Background The rheology of solder paste is a different characteristic from what is generally referred to as "flow". A typical solder paste usually contains 90% alloy particles and about 10% flux medium. The flux medium is one of the main factors determining the rheology of solder paste.

But it's not practical to just look at what's in the flux medium. To understand printing performance, rheological measurements can provide some useful information.

The term "viscosity" in Rheology is interpreted differently in different ways, but the basic concept is to quantify the force required to "move" a fluid substance.

A flowing substance can be represented by the thickness of the liquid between two parallel disks. When a force acts on the top disk while keeping the bottom disk stationary, fluidity can be discussed. Shearing of a flowing substance can be discussed in terms of the cross-correlation of the movement of many layers. Effect. But the viscosity data can only tell us a theoretical value obtained in some predetermined environment, for example, a steady laminar flow state, a fixed temperature, a fixed shear rate, etc.

The properties of substances that flow under different conditions need to be explained by the term "rheology". Solder paste is viscous at low shear rates (slow or no flow), and becomes thinner as fluidity increases and shear rates increase. Likewise, at a fixed shear rate, viscosity also decreases with time. As soon as the shear force ceases, the viscosity immediately returns and eventually returns to its original state, which may take several hours. This characteristic of solder paste is called "thixotropy" in rheology. However, in practical applications, the curves of the two solder paste viscosity changes obtained with the increase or decrease of the shear rate do not overlap, and there is a hysteresis phenomenon.

There are many ways to generate this hysteresis viscosity profile for solder paste. According to the classification of sensors, there are mainly the following three types: rotating mandrel type, cone/plate type, and spiral sleeve type. By using different sensors and measuring the torque or force applied to the measured object, the shear rate can be obtained, and the value of the viscosity can be obtained, so that the hysteresis curve can be drawn. In addition, because the tin powder particles in the solder paste cannot effectively (like a general liquid) enter tiny gaps, the cone/plate measurement method is not suitable for measuring the viscosity of the solder paste.

It is worth noting that the data obtained by using different sensors or different types of viscometers cannot be compared and converted to each other. In other words, simply comparing the data on the product catalog without verifying the test methods and operating procedures will result in a mismatch of viscosity data. For example, using the same sensor type but measuring the same product at a different temperature will give inaccurate viscosity values. Experimental data shows that usually every 10°C increase in the measurement temperature will cause a 15-20% decrease in viscosity. Therefore, when measuring, it is very important to use the same container, the correct temperature of the object under test (solder paste), the correct sensor, and the recommended test method.