What are the key factors influencing solder paste viscosity?
Will finer metal balls in the paste increase its viscosity or vice versa?
Gary Freedman of Colab Engineering lists the five most important factors that influence paste viscosity in his opinion:
- The chemical formula of the paste itself determines its initial viscosity and metal content. The more metal powder there is per unit volume, the more viscous the paste will be. Fine powders have a larger surface area, they are arranged more closely in a unit of volume, so they form more viscous pastes.
- The operation of the squeegees: many pastes have thixotropic properties, i.e. they are 'diluted' as a result of mechanical action as a result of the operation of the squeegees
- Environmental conditions: temperature and humidity.
- When automatic wiping on the underside of the template is used, the wet cloth may gradually release solvents into the paste and change its viscosity. A dry wipe is generally the better option.
- The natural drying phenomenon also changes the rheology of the paste
Tim Jensen of Indium Corp. agrees that smaller balls make a more sticky paste. 'Many factors affect the solder paste viscosity. If all other variables are kept constant, the viscosity will increase as the powder size decreases (e.g. switching from type 3 to type 4). This increase in viscosity is due to the higher packing density of the finer powder (smaller particles can pack more tightly). Solder suppliers take this into account by correcting the percentage of metal: Type 3 solder paste contains approximately 89% metal. On switching to type 4, the metal content drops to 88.5% which gives approximately the same viscosity as type 3 '.
And then Fritz Byle, process engineer at Astronautics: 'If you reduce the particle size of the powder while keeping the same metal percentage, the viscosity will increase, probably very noticeably. […] In the paste production process, almost everything influences the final viscosity. As the powder becomes finer, it becomes more difficult to control the viscosity. Particle size distribution of a certain batch of powder, particle shapes, surface condition - texture, oxides - all of these will make a big difference. Even the method of testing the viscosity makes a huge difference. In order for the customer to obtain data that correlates well with the manufacturer's data, the test parameters must be exactly duplicated. '
Terry Jeglum from Electronic Technology Corporation dives a little deeper into the subject, distinguishing between the viscosity of the paste and its tendency to sag after the print:
'Viscosity can be defined as the degree to which a material resists the tendency to melt. Different viscosities of the solder paste are desired at different levels of the force with which the doctor blade is operated: as the solder paste is spread by the doctor blade on the template, the mechanical action exerted on the paste reduces the viscosity of the paste, thinning it, and allowing it to pass through the apertures of the template. After removing the template, the paste keeps its shape and does not flow over the PCB.
A slightly different parameter is the tendency of the paste to sag, i.e. the tendency of the material to spread after the printing stage. In theory, the sidewalls of the paste deposit should be perfectly straight and remain so until component positioning. However, if the paste has a high sag tendency, its behavior may not be as expected and the deposit walls are no longer perfectly straight. One should try to minimize the phenomenon of sagging of the paste, as this creates the risk of the formation of solder bridges between two adjacent pads. '