Solder Paste Printing Technology and Process Parameter Setting (5)

5 Fluid properties and printability of solder paste

5.1 Fluid properties

Solder paste is a fluid that has rheological properties and flows under external forces. Viscosity is an important physical property of a fluid expressed by the formula K=S/R. Viscosity is defined as the ratio of a constant shear stress S to a constant shear rate R. If K is a constant, it is called a Newtonian fluid, otherwise it is a non-Newtonian fluid.

The flow characteristics of the solder paste are shown in Fig. 12. The abscissa shows the velocity gradient (agitation speed of the solder paste) and the ordinate shows the stress gradient (resistance when the solder paste is stirred). The oblique line of the characteristic shows the viscosity. It can be seen from the figure that the speed gradient is accelerated and the force gradient is close to a constant value, which means that when the stirring speed is increased, the viscosity decreases, and this characteristic is called thixotropic.

5.2 Liquid Flow Characteristics and Printability

The thixotropy of solder paste has a lot to do with printability. When initially filling a solder paste with a narrow printed screen opening, the viscosity of the solder paste is preferably low; and when moving from the opening of the printed screen to the substrate pad, the viscosity is preferably higher. Due to the different printing processes, the required properties of the solder paste are also different. For this purpose, the solder paste must be thixotropic. In the initial process, the solder paste was agitated by a doctor blade to be agitated, and the viscosity decreased due to thixotropy. In the subsequent processes, since the solder paste filled in the openings loses the effect of the blade agitation, the viscosity of the solder paste becomes high due to the thixotropy and a solid-like characteristic is formed.

It can be seen that the flow characteristics of the solder paste are closely related to the printing mechanism. In order to perform high-density, high-precision printing, it is necessary to analyze the fluid flow characteristics of the solder paste in detail.

In addition, when the solder paste is in motion, it also has dynamic characteristics. Let G1 be the elastic modulus, similar to the solid component; G2 the viscous modulus, similar to the liquid component; G* is the composite modulus of the solder paste, then G*=G1+iG2, where i=√-1.

6 Selection of solder paste size and shape

The size, shape, and flux content of the solder paste will affect the print quality. When selecting the solder paste, a proper solder paste should be selected according to the pin pitch to ensure good printability. As can be seen from Table 3, fine-pitch printing often uses spherical, fine-viscosity solder pastes.