By: Mike Peters, Managing Engineer at ACDi
Over my 30 years’ experience working in the contract manufacturing world, I have learned many things. One thing I have learned is that it’s very important to think about what you want to achieve before ordering a SMT stencil. You really have to take a look at the board, Gerber files, and bill of materials to see if you have fine-pitch parts, large components, small components (0402 or 0201), BGAs, QFNs, etc. All of these play a very important role when ordering the correct stencil thickness, stepping, coating, or reducing apertures. We have to always remember that about 80% of failures in SMT are usually caused by SMT prints.
Stepping a stencil usually happens when you have larger components on a PCB board along with smaller components or fine-pitch parts. You may want 6 or 7 mils of paste on connector pads but in turn want 4 mils of paste for a 0201 pad or DFN. You may have a PCB that has all 0402 and 0201 parts on one side and in this case you may choose to have a 3 or 4 mil stencil.
Reducing apertures usually occurs when you have, for example, a fine-pitch QFP and you want to reduce the chance for solder bridges. You would reduce the length, width, or both to help reduce solder bridging between the leads. This is also very effective for QFNs and DFNs to reduce solder bridging.
Nano coating is a great add-on to any stencil that uses small apertures like BGAs, QFNs, DFNs, 0201, 0402, and fine-pitch QFPs. It helps prevent clogged apertures and saves wiping paper. It is also a great add-on for stencils that you plan on using many times. I highly recommend using it on any stencil that meets those specifications.
Home plating and reverse home plating is also ideal to use on all chip cap parts. This method helps tremendously with the prevention of bill boarding and tombstoning.
A correctly made stencil plays a very important role in the beginning of the SMT process. If you need assistance specifying your SMT stencil or want a second set of eyes to validate your stencil specifications, ACDi is here to help. You can also read our other blog about top considerations for solder paste stencils for SMT.