Designing PCBs requires a lot of work. But, you can simplify the process with stencils. If you never had experience using PCB stencils, you’re in the right place. Here, we will cover everything from stencil usage to different types you can use during your manufacturing process.
What Is a PCB Stencil?
Stencils in PCB have a similar purpose as any other type of stencils. They are sheets of material placed over your printed circuit board. They have holes that match places where you will put the surface components later. Sometimes, pads can be relatively small. Therefore, manual soldering can prove to be challenging.
Most commonly, stencils are made of stainless steel, and they are laser-cut to ensure high precision. After all, the stencil holes will need to match component pads perfectly on your printed circuit board (PCB).
Of course, it is still possible to avoid using the stencil altogether, but it will take you a lot more time to do the job manually. Stencils exist to help you apply the soldering paste on all PCB pads at the same time.
Furthermore, to successfully install even the smallest components on your PCB, you will need to ensure that you have the “appropriate” amount of paste on each of the pads. If you choose to do it by hand, not only will it take you a lot more time, but you might also encounter unusual voltage values.
If you opt for using the stencil, the stencil thickness will ensure a uniformed amount of paste on each of the components. Stencils can be quite helpful regardless of whether you are doing a small number of PCBs by hand or plan on using a solder paste printer to speed up the process. For solder paste printers and mass production, you will need a different type of stencil. But, more on that later.
What Is the Purpose of Stencil
The primary purpose of stencils is, of course, to improve efficiency. As we already mentioned, the primary role of a stencil is to assist you in applying equal and uniformed amounts of soldering paste across the PCB. As a result, you will have an easier time placing the surface components on your PCB.
There are three chief roles of the stencil:
- Efficiency — Using a stencil will save you a lot of time since you won’t have to apply soldering paste to each of the pads on your printed circuit board. It can be more than beneficial when making more than one board since stencils are designed for high volume manufacturing by definition. You can easily use the same stencil on numerous PCBs with the same layout.
- Uniformity — Regardless of the type of stencil you get, it will help you apply an equal amount of soldering paste on each of the pads on your PCB. As a result, you will avoid unusual voltage values and damage to your components.
- Convenience — Applying the paste on some pads can be easy. But, it does not apply to every component. Some solder joints or pads can be relatively small (e.g., for oscillators). Thus, you will need high precision to apply the right amount of paste on them. With the stencil, the process is identical for every component, and you can finish it in just a couple of seconds.
As you can see already, stencils are perfect for high-volume screen printing. However, they are also designed to work regardless of the number of copies you plan on doing. You can even use a stencil for a single PCB, but it might be too costly in the end.
Different Types of Stencils
Before we talk about using the stencil, we should talk about different types you can find on the market. There are two terms you might encounter while searching for a perfect stencil. The first one is SMD (short for surface-mounted device). It is any component that you can find on a PCB.
The second one is SMT, and it stands for surface mount technology, which is a method of placing components on the PCB, like SMD, for example.
Types of Stencils Based on the Manufacturing Process
Creating the holes or the “aperture” on the stencil can be achieved in several different ways.
Firstly, there is electroforming. Here, the nickel is electroformed to produce the stencil, and it is commonly known as E-FAB. While the process is a bit more expensive than the others, it offers better wear characteristics than steel. The method also creates smooth tapered walls on the aperture, and it also provides a more consistent paste release.
Secondly, you can get a stencil with apertures designed using laser-cutting. Using this process means that the stencil will be precise and that you can count on high accuracy. That is one of the reasons why laser-cutting is the most popular and favorable option for manufacturing stencils.
Finally, there is chemical etching. Here, the aperture is made through the process of etching. While it is the cheapest method on the list, it provides less precision than the other two.
Types of Stencils Based on the Paste Application Process
There are also different stencils available based on the paste application during the manufacturing process.
The first type is the prototype stencils. These stencils are designed for manual use. The idea behind them is to minimize printing mistakes, and they are usually made from CAD files. It is not rare for the client to provide the prototype stencil to improve the final result.
The second type is framed stencils. These are fixed on a frame, and they receive the necessary tension from the border mesh. Framed stencils are most commonly used during mass production since they can be used with a solder paste printer. They have smooth aperture walls, and they can provide excellent performance and quality. You can also find reusable stencil frames where you can replace the stencil between printing.
The third and final type is the frameless stencils. As the name suggests, they don’t have a frame, and they are a lot more affordable than their counterpart. Frameless stencils are an excellent option for short runs or for designing prototype stencils.
Types of Stencils Based on the Material
Finally, you might encounter categories that have to do with the stencil design materials. When choosing the material, the only important thing is finding something that can withstand working conditions found in the PCB shop.
One of the options is to use Mylar and Kapton stencils. These are polyester and polyimide, and the stencils are made using an etching process. Most commonly, these materials are used during prototyping to ensure optimal performance during manual soldering. Both Mylar and Kapton stencils are quite affordable compared to stainless steel. However, the quality of the stencil and the finished product is not as high as with stencils made of steel.
That leads us to the second option — stainless steel. Companies use both standard and fine grain stainless steel to design stencils. There is also an option to use nickel. As we already mentioned, nickel offers better wear characteristics, but the process is more expensive.
How to Use Stencils
Now, regardless of the material used to design stencils, the process of using it is the same. Once your stencil is ready for use, all you need to do is place it over the PCB. Of course, if you are soldering manually, you will need to align the stencil with the PCB. If you are using a solder paste printer, place the stencil in the frame and the PCB on the printer, and you’ll be ready to start.
The apertures on the stencil will match the pads on the PCB. The thickness of the stencil will dictate the amount of paste that will remain on the PCB. You will need a card to swipe the excess paste for manual use, and you can even use a credit card if you are doing it at home. Just place the paste on the apertures, and swipe the excess paste with the card.
Of course, if you are using a solder paste printer, the machine will do the work for you, and all you need to do is check if everything is fine with the PCB before you proceed to place the components.
Stencils can be an integral part of PCB manufacturing, and they can simplify the whole process. The idea behind them is to make solder paste applications easier and faster, ensuring that each pad gets the same amount of solder paste.
If you have any questions about the stencils or anything else regarding the manufacturing process of PCBs, feel free to contact us. We have years of experience designing high-quality printed circuit boards and countless satisfied clients from all over the world.