How Solder Paste is Used in Circuit Board Assembly Processes
Circuit Board Assembly Processes
The process of circuit board assembly begins with the bare copper board and is then filled (or “stuffed”) with components to make a fully functional printed circuit board (PCB). Depending on whether you’re using through-hole or surface-mount technology, the component leads line up with conductive pads or lands on the PCB; the solder paste applied earlier keeps them in place temporarily. The resulting conductive connections, called a “solder joint,” can be very durable and reliable.
To apply the solder paste, a stencil with aperture openings matched to the PCB pad locations is positioned on the PCB; the printing machine then applies the solder paste through these stencil apertures. The squeegee blade of the printing machine then removes the excess solder paste and leaves only the paste in the aperture openings on the PCB. The PCB is then automatically sent to a pick-and-place machine where SMD components or ICs are mounted. The machine follows the pre-set program set by the engineer to accurately place each component where needed on the bare board.
Once the PCB has all the components placed, it’s ready for a heating process that will melt the solder and permanently bind the SMDs to the board. This is known as reflow soldering. This is the final step in circuit board assembly and requires a lot of skill, particularly when the device is small and the tolerances are tight.
How Solder Paste is Used in Circuit Board Assembly Processes
The success of the reflow process depends on the quality of the paste and the precise application technique. The main issue is that the reflow temperature should be gentle enough to avoid explosive expansion and activate the flux; it also has to reach and remain above the solder’s melting point, which is known as Time Above Liquidus (TAL).
There are many ways to apply the paste to the PCB, but all must involve strict standards of consistency and accuracy to ensure that the bond is a strong one and durable. Using a professional PCB assembly company to apply the paste will reduce your chances of costly errors in production.
Solder paste is a grey powder that contains a mixture of tiny metal balls, typically 96.5% tin, 3.5% silver and 0.5% copper, with added flux, which is a chemical designed to help the solder melt and adhere to a surface. The spherical solder metal is usually mixed with a lead because it lowers the melting temperature and creates strong connections with other metals, but it is becoming more common to use less toxic “no-lead” alloys.
The exact ingredients and formulation of the paste are proprietary to each manufacturer, but it will always contain a combination of these basic components. The storage and transport of the paste is important because the small, minute metal spheres are easily oxidized. The stored paste must be in an airtight container and kept at a low temperature to prevent this from happening. The powder is also often kneaded to improve its rheology, which refers to how it deforms and flows.