![]() Next, the stencil is cleaned and the printing process is repeated for the next board. Once the printing process is complete, the board is inspected with optical scanning equipment to verify the quality of the applied solder paste. A squeegee is used to spread solder over the stencil and fill all of the apertures evenly in an operation that typically takes 15 to 45 seconds per board. The stencils can be plated and coated with materials that will make the aperture openings smooth and help with the even application of the paste. These stencils are often laser cut for precision and can be built from a variety of materials at different widths depending on the components to be soldered and the amount of paste required. Stencil: Using the CAD data from your PCB design tools, the manufacturer will have a stencil made with openings or apertures the size and shape of the surface mount pads on your design. Instead, circuit board assemblers will use either a stencil or jet printing to apply the solder paste. Solder paste can be applied individually to pins with a syringe, but as you can imagine, this is a very slow manual process and would never work for regular production work. How Solder Paste is Applied to Printed Circuit Boards Once the solder paste has been decided on, the next step is getting it applied to the circuit board. ![]() ![]() Solder Paste Grades According to Particle Size These particles must be spherical in shape and can vary in size according to the type standards specified in IPC J-STD 005. Solder paste is also classified by the size of the metal particles that make up the solder powder. For instance, solder paste is available in both lead and lead-free versions to satisfy the restriction of hazardous substances (RoHS) directive. ![]() The solder powder used in the paste can vary in its chemical composition with different material types and percentages used depending on the needs of the board being soldered. The flux not only does its usual job of cleaning the soldering surfaces of impurities and oxidation, but it also provides a temporary adhesive that holds the surface mount components in place. Solder paste is a combination of a powder made up of metal solder particles and sticky flux that has the consistency of putty. Instead, a sticky solder paste is used to hold the parts in place and solder them as well. This new process is known as solder reflow, and it doesn’t use the standard pool of molten solder that the wave process uses. The lack of thru-hole pins in surface mount parts, however, forced the development of a new method of soldering in order to hold them in place on the board until the soldering was complete. Next came the development of surface mount parts which were smaller than their thru-hole counterparts due to their lack of long leads, and eventually they became the dominant component package type used on circuit boards. What is the Paste Used for Solder Paste Application?Įlectronic components were first hand soldered onto circuit boards until the wave soldering process was perfected for the mass production of PCBs. How this is used in different solder paste applications for the assembly of printed circuit boards is something that PCB designers should know about, and which we will take a closer look at here. Solder paste is actually an inventive combination of powdered solder and thick paste-like flux used in the solder reflow process. The paste used for soldering components onto printed circuit boards is a much more complicated recipe then just water and flour, but it is still a paste. You know what else paste is good for? Soldering circuit boards. For years it was the go-to solution for children’s art projects in grade school, and even today people will still use this simple paste to create amazing paper-mâché crafts like piñatas and masks. I once heard comedian Rita Rudner ask the question during a performance “How come when you mix water and flour together you get paste.and then you add eggs and sugar and you get cake? Where does the paste go?” My knowledge of the chemistry of cooking isn’t advanced enough to answer her question, but the basic form of paste is exactly as she described it and can still do quite a lot.
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