The main use of an SMT stencil is to apply solder paste on a bare PCB board. The stainless steel foil is laser cut to make an opening for SMD devices on board. After application of the stencil in the proper way on the board, solder paste is applied on the opening. After separation of stainless steel foil on board, solder paste remains there after SMD components are connected. In this post, we will cover details for smt stencil and learn how its affects PCB assembly.
What is an SMT stencil?
- The PCB stencil is stainless steel foil that comes with holes that are produced with a laser based on SMD components’ pins that are needed to connect on board.
- In this process, we can easily apply solder paste on the board. Then there are different processes applied on board, such as SMD, soldering, automated optical imaging, through-hole, and soldering process in the last.
- SMD component connection on board needed professional skills since it needed accurate handling due to their small size.
- SMT stencil is good for making connections of components on the board without errors.
- SMT stencils are designed based on PCB layout, and solder paste is applied on the board after it is dry structured, and then components are connected.
Process of SMT stencil manufacturing
different steps involved for manufacturing of SMT stencils that are explained here in detail
Designing
- There are different tools used for designing SMT stencils, like CAD, that are used to make the PCB layout. The PCB layout has a different pad design at the accurate point that is good for solder paste application.
- During the design process of SMT, different parameters like thickness, diameter, and board layout are based on the connection made
Materials
- SMT stencils come with different materials according to project requirements, and commonly used materials are nickel and steel. The thickness of the material is about 0.1 mm to 0.25 mm.
Etching
- Based on the created design stencils, cut through using a laser. If stencil materials come with a coating of photoresist, the chemical etching process is used for cutting according to design.
Stencil Cleaning
- Dust particles are removed for surface cleaning.
Stencil Surface Treatment
- stencil surface is polished for removing solder paste and made ready to use.
Inspection
- In the last step, different testing methods are applied on the board, and then it is packaged for delivery to the customer.
SMT stencils feature
The main features of SMT stencils are as follows.
- • An SMT stencil applied with solder paste that minimizes the chances of short circuits, and a board that does not have SMT will face short circuits.
- smt stencil uses laser cutting that make smooth layers of solder paste on board without making gaps and in result we get smooth sodering.
- SMT stencils also make smooth pads for accurate board design.
- With an SMT stencil, we can also get smooth apertures.
- In this process we can get an error-free location of the pad, and it provides accurate printing with the last cut stencil.
SMT stencils working
- • A stencil is basically a thin sheet made with the use of different metal materials like aluminum or stainless steel. It comes with apertures that make it easy to print the board design.
- • The stencil is applied on the upper surface of the board on the frame. After that, solder paste is smoothly applied on the stencil.
- When it passes through apertures, solder paste get dimensions based on the board design. Small-sized holes exist that control paste quantity and help to make an accurate soldering process for the board.
How to determine the pcb stencil size?
- pcb stencil dimensions are comes in two basic parts first one is inner dimensions and other is overall dimension.
- The inner size is based on the board being ready for assembly, and the overall size defines compatibility with factors for the printing machine.
- The accurate design of these dimensions and the stencil makes it easy to use for working.
- Inner dimensions are measured based on these factors.
- • Framed stencil width = PCB width + 100 mm, length = PCB length + 100 mm.
- Frameless stencil width = PCB width + 200 mm length = length of PCB + 200 mm.
Suppose if we have a board size of 50*50 mm and its frame stencil dimensions are 150*150 mm, and the frameless stencil dimensions are 250*250 mm.
It can be easy to work with, and it is good to use for SMT assembly before applications of the printing machine.
before using automatic springing machines, it is good for manual solder paste springing for smt assembly.
PCB size also helps to make stencils through different internal dimensions.
For automatic solder paste printing, it is good to use. The overall size of the stencil is defined with the printing press since stencil work is defined by the type of printing press.
Types of PCB Stencils
main types of PCB stencil are as.
Framed Stencils:
- This type of stencil is configured on a rigid frame that provides support and stability. It is good to use for a complicated board that needs high accuracy.
- Irrespective of the high cost and needing more space, it has high accuracy since it uses automated equipment.
Step Stencils:
- These stencils come in different thicknesses and aperture levels, and it has differential solder paste application on board parts. so it is used for different sizes and pitched components.
- Step stencils are a high-cost and time-consuming process but not effective to use for complex designs.
Frameless Stencils:
- These stencils do not have frames, so they provide flexible design and low cost to use for small boards. It is easy to store and difficult at the time of the process of application of paste and so not good to use for automated high-volume production.
Factors in Choosing the Proper PCB Stencil
Different factors used for choosing an accurate stencil for good PCB assembly:
PCB Size
- Framed stencils are used for complicated and larger-sized boards, and small-sized boards need frameless stencils.
Aperture Design and Size:
- For accurate paste deposition, the aperture should have the same size and design as the component pads.
Material and Construction:
- Stainless steel and nickel are used since they have a long working life and provide high accuracy, and the stencil design affects their working efficiency.
Solder Paste:
- • The stencil must have features for compatibility with solder paste, and particle size and viscosity also matter.
Production Volume:
- • A framed stencil is best to use for bulk orders since it has strong strength, and the low-volume design uses a frameless design that is a low-cost option.
Use of stencils in PCB assembly
There are different uses of stencils for different applications that are explained here.
Precision
- • The stencil has proper and accurate solder paste application on PCB pads. The apertures in the stencil are according to the component pads on board and help to apply the required solder paste.
- This accurate process helps to reduce chances of solder bridges that cause many errors.
Efficiency
- stencils also make aseemblyu simple and streamline. the applications of high speed and smooth solder paste over differnet pads in single term, reduces time for maknig pcb to connect componnts. this helps to use for bulk prodcution where speed is important
Consistency:
- Stencil: Apply solder paste consistently. Every stencil is made for accurate features, and each board is made with the use of a similar stencil and has smooth solder paste application. that provide quality board assembly.
Versatility
- • A stencil can be used for different PCB designs and as a feature to handle different categories for solder paste. Either you have a basic or a complex design stencil, you can apply it to each type with proper assembly features.
Low Waste:
- The accurate use of a stencil also reduces extra solder paste. that make low-cost board assembly and low waste also provide less reworking
SMT stencil thickness
- The normal thickness used for a stencil is in the range of 0.04 mm-0.4 mm, but it is selected based on applications. High applications of solder paste cause bad quality for assembly.
- So ensure that accurate SMT stencil thickness provides final assembly according to requirements.
How to choose an SMT stencil manufacturer
When you are searching for something, smt stencil manufacturer must consider expert and high-quality services provided by a fabrication house like JLCPCB. That is best to use since it helps to manage your different factors such as cost, quality of materials, and customer services.
Cost is a main factor for different workings, as parts and manufacturing costs need to be considered for proper assembly. Material quality also affects the assembly.
JLCPCB has the compatibility to provide frameless SMT stencils and customized outline sizes that start at only $7. There are not frames or types of glue that make the process simple and provide a high-efficiency process. It is transported with a board on shipping charges for customization sizes less than 200×200 mm.
They also offered non-framed stencils that are used for hand soldering and give flexibility for stencil size customization and use other factors for getting the required parameters.
Conclusion
SMT stencil is important for the manufacturing of PCB boards. for choosing an SMT stencil, it is good to have knowledge of components, types, and thickness. Different materials are used for SMT stencils, such as nickel and stainless steel.
The cost of an SMT stencil is based on materials and manufacturers services. So good is that clean SMT stencil at the time of use. These factors help to provide required assembly for applications. The selection of reliable SMT stencil manufacturers helps to provide consistency for getting high-quality products at competitive rates.
