Comparing the viscosity properties of different solder paste types consisting of different diameter metal particles

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Dr. Krammer Olivér
Department of Electronics Technology

Since the solder paste is the suspension of flux and solder powder, during the measurements of the rheological properties we have to be thorough. Powders in the suspensions have varying size and concentration depending on the type of the solder paste. Therefore, we need to adjust the measurements depending on these properties, which means that we need to take into account for example the wall slip effect, and to carry out the measurement method based on that. The wall-slip effect influences the viscosity of the solder paste, and the results of the measurements, which could lead to false model results in the simulation based on measurement data. Other than that, the actual viscosity of the different kind of solder pastes can differ, which could influence their application, and thus it needs to be taken into account while designing the manufacturing processes. Furthermore, the viscosity of the solder paste could show changes during the printings, which also needs to be taken into account.

I found that the common point in the processes and results of the literature regarding the subject was, that for every measurement the solder paste sample was fresh, and based on those results, they drew conclusions regarding the property changes of the solder paste during stencil printing.

While designing the experiments, I tried to develop a method that gives the best approximation for the rheological property changes of the solder paste during the stencil printing. In my experiments I measure the samples multiple times, so it gives better results.

During my measurements it was shown that with increased idle-times (between the measurements like between the stencil printings), the solder pastes viscosity increased in stationary state. As expected, the greatest viscosity was at 60 second idle-time. The viscosity of the Type 5 solder paste was the greatest, and for Type 3 was the lowest.

During oscillatory stress test the cross point tension gives more information for stencil printing application. I experienced that this tension was the greatest when 30 second idle–time was applied.


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