How to Choose a SMT Stencil What is a SMT stencil?

SMT Stencil is a thin metal sheet used in the soldering process for SMT (Surface Mount Technology) and it plays an essential role in the SMT soldering process. SMT Stencil allows the direct placement of solder paste onto the SMD pads of PCB, which can help prevent errors and defects during the reflow soldering process. This results in a precise amount of tin coverage once the work is completed.

Types of SMT Stencils

Depending on the application of solder paste, three different types of stencils are generally used.

Framed SMT Stencils

Framed PCB stencils are mostly ready to use. The framed stencil consists of a normal laser-cut stencil sheet stained to the metal frame. This type of stencil is most suitable for high-volume SMT assembly due to the frame's ability to provide better stability and reusability of the stencil itself.

The frame of the stencil also allows a better and faster process of alignment between the PCB and stencil. The cost of the frame stencil is relatively higher due to the need for additional frame support. Moreover, frameworks require additional storage space and can be more complex to manage. It also makes stencil heavier, increasing shipping costs.

Frameless Stencil

Unlike framed stencils, frameless stencils do not have a surrounding frame, making them more economical than framed stencils. This design allows for the direct placement of the stencil onto the PCB and it’s mostly suitable for hobby and nonrepeatable usage. Frameless stencils are very light which makes them great for shipping.

Due to no additional frame support being required, it is more suitable for small-batch PCB printing and allows for custom sizes. Like framed stencils, frameless stencils can be customized to accommodate specific PCB designs and component layouts.

Step Stencil:

There are various components on the PCB, and each component has different solder paste requirements during the soldering process. Regular laser stencils can only regulate the amount of solder paste by adjusting the size of the openings while the overall thickness of the stencil is the same everywhere.

Step Stencil by jlcpcb

To meet higher standards and requirements for soldering effectiveness and quality, some engineers require multiple solder paste thicknesses on the same SMT template to achieve precise control of the solder paste amount. This requirement leads to the use of a step stencil. It allows for printing varying solder paste thicknesses on the same stencil, enabling precise control of the solder volume.

How to Choose Your Own SMT Stencil?

The thickness of the SMT Stencil

The thickness of the stencil is a crucial factor in determining the quantity of solder paste that is being placed onto the PCB pad. During the printing process of the stencil, the bottom of the stencil sheet comes into contact with the top surface of the PCB board. Therefore, the thickness of the stencil determines the thickness of the solder paste that is printed onto the surface of the PCB pad.

• The thickness of the SMT stencil should be determined based on the density of PCB components, packaging form and size, and the spacing between pins (or BGA solder balls). If the spacing between components is small, use thinner ones. If the components on the entire PCB are relatively large, a thicker stencil can be used.

• On the same PCB, there are both general spacing components above 1.0mm and narrow spacing components. The thickness of the stencil should be selected based on the condition of the majority of components on the PCB board. A popular way to reduce or enlarge the amount of the solder paste applied on the solder pad is by adjusting the pad opening on the stencil. In the PCB design software, this is being done in the “Solder Paste” layer.

• When components require significantly different amounts of solder paste, localized thinning of the stencil for components with narrow spacing can be employed. However, the thinning process incurs higher manufacturing costs. Therefore, a compromise can be made by choosing an intermediate thickness for the stencil to determine the solder paste requirements.

• For general pitch components, the opening can be 1:1. However, for larger components that require a significant amount of solder paste, the opening area should be increased by 10%-20%. For OFP and other components with pin pitches of 0.65mm and 0.5mm, the opening area should be reduced by 10%.

SMT Stencil material

The main materials used for laser-cut SMT stencils include stainless steel and copper. Stainless steel has a higher hardness, longer service life, and better corrosion resistance. On the other hand, copper has better electrical conductivity, which can enhance production efficiency. Stainless steel is the most commonly used material for making PCB stencils. It is undoubtedly the best material for producing a PCB stencil.

Size of a SMT Stencil

The outline size of the SMT stencil needs to be selected based on the actual size of the PCB board. Specifically, the appropriate size of the stencil sheet should be selected based on parameters such as the shape and spacing (line width/line spacing) of the PCB board to prevent errors during the production process.

Your stencil needs to be sufficiently large to accommodate the board data within the valid area of the stencil, rather than the stencil size. The valid area refers to the central region where SMT pads can be cut into the sheet, while the remaining space forms the stencil margin and cannot be cut. But how to choose?

SMT size specifications are diverse.

Below this text you can find commonly used specifications and sizes at JLCPCB about framed stencils:

For some electronic engineers, their PCB prototypes are sometimes within a size of 10x10cm or less. If the current conventional stencil is used, a significant portion of the stencil sheet area will be wasted as shown below, resulting in a highly uneconomical situation. It is very large and heavy, which makes it inconvenient to transport and store.

To avoid such situations and reduce costs, JLCPCB supports the production of frameless SMT stencils with customized outline sizes, Prices start at only $7!There are no frames or different types of glues, which simplifies the process and enhances production efficiency. It can also be transported together with the PCB to save on shipping costs if the customized size is smaller than 200x200mm.

Mini stencil produced by JLCPCB

Conclusion

SMT Stencil plays an important role in the PCB assembly process. It can control the amount of deposited solder paste and achieve precise soldering of electronic components. The appropriate selection of stencil can significantly impact the quality and reliability of PCB assembly.