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  • Laser Cut and Electropolishing Stencil (E.P.Stencil)
  • Surface Mount (SMT), Thru-hole(PTH) & Mixed Technology
  • Rigid, Flexible PCB and Rigid-flex PCB Assembly
  • Small to Medium Volume Production
  • ISO9001:2015, ISO 13485:2016 Certified & UL Listed
  • 100% E-test, AOI, X-RAY, ICT & FCT

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:

  • Put the stencil on the PCB, and then align the stencil to fit with the PCB pads;
  • Add some solder paste on the stencil, but make sure the solder paste is properly stirred;
  • Brush the solder paste with pressure to force the solder paste to run through the SMT stencil openings and stick on the PCB pads. It can be done manually or by automatic printing equipment.  

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    Why PCB Stencils Are Important?

    PCB stencils have an important role in PCBA processing. They are used to accurately print the solder paste onto the well-made PCB.

    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 Stencils

    SMT stencils are applied in every kind of SMT assembly. It is indispensable in the SMT process.

    SMT/PCB Stencils Manufacturing Capabilities

    Viasion’s SMT/PCB stencils manufacturing capabilities match the following PCB assembly types. 
    • Surface Mount (SMT)
    • Thru-hole
    • Mixed Technology (SMT/Thru-hole)
    • Single or double-sided placement
    • Rigid, Flexible PCB, and Rigid-flex PCB Assembly
    • High pin count press fit capable

    The Different Types of SMT Stencils

    According to how the SMT stencils are produced, we can divide them into the following categories.

    Laser stencil

    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)

    In some PCBs, IC chips with QFP/BGA/ūBGA/CSP packages are used, normal laser cut stencil cannot meet the high accuracy requirement. So an electropolishing process is added after the laser cut. Then the opening hole wall is very small which is good of find pitch PCB assembly. We always use this kind of PCB stencil in our PCB assembly process.

    Step stencil

    In some cases, a different amount of solder paste is needed in special PCB assembly, which cannot be realized by adjusting the PCB stencil thickness and opening size. Then step stencils are invented. There are 2 kinds of step stencils: STOP-DOWN and STEP-UP. STOP-DOWN means the specified place is thinner than the other parts, and the STEP-UP part is higher than the other parts. The height accuracy can be as small as 0.005mm.

    Etching stencil

    In short, it means using chemicals to etch out the opening on the steel mess according to the solder stencil file. It is very cost-effective and also time-saving. But the problem is the positional and dimensional accuracy is not good. The minimum IC PADs is 0.4mm.

    The Differences between Frame-less SMT Stencil and SMT Stencil with Frame

    1
    Frame-less PCB stencil is a stencil without a frame

    Frame-less stencils are normally used in manual solder paste printing, such as very simple PCB assembly or BGA repair. The advantage is the price is very cheap because no metal frame(normally is an aluminum frame).

    2
    Stencils with frame is commonly used
    Stencils with frame is the most commonly used ones. This kind of SMT stencil is very easy to load and unload from the automatic solder mask printing machines which are normally used in the SMT PCBA industry in China. But the disadvantage is the price is more expensive than frame-less SMD stencil.
    3
    Removable thin frame SMT stencil
    There is another solution that combines the advantage of SMT stencil without frame and SMT stencil with frame. A small frame SMT stencil is produced for every PCB PCB assembly, but these PCB stencils cannot be loaded in the automatic solder paste printing directly. These small-frame SMT stencils need to be plugged and locked on an aluminum frame first and then load on the solder paste printing machine. This kind of SMT stencil is called removable thin frame SMT stencil in the PCB assembly industry.

    The Advantages of Removable Thin Frame SMT Stencils

    Cost-effective

    Since only, a very thin and small frame is needed for the individual SMT stencils, and the frame can be reused many times, the cost for each PCB stencil is much cheaper than the normal SMD stencil with a frame. This will help PCB assembly manufacturers to reduce costs.

    High production efficiency

    In the SMT production process, we can exchange the individual SMT stencils easily since the frame is the same. This is helpful for prototype PCB assembly.

    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

    Such as us in Viasion, we have so many stencils that need to be stored after years of PCB assembly and electronic manufacturing service. Since the normal stencils have a frame width of 40mm, we need a very big room to store the used PCB stencils. But the removable think frame PCB stencil only has a 6mm frame width, which means we can save 80-90 % of storage pace.

    How to Produce a Laser Stencil?

