Stencils are important in electronic manufacturing. These days, electronic manufacturers use stencils to print solder paste. The paste creates deposits on the printed circuit board to firmly hold electronic components. It is important to apply the paste in the right volume and position. This precision is very important when applying paste.
Therefore, the use of stencils is crucial in applying solder paste. These stencils help to regulate solder paste printing. Also, stencils make solder paste application easier for surface mount device (SMD). Stencils offer great benefits to electronics manufacturing. In this article, we will discuss the role of stencils in electronics manufacturing.
What is PCB Stencil?
A stencil is a sheet of steel used to apply solder paste on a circuit board for SMT component placement. Also, the PCB stencil features laser-cut openings. This stencil deposits the paste on marked areas on a bare circuit board. Therefore, it keeps components placed on the circuit board firm.
The main function of a PCB stencil is to deposit the appropriate amount of paste on SMT pads. This enables the solder joint between the component and the pad perfect as regards mechanicals strength and electrical connection. A PCB stencil makes the application of paste easier and faster. Also, it ensures accuracy and precision when applying paste.
PCB stencil manufacturers design stencils by creating holes via lasers on a stainless steel foil. These holes are created at the positions where PCB assemblers need to place surface mount components on the PCB. This allows the PCB assembler to place the stencil on top of the circuit board and align it using position points. These positioning points are also known as fiducial marks.
After proper alignment, the assembler applies the paste using a metal squeegee blade. Therefore, the paste is generously and precisely applied to the bare circuit board. After this, the assembler can place the components on the circuit board.
A PCB stencil manufacturer creates apertures or openings through laser beams on a hard steel-foil based sheet. The stencil is then placed on top of the circuit board. Solder paste stencils, reusable stencil frames are permanently glued. The metal stencil and lead free solder paste. Laser cut stencil are the best option for PCBs. Electroformed stencils offer good quality.
Important Considerations for PCB Stencil Design
Stencil thickness
The thickness of the sheet determines the amount of solder paste released through the opening. With the accurate thickness, the PCB stencil will release the desired amount of solder paste. The aperture size and the sheet thickness are crucial factors that impact the amount of solder paste deposited on the PCB.
Also, deposition of extra solder can result in solder bridging. Meanwhile, deposition of less solder can cause weak solder joints. Therefore, this can affect the performance of the final printed circuit boards.
Aperture design
Another crucial factor that can affect solder joint is the aperture design. The chances of defects like solder beads and solder bridging depend on the aperture design. The opening of the stencil is usually smaller than the pad size of the circuit board. It is possible for the solder paste to stick to the aperture wall after solder deposition. Therefore, the PCB pad’s area should be more than two-thirds of the area inside the aperture wall.
Stencil alignment
Stencil designers add registration marks known as fiducial marks to the stencil and PCB. These marks help to ensure proper alignment between the stencil and PCB. Also, it helps to achieve a perfect print of solder paste.
Stencil material
The material of the stencil will impact its ability to provide paste from the aperture to the PCB pad. The best material for stencil is stainless steel. This is a major reason why stainless steel stencils are commonly used in the PCB industry. Also, the stencil material determines solder paste release via the apertures.
Types of PCB Stencils
There are different types of PCB stencils.
Framed PCB stencils
These stencils have a casing around them. The stencil frame ensures the stencil sheet is tight enough for the appropriate exactness. Also, framed stencils are specifically designed for a high degree of screen printing of PCBs. Framed stencils feature smooth aperture walls. Also, these stencils have clean laser-cut apertures. Therefore, it creates permanent non-fading fiducial.
Frameless stencils
Frameless stencils are laser cut solder paste stencils. These stencils work with stencil tensioning systems. Frameless SMT stencils are less expensive than framed stencils. Also, they deliver great quality and performance despite their low cost. These stencils don’t feature a fixed casing around then.
Prototype stencils
Prototype stencils are usually made from Gerber or CAD files to match prototype printed circuit boards. Also, these stencils are frameless. Prototype stencils are specially designed for manual printing.
