What is a Stencil for PCB Assembly and why do you need it?

What is a stencil in PCB?

A Printed Circuit Board (PCB) stencil is typically a thin sheet of stainless steel with openings that are generally laser-cut and is used for the application of solder paste to a circuit board to attach surface-mounted components. PCB stencils are utilised for solder paste coating over specific locations on the circuit board, which allows these components to be placed accurately in position and perfectly aligned. Its main purpose is to deliver the appropriate amount of solder paste to the surface-mount technology (SMT) pads to ensure that the joints are in perfect harmony with aspects of mechanical strength and electrical connectivity. They speed up the overall assembly process.

What is a PCB Stencil made of?

PCB stencils are composed of several key parts that contribute to their performance and durability.

Frame

Screen frames are majorly categorised into movable frames and fixed frames. The movable ones attach the steel plate directly to the frame, and the template frame can be used repeatedly. The fixed frame attaches the wire mesh to the formwork with an adhesive. A strong frame is easily achieved with a uniform steel tension, typically between 35 and 48 N/cm2.

Sheet

Copper plate, stainless steel plate, nickel alloy, polyester, etc. The stencil generally uses high-quality 301/304 stainless steel plates that have excellent mechanical properties, greatly increasing the service life of the stencil.

Mesh

The purpose of the mesh is to fix steel plates and formwork. There are two types of mesh; stainless steel and polymer polyester mesh. Stainless steel wire mesh is usually around 100 mesh to ensure a sufficient amount of stable tension. However, they tend to deform and lose tension when utilised for long periods.

Glue

The glue is the binding material to attach the trellis frame and steel sheet. It plays a significant role in the template. The adhesive maintains strong adhesion and can withstand heavy cleaning with a variety of plate cleaners.

What are the different types of Stencils?

Stencils for PCB assembly come in various forms depending on their structure and manufacturing method.

Framed SMT/PCB Stencil

It is supplied as a solder paste-stencil which has been laser-cut and then permanently attached to the frame. Framed stencils are designed for high-volume PCB screen printing. Ideal for stencil printing on high-volume printed circuit boards, using framed surface-mount technology, the stencil is double-bonded to oppose extreme wear and tear.

Frameless SMT Stencil

This type of stencil, also called the foil, is a laser-cut stencil designed to work with a reusable stencil frame known as the stencil clamp system. Frameless SMT stencils are less expensive than framed stencils. And besides offering advanced quality and great performance, it also saves you money in terms of storage.

Electroformed SMT Stencil

Electroformed SMT stencils or electroformed foils are nickel-based stencils permanently attached to a stencil frame by a mesh rim that stretches the foil within the frame. Electroformed stencils offer one of the best paste release attributes available. As a result, it is used in fine-pitch SMT applications (12 mils to 20 mil pitch) on printed circuit boards.

How are stencils manufactured?

All circuit-board stencils are made by forming openings or holes within a steel sheet or a foil. The process is assisted by lasers and the positioning indicates where the SMT components would be inserted.

Three primary methods exist:

Chemical Etching

This is a natural subtractive process that uses a chemical etching process to create openings. It can withstand all established standard cleaning procedures. Although relatively inexpensive as a process, it tends to produce poorer results (due to rougher orifice sidewalls).

Laser Cutting

Laser cutting is the most widely used method for stencil production. It also occurs as a subtractive process where the stencil sheet material is removed by laser cutting. Standard laser cutting often results in finer output, which helps achieve greater precision.

Electroforming

This includes an additive process where stencil foils are developed by nickel electroforming. This process comes with a high cost of acquisition, but... offers greater precision but incurs high initial costs and also requires greater processing time.

Do you need a PCB stencil?

Traditional PCB assembly processes often involve manually placing electronic components before soldering. However, as the complexity of electronic printed circuit boards increases, mechanical assembly and the application of solder paste using SMT stencils have become essential.

Stencils replace the manual soldering of surface-mount components, ensuring consistent and precise solder paste application. They create uniform solder deposits and act as guides to help position and align components accurately on the circuit board. This process improves efficiency, accuracy, and the overall quality of the assembly.

What is a stencil mask in PCB?

A stencil mask in PCB assembly refers to a thin sheet, typically made of stainless steel or a similar material, with cutouts that match the shape and size of the SMD (Surface Mount Device) pads on the PCB. These stencils are used to apply solder paste onto the pads in a precise and uniform manner. The solder paste is applied through the stencil onto the pads, ensuring proper coverage. Afterward, components are placed onto the paste-covered pads, and the solder paste is melted to attach the parts to the PCB, typically using a reflow soldering process.

What is the difference between a solder mask and solder paste?

Solder Mask

A solder mask is a thin lacquer-like polymer layer that is usually applied to protect them from oxidation.

Solder Paste

Solder paste, also known as solder cream, is a mixture of fine solder particles suspended in flux. It is used to attach electronic components to a PCB during the soldering process.
How to choose the correct stencil?

When choosing a stencil that would satisfy all your requirements, the following are some important points to keep in mind before ordering the desired PCB stencil:

Thickness of the stainless steel sheet

The precise thickness of the stencil sheet ensures that the required amount of solder paste is expelled from the opening. Board thickness and aperture size are important factors that affect the amount of solder paste applied to the board. This is because additional solder deposits can cause solder bridging. On the other hand, poor solder coverage can lead to weak solder joints and affect the functionality of the finished PCB.

Aperture size of the created opening created

The stencil opening is designed slightly smaller than the PCB pad size to avoid soldering defects such as bridges and solder balls. When the stencil is removed from the board after soldering, the solder paste can migrate to the board or stick to the walls of the stencil opening. To avoid this issue, the PCB pad area should always be greater than two-thirds of the area inside the stencil hole wall.

Stencil Material

The stencil material also affects the ability to transfer the solder paste from the opening/apertures to the PCB pads. Stainless steel is the material usually used for stencils. But other nickel-based materials can also be used for stencil production.

Stencil Alignment

Fiducial marks are added to both the PCB and the stencil to print perfect solder paste on the PCB pads. These registration marks ensure alignment between the circuit board and the stencil. If the surface-mounted parts are out of alignment, the functioning of the device can be impacted negatively. Therefore, the stencil alignment is important before proceeding to the paste application.

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FAQ

Q: Can I reuse my PCB stencils?
A: Yes. Especially frameless SMT stencils designed for use with universal frames allow reuse across multiple projects.

Q: How do I know what thickness my PCB stencil should be?
A: It depends on pad sizes; thicker sheets deposit more paste but may risk bridging—consult your assembler’s recommendation.

Q: Are electroformed stencils worth it?
A: Yes—for ultra-fine pitch applications requiring superior paste release properties.

Q: Does every board need its own unique stencil?
A: Generally yes—stencil apertures must align precisely with pad layouts specific to each design.