Optimal solder paste viscosity depends on the application and the assembly process. A good range is 500-1000 Kcps. This can be adjusted by the end user or by the manufacturer.
There are several different methods for measuring the viscosity of solder paste. One of the most common methods is using a spiral viscometer. Spiral viscometers spin at speeds between three and 30 rotations per minute. These devices are used to measure the viscosity of a sample under a wide range of shear rates.
Another method is called Brookfield viscometry. Brookfield viscometry is not often used for measuring the viscosity of solder. It requires multiple measurements under different shear forces.
A third method is a thixotropic index. This index measures the viscosity of the paste at rest and while it is applied. The thixotropic index is more helpful to determining the working life of the paste.
Some manufacturers of solder paste recommend a specific viscosity range for their product. For example, if you are printing a fine pitch SMT, you will need Type 4 solder paste. Alternatively, if you are using water-soluble solder, the recommended range for the product may be different.
As a result, it is important to keep your equipment calibrated. Manufacturers should test their products regularly.
The best way to ensure you get the proper paste is to follow the manufacturer’s handling procedures. These procedures include shipping, receiving inspection, and a test for viscosity.
You should also check for any problems with your paste. If you find a large air bubble in the paste, for example, you should remove it and replace it with a clean container.