When constructing circuits boards that use surface mount components, precision and repeatability of laying down solder paste are key. While a syringe can be used to accomplish this, boards with large numbers of close together parts can become tedious to work through using this method. One alternative option is to turn to SMT stencils (http://www.solder.net/prometal-smt/metal-smt-stencils/). What these stencils allow is for solder paste to be rolled out onto every pad of the PCB at once, greatly reducing the amount of time it takes to apply solder paste to the PCB and making the process easily duplicated. Should you choose this method, selecting the correct SMT stencil can have a major impact on how well your project performs. To aid in selecting the correct stencil for your product, this guide will provide a quick overview of how these stencils are made, the three main types of SMT stencil, and the own benefits and drawbacks of each stencil type.
Step 1: How the Stencils Are Made
Most solder paste stencils you could buy online (http://www.soldertools.net/categories/Metal-Stencils/) are made by laser cutting. In this method, a computer controlled, high-precision laser is used to cut the apertures in the stencil according to a design given by a CAD or GERBER file. High quality lasers can allow for spacing as tight as 0.15mm between apertures in the solder paste stencils.
Step 2: Foil and Prototype Stencils
The first type of stencils are foil and prototype SMT stencils. Foil SMT stencils are laser cut solder paste stencils designed for hand printing or with stencil tensioning systems. These laser cut stencils do not need to be permanently glued in a frame. Because of this, these stencils are less expensive than framed stencils while at the same time reducing the storage space requirements. Unlike framed stencils, foil SMT stencils require no special machines to use.
Prototype stencils, like foil stencils, do not need to be glued into a frame. Designed with manual solder paste application in mind, prototype stencils allow for a design to be tested in small quantities and at low costs.
Foil and Prototype stencils are a better option when the cost of a project must be kept low. Unlike framed stencils, they can also be manually aligned, making them easier to use. However, one must take care that they are lined up correctly, because manual application of solder paste is less consistent than when it is applied using a machine.
Step 3: Framed Stencils
SMT framed stencils are laser cut solder paste stencils designed to work on SMT printing machines. This type of SMT stencil is permanently glued into a frame, allowing for highly repeatable continuous operation in volume production. The frame of these stencils allows them to be installed in a special machine, allowing for easy and accurate repetition of the solder paste application process for large scale PCB production. When quality is valued above all else, framed stencils are a good choice for their repeatability and accuracy.
Step 4: Wrapping Up
While, this is not an all-inclusive guide, hopefully it has provided enough information on choosing the type of SMT stencil that is best for your project. See www.soldertools.net for online ordering featuring 24 hour or less turnarounds.