What are Stencils and How to Use Them?
As Printed Circuit Boards (PCBs) become smaller and more sophisticated, it is becoming increasingly difficult for operators to solder SMDs manually. But with the help of automated machinery, the process of mounting small components has become simpler and more efficient.
With SMT stencils this process becomes much simpler as it allows for accurate transfer of solder paste when assembling printed circuit boards with Surface Mount Technology (SMT) components.
Stencil and Component Assembly
When assembling PCBs with SMDs, stencils are created from a steel foil where a laser is used to create openings for each SMD on the board. Then, the stencil is properly aligned on top of the board where solder paste is then applied over the openings. Once the stencil is removed from the board, the solder paste will remain and the board is now ready for placement of SMD.
Characteristics of SMT Stencils
The characteristics of SMT stencils vary depending on their type. However, they all share the same purpose. Some of the most common characteristics found in SMT stencils are:
- SMT stencils are typically made from stainless steel or metal, which makes them very sturdy and durable.
- The thickness of the stencil is usually kept between 0.005” to 0.007”.
- The area and apparent ratio are used to determine the approximate size of the hole
- The area ratio is typically kept greater than 0.66 and the apparent ratio is typically kept greater than 1.5 to allow for proper solder paste release.
How are SMT Stencils Created?
Three stencil technologies are commonly used within the industry to create SMT stencils: laser-cut, electroformed, and chemically etched plastic.
Electroforming is an additive process, this means that manufacturers build up the stencil by electroforming nickel. The advantage of electroforming is that it can simplify the process of forming smaller apertures with a finer pitch on electroformed nickel stencils. This creates aperture walls that are smooth with a reflective surface, allowing for effective solder paste release.
However, the processing time for electroforming can be high, as well as the initial cost of manufacturing. But, this initial cost remains the same regardless of the number of apertures that it contains. This means that the electroforming process is ideal for stencils with large aperture counts as it allows for quick turnaround times.
Chemical etching is a subtractive process, where apertures are formed using chemical milling. This is a low-cost process compared to the other two methods, but the results are not as ideal as the etchings often leave rough aperture sidewalls.
The process of chemical etching creates apertures into metal sheets using acid. The advantage of this is a quick and cost-efficient method of manufacturing stencils. However, it is important to note that this method should only be used to manufacture stencils with large openings, as this method tends to form sand clock shapes on the walls which are not ideal for forming stencils with finer pitch openings.
Similar to chemical etching, laser-cut is a subtractive process. During this process, unneeded materials are removed from the stencil foil via laser. The advantage of laser cutting is to form quality stencils that are consistently made with high levels of precision.
To achieve this, manufacturers utilize the Gerber data of the PCB to create laser-cut stencils. The aperture is cut out with a high degree of accuracy by moving a laser. The laser accurately cuts out the aperture from the metal one at a time. When using laser cutting, there are a few variables to the smoothness of the cut, such as cut speed, beam spot size, laser power, and beam focus.
Framed vs Frameless Stencils
When we think of stencils two types often come to mind, which are framed and frameless stencils. The choice between the two often comes down to your budget and the storage space available.
Framed SMT Stencils
Framed SMT stencils often come ready to use. The SMT stencil is mounted to a piece of mesh on one side of the frame to provide a support tension. The thickness and style of the frame will vary depending on your printer.
Some of the advantages of framed SMT stencils include:
- Lower short-term cost: Framed SMT stencils require no additional attachments meaning they are ready-to-go as they are.
- Simple setup: Framed SMT stencils can be immediately used. Simply load it up in the printer and you are good to go.
- Recyclable: Framed SMT stencils can be reused and can be recycled once you are done with them.
Frameless SMT Stencils
Despite its name, frameless SMT stencils are not exactly frameless. When it is being used, the SMT stencil must be placed into a reusable frame. Multiple stencils can be used with a single frame.
Some of the advantages of frameless SMT stencils include:
- Easier storage: Frameless SMT stencils can be stored more easily because you are only storing a flat piece of foil. Compared to framed stencils, frameless SMT stencils allows for four times the storage capability within the same storage space.
- Long term efficiency: When you are working with different types of PCBs, frameless SMT stencils can be advantageous as it allows for less wastage, and easy changeover. Over time the cost of each additional SMT stencil will be lower once you have a system in place.
The SMT Stencil Printing Process
For low-volume production, manual printing can be used to achieve better results. There are three key processes when printing with an SMT Stencil manually.
The first being the aperture-fill process where solder paste fills the aperture. Filling the stencil aperture with solder paste will require a metal squeegee that is used to force the solder paste into the aperture.
Secondly, the paste transfer process where the paste accumulated in the aperture is transferred to the PCB surface.
Lastly, the positional location of the deposited paste. The positioning of the aperture with respect to the squeegee blade can affect the filling process. For example, apertures that are positioned on a shorter axis in the direction of the squeegee will fill better compared to those that are filled on a longer axis.
To achieve the ideal results, it is recommended to always consider these three key processes when manually printing with an SMT stencil.
Automated SMT Stencil Printing
When you want to produce in high volumes, it is more efficient to use an automated SMT stencil printer. With automation, PCBs are transported through the printer through a conveyor belt system.
As the PCBs enter the printer, they are locked into place, where stencils are then dropped down onto the PCB so that the machine can register the fiducial marks on both the stencil and the PCB.
Following that, just the right amount of solder paste is dispensed for the PCB, where an automatic squeegee is used to drag the solder paste across the stencil.
After, the machine automatically lifts the stencil away from the board and removes any excess paste from the underside of the stencil. The PCB is then moved out of the machine so that it can bring the next blank board for the whole process to be repeated.
The benefits that come with automation are that the angle of the squeegee, as well as the pressure applied while dragging the solder paste, remains constant and can be repeated throughout the whole process. This ensures consistent performance across all boards.
Advantages and Disadvantages of SMT Stencils
Now that we have a good understanding of what SMT Stencil is and how it can help in the manufacturing of PCBs. Let’s take a look at the advantages and disadvantages of SMT stencils.
Advantages of Using SMT Stencils
- They are easy to set up and use.
- When producing in high-volumes, stainless steel stencils are convenient as they have a long-life span which allows for multiple usages for solder paste applications.
- Stencils allow for on-contact and off-contact printing on PCBs.
- They are effective when used for multi-level printing of PCBs.
Disadvantages of Using SMT Stencils
- May incur higher costs during production if the design of the PCB changes frequently.
- Stencils require proper storage as the material may bend and affect the apertures, which may affect the final product.
- When printing on-contact there may be cases of solder smear when lifting the stencil from the PCB.
SMT stencils that are properly designed enable highly efficient production of PCBs that are consistent and repeatable. This not only helps to save costs in the long-run, but it will also allow for a higher-quality final product.