Mini Micro Stencils
When it comes to PCB rework of complex device the method that has been most widely accepted over time has been the use of mini micro stencils. These stencils are miniaturized version of the full metal SMT stencils used in the PCB assembly industry. Given that the objective in any rework process is to emulate the original manufacturing process, this type of stencil is the way to emulate this process the most closely.
Mini micro stencils refer to metal (in this day and age stainless steel) stencils that will allow you to paste print a site location or an area of a PCB. The stencil itself comes in a variety of configurations. The “foil only” stencil, like the name implies, is a section of the metal stencil which has been “cut out” from the production stencil. In some versions of the min micro stencil flaps are added to (2) sides of the stencil. The purpose of these flaps is to make sure the solder paste does not “bleed” to others of the PCB. In still other versions of the stencil mechanical bolding piece is attached which allows it to be stabilized and oriented in the right location of the rework area.
Mini micro stencils have some shortcomings and trials based on their usage. The stencil is very difficult to keep coplanar to the PCB as it gets bent during usage and cleaning. This lack of co planarity causes solder paste to “smear” underneath the underside of the stencil. In addition this type of stencil has the weakness of being a “one swipe and you are done” stencil. You do not get a second chance. Finally, these metal stencils are very difficult to fit in around very tight areas of the PCB as they are very rigid on their design. More commonly plastic film stencils with an adhesive backing are being used for reworking component locations with the paste printing method.