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What is PCB Stencil?
PCB stencil is the mould used to print solder paste on PCB pads in the PCB assembly process. SMT stencils are usually made of stainless steel, with openings on the area where solder paste can go through to stick on the pads of the PCB.
There will always be a frame around the stencil, which makes it easier to load the stencil on the automatic printing machine. But frameless stencils or removable thin frame stencils are also commonly used to save costs.
According to its production methods, stencils can be categorised as laser stencil, laser cut and electropolishing stencil (E.P.Stencil), step stencil, and etching stencil.
Typically, there are three steps when printing solder paste on the PCBs with an SMT stencil:
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Why PCB Stencils Are Important?
How to use PCB stencil?A stencil is a thin piece of steel that has holes etched into it matching the pads on the PCB. The stencil is placed on the PCB with the pads just exposed. Use the blade to scrape the solder paste to the stencil, and then pick up the stencil, so the pads on the PCB will have solder paste on them, and then proceed to the steps of mounting components and reflow soldering.
Applications of SMT StencilsSMT stencils are applied in every kind of SMT assembly. It is indispensable in the SMT process.
SMT/PCB Stencils Manufacturing Capabilities
The Different Types of SMT Stencils
According to how the SMT stencils are produced, we can divide them into the following categories.
Laser PCB stencil means the opening on the steel mess is cut by laser. Because of the development of high-technology laser cut machines, the position accuracy and the dimensions are relatively high (tolerance less than 3 μm). And the production speed is also very quick, PCB stencil can be cut in a few minutes. So laser stencils are the most common stencils in PCBA manufacturing companies.
Laser cut and electropolishing stencil (E.P.Stencil)
The Differences between Frame-less SMT Stencil and SMT Stencil with Frame
The Advantages of Removable Thin Frame SMT Stencils
High production efficiency
Very good tension
Since the small individual frame is plugged and locked to the main frame which is very strong. The SMT stencil can be tightened from the 4 sides with 35-50N tension, which is enough to keep the PCB stencil smooth and the solder paste printing process in SMT production lines.
Compatible with all solder paste printing machines
These removable SMT stencils are developed from traditional SMT stencils, so the designer considered the usage fully. So the PCB assembly manufacturer and electronic assembly manufacturer never need to worry about if the removable SMT stencil will be compatible or not.
Easy for storage
How to Produce a Laser Stencil?
To make SMT stencils, Gerber files or PCB engineering files are generally provided. Of course, most of the SMT stencils are opened after PCB manufacturing has been made. Therefore, the general PCB files are ready-made, just use this!
- If there is a Gerber file, it is best to have a patch layer, because the steel mesh is opened following this file.
- If there is no PASTE patch layer, silk screen, solder mask, and trace layers are required. Of course, drilling holes are the best.
- Finally, if you only want to resist soldering, you can only do it according to this. Of course, the final effect cannot be guaranteed.
In addition, Gerber, that is, the working files must contain some data to ensure production, and must contain the shape data of the financial mark and the printed circuit board. Included the silkscreen data for checking the components and so on.
More about PCB Stencil?
To check whether you know about stencils, here is a small test for you. Please answer the following questions with “Yes” or “No”: Is a stencil mandatory in the SMT process of a PCB assembly factory? Its function is to print semi-liquid and semi-solid solder pastes on PCBs by automatic printing machines, right? The answer is “Yes”.
For those of you who answer this question correctly, it means that you indeed know something about stencils and even SMT. Except for the extremely easy power supply board which only has PTH holes, most of the current printed circuit boards use surface mount technology (SMT), and there are many solder pads on the PCB board. The pads are copper traces that are not covered by a solder mask but covered by a thin layer of metal that can be soldered, such as Tin, gold, silver, etc. The openings on the PCB stencil should be the same as the pads on the PCBs, especially for the IC and BGA chips, any slight mismatch may cause a serious quality issue.
Some people may say, “I know some of the SMT processes! Semi-liquid and semi-solid solder pastes are printed on the pads of the PCB through the openings in the SMT stencil and then the components are mounted together with an automatic machine and then go through the reflow soldering, is that correct?” Of course, you quite know about that. Finally, the so-called PCBA circuit board comes out! It is indispensable in PCB assembly. If you want to control the precise amount of solder paste at the specified position, you need to use a PCB stencil with different thicknesses or different sizes of openings. Even with the continuous development of SMT assembly, SMT stencil is always used in the PCB assembly service process.
SMT stencil is not the current stainless steel process from the beginning. At first, SMT stencil was made of intolerable nylon and then evolved into steel. Although the tolerance and accuracy were improved, there was still the problem that it was easy to rust. Therefore, with the continuous improvement of smelting technology and industrial technology, it finally evolved into the current stainless steel SMT stencil. It combines all the advantages of the former, which is accurate, durable, and not easy to rust.
