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Knowing PCB Stencil's Role and Available Options

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Published by Pinsheng Electronics Co., Ltd February 20,2020

What is The Use of PCB Stencil?

We've already introduced PCB stencil in the previous article, which is used in solder paste printing. Obviously, PCB stencil must work together with solder paste, and usually an automatic machine in an SMT assembly line, which sometimes can be a brush if you printing at home by hand.

hand printing
Hand printing

The process is simply descript as the following figure, solder paste will be printed onto the SMDs' pads through the PCB stencil. Solder paste is sticky so that the SMDs will be on their locations in soldering, sometimes we will apply a few red gums on the pads to strengthen the connection.

Stencil printing is critical to PCB assembly, because the volume and wettability will affect the quality of solder joints, in which wettability is determined by the quality of the solder paste, as well as the machines employed and the solder flux. PCB stencil plays an important role in determining the dosage of solder paste.

  • For big components, it will require high volume solder paste - thicker PCB stencil is required.
  • For small devices such as 0201s, it will require low volume solder paste - thinner PCB stencil are required.

In the case of there are large components together with some extreme small devices on the same board, a stepped PCB stencil will be the choice for better solder printing. The stepped stencil means the target area of the PCB stencil will get processed to thicken or thinned before cut the opening. Except for the stepped PCB stencil, there are several types of commonly used PCB stencils, such as laser cutting stencil, EP Stencil, etc. Please refer to the previous article - What is SMT Stencil for that.

Step up PCB stencil
Step up stencil
 
Step down PCB stencil
Step down stencil

 

How to Choose PCB Stencil?

The main option is the thickness (regardless of frames), which should be determined involved the min chip pitch on the target PCB board, which can be referred to as the following figure.

Recommended Thickness of PCB Stencil
Recommended thickness of PCB stencil

Another available option is frame

Better for handly printing, there is no-frame PCB stencil, except for the framed PCB stencil. There are in the same use, only different in shapes and weight. Clarify this in the email sent to your vendors, you'll get the desirable one. But just note that framed PCB stencils may incur more shipping fee if you buy it abroad.

framed pcb stencil   No frame pcb stencil

3 situations for the mark points on PCBs

  1. Laser out a through-hole, the mark points on the board could be told apart easily. This is mainly used for handly printing or half-automatic machines.

  2. Laser half depth of the PCB stencil, the mark points are not visible, you'll see a valley instead, which is used for automatic machines.

  3. None opening, only for the boards without mark points.

Marked points lasering

Electropolishing or not

Electropolishing will remove the burrs on the wall of openings, improve surface smoothness, good for fine-pitch devices, such as uBGA or CSP standard packages. Comment it when electropolishing is necessary, otherwise polishing is the default.

Electropolish or not

 

What is Available in PS Electronics?

We provide turnkey PCBA services, the SMT stencil will be included in each PCBA quote, while customize stencils are also available, thickness between 0.1-0.3mm. 

But be aware, the stencil is not a part of the package by default for saving shipping costs. Please leave a note if you require it, as well as the requirements for PCB stencil in the email. 

 
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