of boards with tiny pitch parts like 0.4 mm pitch.
Using A Machine
A PCB stencil printer uses framed stencils for screenprinting. The bolted frame to a hinge allows you to maneuver the stencil in different directions and also you can replace the PCB by this method.
With a 5 second gap between screenprints, about 10 to 50 PCB panels can be done through this technique. As the stencil is flat and stretched, the bolting situation allows minimal movement which is ideal for making multiple units at a single time.
Unframed PCB Stencils And Framed Stencils
A PCB stencil maker must know that being pre-stretched is one big advantage of framed stencils. The larger panels with stencil-board-alignment being very sensitive to an even an mm for opening and closing bridges.
A frame stencil can laser cut the stainless sheet of thin steel and stretch it into an aluminum frame of a solid cast. With the proper equipment, you can properly frame metal unframed stencils by yourself but beware that this method is not cheap.