SMT Stencil Thickness Design
The specific thickness options of the stencil are 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, and 0.3mm. The thicknesses in the range of 0.1-0.18MM are generally chosen a lot. Of course, because the production situation or demand is not the same, there are some slight differences in thickness.
SMT Stencil Thickness Design
Different methods in SMD Stencil production
Among them, the most affordable is a chemical etch, the highest cost is electroform, and the price of laser cutting is the most moderate.
In addition, from the perspective of environmental protection, laser cutting is also the most environmentally friendly. The other two will pollute the environment.
In terms of production, chemical etch is characterized by fast production and forming speed, and high precision of laser cutting but too slow production speed. The production cycle of electroform is the longest. Therefore, chemical etches or laser is usually selected.
As for the manufacturing process, chemical etch is similar to electroform, in which the latter needs to be coated with photosensitive film and needs to be electroformed with nickel. Chemical etch is more convenient. Laser cutting is special. It only needs laser cutting, and then it can be polished and netted. In addition, it does not need to clean the PCB stencil, to avoid pollution in the circuit board assembly process.
Post-Processing of Laser SMD Stencil
How to Get A Good- Quality SMT Stencil?
Including how to determine the quality of the SMT PCB stencil, three main factors determine the quality of the SMD stencil: the size of the opening of the SMD stencil, the thickness of the steel sheet, and the cutting hole’s wall. You can select the best PCB Stencil Manufacturers from these paramters.
This is very important. If the large components such as the cardholder press the key, etc., it is opened according to the document 1:1, it is easy to have less tin and false welding. The opening of the SMD stencil should be appropriately enlarged for large components. , increase the amount of tin to ensure that the tin is solid. For precision components, if the opening is too large, there will be a short circuit with the tin.
In the design of CHIP components, if the inner distance is too small, it is easy to connect tin or tin beads internally. Too big, and the component will be erected again! Guaranteed spacing is also important.
Depending on the amount of tin, the thickness of the SMD stencil required is also different in SMT PCB assembly. For large components, the required thickness is thicker, and for precision components, the required thickness is thinner. Because the tin of the SMD stencil is related to the aspect ratio. IPC regulations, W/T> 1.66 In fact, it is more than 1.5, which is enough. It can ensure the tin.
Thickness selection reference
When there are 0.4 pitch IC and 0402 components, the thickness of the SMD stencil is 0.1-0.12MM. When there are 0.5Pitch IC and 0.65Pitch BGA IC, the thickness of the PCB stencil does not exceed 0.13. If it is too thick, it will be no enough tin in the BGA assembly, chip assembly, or IC assembly.
If there are burrs, it will affect the lower tin. The role of electropolishing is to smooth the cutting hole wall. That is, deburring.
Also, regarding the cleaning part I just mentioned, Cleaning must be done quickly, to avoid clogging, it must be cleaned immediately after use. After cleaning, it will involve the storage of solder stencils. As mentioned in the previous article, it is necessary to bump into the packing box. In addition, it is best to set up a locker, so that the serial number of the goods can be easily sorted out and it is not easy to be confused, but pay attention to it separately. Separate to avoid damage.
In terms of technology, the laser is the best, and if electropolishing can be used for subsequent processing, the best quality will be obtained. Finally, when using it, you must pay attention to stability and proper force to avoid damage.
SMT stencil is indispensable in the PCB assembly and Electronic Manufacturing Service process. In addition to the choice of materials and craftsmanship, you should always pay attention to maintenance and placement in post-processing and preservation.
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Frequently Asked Questions
Take the method of laser cutting method as an example.
Process flow: PCB files ready→take coordinates→data file→data processing→laser cutting→grinding→mesh
PCB stencils can be recycled and then reused.
Before placing the components on the PCB, a certain thickness of solder paste is printed on the pads through the stencil, which is called PCB stencil printing.
The material of the stencil is usually stainless steel foil and depending on the process used there are also some other materials such as nickel, etc.
We need a stencil in the SMT assembly process. In this process, we need stencil to cover the PCB surface and print solder paste on the pads of the circuit boards.
PCB stencils have several standard dimensions, usually costing around 50-150 USD.
Laser cut and electropolishing stencil (E.P.Stencil)
The following factors should be considered in the stencil design.
Mark the type of the product, thickness, date of manufacture, etc.
The size of the frame is to be decided according to the structure of the printing machine and the specification of the product.
The opening method and the size of the opening are to be determined according to the type of each component.
Care should be taken when using stencils.
Take it lightly and place it gently.
Before using the stencil, it should be cleaned (wiped) to remove the dirt carried by the transport process.
Solder paste or red glue should be stirred well to avoid blocking the openings of the stencil.
The printing pressure is adjusted to the best: the pressure is best when the squeegee can just scrape all the solder paste (red glue) on the stencil.
After the scraper step, if possible, it is best to stop for 2 to 3 seconds before demolding, and the speed of demolding should not be too fast.
Do not use hard objects or sharp knives to hit the stencil.
The stencil should be cleaned in time after use, put back into the packing box, and placed on the special storage rack.