Viscosity of Solder Paste

Solder paste is a complex viscous fluid that is non-newtonian in nature. This means that the viscosity is not constant and varies based on the shear stress applied to it. The viscosity of a fluid is defined by its resistance to flow. In the case of solder paste, the viscosity is inversely proportional to the amount of shear stress applied to it. If the applied stress increases, the viscosity of solder paste reduces and it can move through the stencil aperture onto the pads on the PCB. Then when we remove the stencil, and no stress is applied to the paste, the viscosity increases, and the solder paste holds its form and remains stationary on the circuit board. This property of solder paste is called thixotropy.

This property of solder paste is a crucial requirement for printing, as the paste must flow in and out of the stencil apertures during the print process, but afterwards remain in a stagnant position.

The viscosity of solder paste is an important property to evaluate when selecting a solder paste. Solder paste cannot be spread through the stencil if the applied stress is below the viscosity breakdown limit. This is usually specified by the manufacturer of the solder paste.  

The viscosity of the solder paste also reduces with the temperature because of the change of state of one of the material composition due to heat.

What is the optimal viscosity of solder paste?

The optimal viscosity of solder paste usually depends on the application and assembly process. The viscosity should be low enough so that the solder paste can print and release properly at high print speeds, but at the same time it should not slump after printing.

A rough range for solder paste viscosity is 500 to 1000 Kcps. No clean solder usually has viscosities from 500 to 800 Kcps and water soluble solder pastes have viscosities from 700 to 1000 Kcps. These are just example ranges.

What is Shear Thinning?

Solder paste is often re-used during the PCB Assembly process. For example, once paste is applied through ta stencil, the excess paste is collected and reused. This property of solder paste is called Shear thinning. The viscosity of solder paste decreases as the paste is reused or repeatedly printed. This drop in viscosity can result in a number of defects like short circuits or solder balls. The level of degradation of viscosity with reuse is what differentiates a good solder paste from a bad one.

This graph shows the viscosity of two different solder pastes each time it is reused. The x-axis is the reuse count and the y-axis is the viscosity.