Electroformed stencils
These solder paste stencils are usually nickel-based. An electroformed stencil provides the best release of solder paste. Also, these stencils are widely used for fine pitch. The nickel in electroformed stencils feature a low coefficient of friction compared to that of stainless steel. Also, electroform foils are much harder than hard stainless steel. This offers a longer stencil life.
How to Manufacture A PCB Stencil
A PCB stencil feature hundreds or thousands of opening created with high precision. Therefore, it is not advisable to manually create these holes. There are three methods for making circuit board stencils. These methods are electroforming, chemical etching, and laser cutting. The laser cut stencils is the most popular PCB stencil.
Here, we will go through the process of PCB stencil manufacturing by using a laser cutter.
Stack the mylar sheet
You need to stack two sheets together. Then, heat the mylar sheets until you detach the first sheet from the stack. The second sheet will suck the melted pad of the first sheet. Therefore, this allows you to pull the first sheet off.
Export design files from EagleCAD
Export your cream layers via a cam file. Using ExpressPCB, open the file menu and choose the “Export DXF Mechanical Drawing.”
Swell pads in viewmate
You will have to compensate for the melting caused by the cutting process. This is because meting enlarges the pad size. Therefore, you will have to cut down the size of the pad. To import the cream layers into ViewMate, you need to choose File>Import>Gerber. After this, select Setup>D codes. Choose all columns and open Operations>Swell. Input the value for the size adjustments based on the laser specifications.
Use Autocad to swell pads
You can use AutoCAD to scale down the DXF’s content. Also, you will need to highlight the drawings in the AutoCAD window. Key in the proper scale figure when the Scale Factor dialog comes up. This will place the drawing in the appropriate scales. After doing this, you can now print out the finished drawing.
Cut the stencils
You will need to adjust the settings on the laser cutter to appropriately cut through the already stacked Mylar sheets. The laser’s heat will melt the first sheet. However, the second sheet will absorb the first sheet’s melted pad. Therefore, this enables you to neatly pull out the first sheet.
How to Ensure Quality Solder Paste Printing
PCB assembly involves solder paste printing. Therefore, precise printing of this paste is crucial for accuracy for a PCB assembly. However, some crucial elements help to ensure quality paste printing.
Stencil installation
Pool alignment of stencil can result in solder bridge. Therefore, it is crucial to ensure there is precise alignment between the circuit board and the stencil.
Blade stroke speed
Blade stroke speed will impact the amount and shape of the paste deposition. Also,this affects the solder paste printing quality. Generally, you should set the stroke speed between 20 to 80mm/s. You must adjust the blade’s speed relative to the solder paste’s viscosity.
PCB mounting
Before you start solder paste printing, you need to mount the target PCB to the work table using a vacuum or mechanical clamp. Also, you should secure this circuit board with support pins at the center of the circuit board. This ensures it doesn’t move during printing.
Squeegee pressure
The pressure of the squeegee blade will impact the amount of paste deposited. If the pressure is more, there will be less solder deposition. Also, this is because the pressure squeezes the gap between the circuit board and the stencil.
Squeegee blade set up
After mounting the stencil firmly on the printed circuit board, you will apply the solder paste to the stencil. You will need a squeegee blade to fill paste in the stencil apertures.
Thorough cleaning
Thoroughly cleaning the stencil will get rid of any solder paste. Also, this will get the stencil prepared for the next printing.
The Importance of Stencil in Surface Mount Assembly Process
Stencil is widely used in the surface mount assembly process to achieve an accurate and repeatable deposition of solder paste. High volume screen printing can impact PCB board functionality. Also,the stencil design is crucial to achieving a high-yield assembly process. Low amounts of solder can cause dry solder joints.
The type of surface mount devices on the PCB determines the optimum foil thickness. The thickness of the stainless steel foil varies. Also, laser-cut stencils are commonly used for surface mount device.
Conclusion
Stencil printing is an integral aspect of PCB assembly. Also, this process could be very tricky. The slightest error can ruin the whole PCB assembly process. Also, PCB stenciling helps to mount all solder joints in the PCB. This helps to save time and eliminate error in the solder joint. The PCB stencil helps to achieve smooth aperture walls. Printed circuit boards perform better with accurate stencil printing.
