Seven Important Concepts to Know About Solder Stencils

The placement of solder is a crucial stage in the PCB assembly process. Poor or inaccurate placement of SMT components can cause the failure of the PCB board. To avoid time-consuming manual rework on boards you have already assembled, you need to ensure the effective deposition of solder paste. It should be clean, uniform, and precisely positioned on the PCB board. Correct soldering will help you save hours of rework using the desoldering process.

Using solder stencils for assembling and placing components on the board can help you get a suitable yield. The stencils offer superior flexibility to achieve the correct solder paste volume deposit for components with diverse paste requirements. It gives you optimum control over solder paste volume and achieves good results. It enables fast assembly with minimal cost. Solder stencils are precision tools for effective surface mount components assembly.

Fiducial Marks

Solder stencils are manufactured by creating laser openings on a stainless steel foil sheet. These openings occur at positions where you need to place Surface Mount components on the board. You have to set the stencil on the top of the circuit board and align it correctly with the help of positioning points. It is also called fiducial marks.

These are recognition marks, which act as reference points for solder stencil alignment. It enables the automated component placement using the Pick and Place machines. The machine recognizes the mark and uses it as a frame of reference to place components.

Squeegee Blade

Once you align the solder stencils on the PCB, you must apply solder paste using a metal squeegee blade. The squeegees are tools to spread the necessary force to move the solder paste onto the circuit board across the stencil.  These are made from metal or polyurethane. It facilitates uniform deposition of solder paste on the board to place the surface mount components. On removing the stencil after depositing it, you can place the components on the board.

Cleaning Of Stencil

You need to regularly clean the stencils during use with the help of manual or automatic processes. Most automatic printing machines have a system that helps quickly clean the stencil after a fixed number of prints is achieved. It applies a lint-free material using a cleaning chemical like IPA. The automated cleaning system carries out two functions. The primary one is to clean the stencil’s underside to stop smudging. The second one is to clean apertures using a vacuum to prevent blockages.

Stencil Condition

You need to carefully store and maintain the stencils and squeegees so as not to undergo any mechanical damage. Such damages can lead to undesired results. Check the stencil before use and clean it thoroughly after it. You can use an automated cleaning system to remove any solder paste residue. If you notice any damage on the stencils, you must replace them to ensure a reliable, accurate, and repeatable process.

Stencil Separation Speed

Stencil separation speed is the speed at which the stencil separates from the PCB after printing. You can use a speed setting of up to 3mm per second. The aperture size also governs it within the stencil. If the speed is too fast, the solder paste may not fully release from the apertures. It may cause the formation of high edges, “dog-ears,” around the deposits.

Which Stencils?

Polyimide stencils are useful for various projects, but their lifespan only expands to a few dozen uses. If you carelessly handle it, it will decrease its lifespan. The best standard for solder stencils is the “stainless steel” standard. These are more durable and precise than polyimide. But it has a higher price tag than the polyimide stencils.

Designing a Solder Paste Stencil

To design a solder paste stencil, you need to use PCB layout software. It is generally known as the paste layer, based on the shape, size, and position of the surface-mount components pads in the design layout. The process of generating a Gerber file for the paste layer is dependent on the software you use. The holes in the stencil are slightly smaller than the SMD pads. The stencil thickness also affects the size of the holes.

• Stencil Thickness: Accurate thickness of the stencil sheet helps release the desired amount of solder paste via the opening. The sheet thickness and aperture size are important parameters for effective solder paste deposition. Any extra amount of solder paste causes solder bridging or creates weak solder joints, affecting the functionality of the final PCB board.
• Aperture Size: The stencil openings must be slightly smaller than PCB pad size to avoid solder defects such as bridging and solder beads. Once you complete with solder deposition, you need to remove the stencil. The solder paste may get transferred to the board or stick to the stencil’s aperture walls during this process. The PCB pad area must be greater than two-thirds of the inside area of the aperture wall to eliminate this problem.
• Stencil Material: The stencil material also determines the capability of solder stencil to effectively deliver the solder paste from the aperture to the board pad. Stainless steel is mainly used for stencils.
• Stencil Alignment: Add ‘fiducial marks’ to both the PCB and stencil for accurate solder paste printing on the PCB pads. The fiducial marks must ensure good alignment between the stencil and board.

To Conclude:

These are a few essential things to understand how the stencil works and is vital to the PCB printing process. Also, read about Electronic Circuit Board Repair Service and many more articles on Viral Sant.