PCB Stencil Selection for Beginners
Posted: Mar 15, 2020
PCB prototype assembly of surface mount components, depending on complexity, can either be done via hand soldering or with SMT stencil printing of the solder paste.
Hand soldering for PCB prototype development,as long as the design does not contain a large component count or has fine-pitched components, can be hand soldered. Hand soldering has several advantages for getting your first build completed as "beginner" electronics assembler. One of the advantages of this approach is that the parts can be reasonably quickly attached to the board in order to get the design tested. In addition, a skilled hand solderer can make adjustments on the fly when new or different parts needs to swapped out for same footprint component body styles. The downside to this approach for beginner electronic assemblers is if your soldering skills are not up to the task there can be serious project consequences. If you are not trained or experienced in electronics then the boards and components can be easily destroyed. If not properly soldered circuit debug can take a long time as you sort through whether there is an assembly or circuit design problem.
The other approach to the PCB electronics assembly process is to use a PCB stencil which allows you to print solder paste onto the component solder locations. The stencil has solder paste rolled through the apertures after alignment. The alignment, handling of the paste and the rolling of same all take a degree of skill to get right. Once paste is applied to the PCB, the components can be hand-placed and then sent through a controlled heating source such as PCB prototype oven in order to reflow the solder paste and get the mechanical attachment of the components to the PCB.
PCB stencils for prototype development come in a variety of configurations including a variety of materials, thicknesses and methods for holding and alignment. Each of these choices will make the process of printing the solder paste onto the PCB either more or less difficult.
There are a variety of materials which can be used for solder paste printing of for "beginner" electronic developers. The materials are generally either a form of stainless steel or plastic film. The majority of the plastic film stencils are either fabricated from Mylar™ or Kapton™. Mylar™ stencils are generally cut for a PCB prototyping job when the board has very simple aperture patterns and precision is not required. Kapton™ stencils are used when higher precision is required or there are larger print areas. Both of these plastic film stencils "stretch" in X and Y when pressure is applied via a squeegee thereby causing a degree of variability for the "beginner" assembler of electronics. Stainless steel stencils offer a degree of resilience and a larger process window for "beginner" assemblers. The flatness of the stainless steel stencil helps to keep the stencil in intimate contact with the PCB thereby preventing "smearing" of solder paste when the stencil comes off of the PCB.
The material thickness choices for stencils also varies and is a function of the size of the openings or apertures required. A majority of the stencil thicknesses fall into the 3, 4 and 5 mil thicknesses (0.076 to 0.127mm). Thinner stencils make it more challenging for the "beginner" assembler to print and release the stencil from the surface of the PCB. Smaller apertures can be printed with these thinner stencils. Thicker stencils provide for increased rigidity but are not able to release paste into small apertures. Stencil suppliers will work with the assembler to help guide and determine which stencil thickness is the most appropriate given the PCB design parameters.
Finally, the configuration of the prototype printing stencil is varied and drives the relative ease or complexity for a beginner electronics developer who is also assembling the board. Foil only or foil stencils with prototype flaps make it challenging for the beginner to both aligning and holding the stencil in place while rolling the solder paste through the apertures. Framed stencils, which fit into manual printing machines, rely on the machine/stencil mechanics to provide alignment and holding force which makes this type of stencil the simplest for beginning electronic assemblers.
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