    As the most commonly used method at present in the printed circuit board assembly process, laser-cut PCB stencil has many advantages. One of the most important points is that it is very accurate and has a small error because it only needs to directly apply working files during production. 
    • The production of a laser SMT stencil only needs three parts, namely, the printed circuit board, the film, and the final working files.
    • But the three parts have their requirements. For example, as for the printed circuit board, there should be no problem with appearance.
    • Regardless of any damage, the appearance must also be accurate. As for the film part, it is very similar to the PCB part.
    • First of all, there should be no problems with appearance, no damage, folding marks, etc. and there are also high requirements for temperature.
    • Whether it is overheated or too cold, it will affect production, so it needs to be strictly preserved.
    • The last working files part usually accepts many CAD formats, but Gerber is the most commonly used. The second most commonly used compression format is the traditional ZIP format.

    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

    • Three factors should be paid attention to in opening design. The most important factor in SMT stencil production is the width-thickness ratio and area ratio from our SMT production experience.
    • What are the width-thickness ratio and area ratio? Width thickness ratio is the ratio of width to thickness of PCB stencil, and area ratio is the ratio of area to the cross-sectional area of the hole's wall.
    • We also need to pay attention to the shape of the openings which need to be the same as the Gerber design, and the hole's wall of the metal SMD stencil opening should be smooth enough for SMD assembly.
    • These three points are the most important points for opening design. We need to make sure that the width-thickness ratio is greater than 1.51 and the area ratio is greater than 0.65, to achieve the best effect in solder paste printing and also SMT assembly.

    Different methods in SMD Stencil production

    Three methods are usually used. They are laser cutting, chemical etches, and electroform. Each has its advantages and disadvantages. 

    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

    Generally, only laser SMT stencil needs post-processing. 
    • The usual treatment method is to remove the residue left after laser cutting by grinding, and another method is to polish by electric polishing, which will cost more, but the effect will be better, and the residue can be almost completely removed. The usual way is to polish.
    • After talking about the post-treatment of laser stencils, let's talk about the cleaning of PCB stencils after SMT manufacturing.PCB stencil usually has a dedicated cleaning machine, and each circuit card assembly process usually needs to be cleaned three times.
    • The commonly used cleaning method is manual wiping. There are two tools, a rag, and a special wiping paper. The special test paper can also be used in some machines to achieve automatic cleaning. However, generally speaking, in the beginning, the rag is used more and more conveniently.
    • It can be carried out at any time. The time can also be controlled freely, but the cleaning degree is not particularly good. The use of rags can also be applied to special wiping paper.
    • The other way is special machine cleaning, which is powerful and efficient, but the disadvantage is that the cost is too high.
    • Here are some things you should pay attention to when using solder stencils. A solder stencil is still a fragile appliance, so there are many things you should pay attention to when using it to protect it.
    • As mentioned before, the SMT stencil is usually cleaned three times in each production, and each piece of PCB stencil should be provided with a packing box, which should be cleaned immediately after printed circuit assembly, and put back into the packing box for preservation to prevent damage and loss.
    • And it should also be cleaned before reuse, especially after transportation to the customer, to eliminate the dust and stains on it. It also needs to pay attention to the strength when using it in SMT production.
    • It should be as light as possible whether it is installed or removed to avoid damage.
    • In addition, after the production is completed by the PCB assembly manufacturer, it can be placed for a while before removing from the mold, and during the operation, it is necessary to avoid scraping and impact of hard things on the PCB stencil, which will affect solder stencil. No matter is it a prototype PCB assembly or high volume PCB assembly.

    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.

    Viasion’s Advantages of the One-Stop SMT Stencils Manufacturer

    1
    High quality
    There is a scientific and strict production management system and a superior after-sales guarantee process in Viasion, which fully ensures product quality and after-sales service.
    2
    Experienced team

    Viasion relies on advanced research & development technology and rich industry experience to continuously provide customers with reasonable, practical, and efficient product solutions.

    3
    Good customer service
    Viaison has a good sense of service, which is the only way to be able to take the initiative to take responsibility and solve problems when customers encounter them. Through the research of customer needs and in-depth communication with customers, we analyze the specific needs of customers comprehensively and provide products customized to meet their special needs. We are dedicated to providing our customers with high-quality products and services in all aspects.
    4
    Trusted by customers for many years
    Viasion has won the trust and affirmation of our customers through our exquisite technology, excellent quality of the product, and perfect comprehensive service, which strengthens our confidence in the continuous development of our company.
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    Viasion Technology is your trusted partner & one-stop shop for PCB fabrication, components sourcing, PCB assembly and electronic manufacturing. With more than 16 years of experience, we have been supplying high-quality PCBs with competitive pricing for 1000+ customers worldwide. Our company is ISO9001:2015 Certified & UL Listed, and all our products are 100% E-tested and inspected by AOI & X-RAY to meet the highest industry standards. So please get an instant quote from our sales team now, and we will take care of the rest.

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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.

    As mentioned above, 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.
     

    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 stencil
    Laser cut and electropolishing stencil (E.P.Stencil)
    Step stencil
    Etching 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.