Properties of Solder Paste
In using solder paste for circuit assemblies, one needs to test and understand the various rheological properties of a solder paste.
- Viscosity
- The degree to which the material resists the tendency to flow. In this case, varying viscosities of solder paste are desired at different levels of shearing force. Such a material is called thixotropic. When solder paste is moved by the squeegee on the stencil, the physical stress applied to the paste causes the viscosity to break down, thinning the paste and helping it flow easily through the apertures on the stencil. When the stress on the paste is removed, it regains it shape, preventing it from flowing on the circuit board. The viscosity for a particular paste is available from the manufacturer's catalog; in-house testing is sometimes needed to judge the remaining usability of solder paste after a period of use.
- Slump
- The characteristic of a material's tendency to spread after application. Theoretically, the paste's sidewalls are perfectly straight after the paste is deposited on the circuit board, and it will remain like that until the part placement. If the paste has a high slump value, it might deviate from the expected behavior, as now the paste's sidewalls are not perfectly straight. A paste's slump should be minimized, as slump creates the risk of forming solder bridges between two adjacent lands, creating a short circuit.
- Working life
- The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.
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