Stencil Efficiency for Solder Paste Deposit


Assembling PCBs or Printed Circuit Boards requires the operator to deposit solder paste on the pads of the board to connect surface mount components for a better result. The solder paste deposit must be even, and the execution must be precise. For achieving this, the operator makes use of stencils. A stencil is an important and essential piece of equipment in the surface mount assembly process for repeatable and effective deposition of solder paste.

The use of a stencil allows faster, easier, and more accurate deposition of solder paste. It minimizes the error-prone manual method of solder deposition. The stencil is a foil, usually made from a thin sheet of stainless steel. The stencil manufacturer uses a laser to make cuts in the foil matching the places where
surface mount devices will appear on the board.
Before depositing, the operator must register the stencil with the PCB underneath it. Then they use a metal squeegee blade to draw solder paste across the entire length of the stencil. In the process, solder paste passes through the cuts in the foil and deposits on the PCB pads underneath. When the operator removes the stencil from the board, the solder paste remains on the pads. Therefore, the stencil enables placing the solder paste accurately on the board, allowing mounting of the necessary surface mount devices.

Stencil Efficiency for Solder Paste Deposit



Stencil Efficiency


The amount of solder paste deposited on the PCB depends on:

  • The thickness of the stencil and the dimensions of its apertures—they determine the quantity of solder paste the stencil will transfer to the PCB pads.
  • The surface finish on the aperture wall of the stencil—this determines the ability of solder paste to free itself from the surface of the aperture wall.
  • The registration of the stencil with the PCB—this determines the positional accuracy of the solder paste deposit on the PCB pads.

During each print cycle, the squeegee blade, traversing the stencil, pushes solder paste across the stencil apertures, thereby depositing the paste onto the board. Preferably, with the removal of the stencil, all the solder paste in the aperture must detach itself from the stencil and deposit on the pads. However, this transfer quantity depends on the thickness of the stencil and the dimension of its apertures.

The ability of the solder paste to free itself from the surface of the aperture walls depends on the surface finish and coating on the surface of the aperture wall. With laser-cut apertures and electropolished and electroplated surfaces, the paste transfer improves substantially.

The registration of the stencil with the PCB, coupled with the dimensional precision of the apertures, defines the accuracy of the solder paste deposited on the pads. With modern technology and CAD techniques for making stencils, this is no longer a factor. However, dimensional accuracy also depends on the temperature of the stencil throughout the printing process.

Important Stencil Elements


To improve the stencil efficiency in the PCB assembly process, considering a few key elements of the stencil during its design is important:

Thickness


The required thickness of the stencil foil comes from the aspect ratio and area of the aperture. The aspect ratio is the width of the aperture divided by the thickness of the stencil. For an acceptable solder paste transfer, an aspect ratio of 1.5 or greater is suitable. Achieving an accurate thickness is necessary for a proper release of solder paste through the aperture, especially for fine-pitch BGAs and QFPs.

Aperture Design


The aperture design on the stencil must match the land area on the printed circuit board. With increasing miniaturization, the pressure on the stencil printing process is also increasing. Regulating the aperture design is essential for proper solder paste transfer efficiency. For this, one must consider the aspect ratio and the area ratio as crucial aspects. The right aperture size decides the volume, uniformity, and definition of the solder paste deposit on the board’s surface.

Positional Accuracy of Aperture


The positional accuracy of the apertures on the stencil defines the effective placement of the solder paste on the PCB pad. That necessitates the proper alignment of the stencil apertures to the pads on the PCB.

Stencil Material


The release of solder paste from the aperture also depends on the material type of the stencil. Although stainless steel is the most common material for making stencils, Mylar and Kapton are also popular alternatives, and they cost less than stencils made of stainless steel. However, it is possible to electropolish and electroplate stainless steel surfaces at the apertures to improve the release of solder paste. In addition, stainless steel stencils can withstand harsh and rough working conditions.

Stepped or Multi-Level Stencils


Some boards consist of both, fine-pitch SMD components and regular SMD components. While fine-pitch components require thin stencils, regular SMD components require the stencils to be thicker. To have a single stencil, therefore, such boards require a stepped or multi-level stencil design. Manufacturers add materials to the stencil to create step-up areas. This helps to increase the volume and height of the solder paste deposit in the requisite areas.

By removing material from the stencil, they can create step-down regions, thereby reducing the size and volume of the solder paste deposit in the requisite areas. They design the apertures at a minimum distance from the step edge.

Conclusion


Rush PCB Inc considers the above key elements of a stencil to be important enough to be taken care of during its design. A properly configured stencil ensures that the solder paste deposit on the PCB is of adequate size and shape, thereby increasing its operational efficiency.


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Stencil Efficiency for Solder Paste Deposit Stencil Efficiency for Solder Paste Deposit Reviewed by Author on 9/10/2022 Rating: 5

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