Prototyping with Solder Stencils
Solder stencils are most commonly used in automated printed circuit board assembly (PCBA) processes. Bare PCBs are fed into one end of an assembly line where solder paste is applied automatically, followed by precisely placed components on the correct solder pads. Finally, the solder paste on the entire assembly is reflowed and allowed to cool before dispensing the completed board.
To ensure the resulting assemblies have quality electrical connections, the amount and placement of solder paste is critical. If solder paste is not applied appropriately, the resulting board may be subject to defects, including bridged connections, open connections, solder balling, tombstoning of components, and insufficient fills. All of which will impact the performance and reliability of the final assembly. Solder stencils simplify the application of solder paste by acting as a mask to ensure the paste is applied only to the correct areas; this process is often referred to as solder paste printing because it is similar to silkscreen printing.
Solder stencils are thin sheets of metal or polymer with a series of holes matching the footprint pads of a PCB. Standard footprint stencils are available for SMD breakout boards, with many PCB manufacturers offering custom stencils to match a customer’s PCB design. When used in automated PCBA systems, stencils are held firm in a frame. Then, once the stencil is placed onto a blank PCB, solder paste is pulled across the stencil using automated squeegees. This process provides a uniform deposition of paste onto each board and with a little practice, the same advantages stencils provide are realized when manually creating SMD prototypes.
Here are a few things to keep in mind when prototyping with solder stencils to achieve the best results:
· Solder paste should be at room temperature before spreading to provide optimal flow into all cutouts and not pull away with the squeegee.
· Position the PCB in a sturdy frame. Match the frame thickness with that of the PCB to which the solder is to be applied. This provides good coplanar alignment between the stencil and the PCB, preventing excess solder from seeping under the stencil. Attach one side of the stencil firmly to the frame with tape to prevent misalignment and movement.
· Hold the squeegee at a 45-degree angle to the stencil surface. Spread the solder paste across the entire stencil in one steady motion while keeping constant pressure against the stencil. Avoid multiple swipes which increase the possibility of pushing solder under the stencil or pulling solder off the pads.
Keep in mind that solder paste is quite forgiving, and surface tension often corrects minor errors during reflow. If defects are found following reflow, these may be corrected by hand. If the paste application process doesn’t go as planned, be aware that attempting to clean all the paste from the board and starting over is not as easy as it sounds. Often the process pushes solder into vias and the corners of pads, which may cause defects during reflow. When finished with a stencil, clean it well with isopropyl alcohol. Store stencils supported between flat surfaces to prevent damage or bending.
Once you’ve mastered using a stencil to apply solder paste for your SMD prototypes, you will be able to create higher-quality projects in less time, allowing for faster design iterations with less post reflow troubleshooting.