Rheological Characterisation of New Lead-Free Solder Paste Formulations for Flip-Chip Assembly

The study of developmental products is of great importance when attempting to identify solder pastes that produce a good stencil print. This study concerns rheological characterisation of two such developmental pastes (labelled A and B for simplicity), which were investigated to identify their printing performance characteristics. Paste "A" was formulated with a higher metal content (0.5% higher than paste "B"). Two different types of test were conducted in the study: firstly an oscillation sweep test was carried out to identify the viscous and elastic properties of the samples, and secondly a viscosity test was used to identify any shear-thinning behaviour. In the oscillatory shear test experiments, the pastes were subjected to varying stresses between 1 and 500 Pa, at a constant frequency of 1 Hz. This was done in order to measure the storage and loss modulus of the pastes, as well as indicating the phase angle, which identifies the dominant property (viscous or elastic) of the material at a given point. In the viscosity experiment, the paste samples were subjected to 3 different tests, in the first, the shear rate was varied between 0.01 and 0.5 sec
^-1
, in the second between 0.5 and 5 sec
^-1
, and the third from 5 to 15 sec
^-1
. The focus of these tests was to measure the resulting viscosity and hence the shear-thinning behaviour of the pastes. The result of the oscillatory shear sweep test showed that both pastes exhibited a dominant solid-like behaviour, with paste "A" demonstrating the higher solid characteristic of the two, whilst the viscosity test identified both pastes as being shear-thinning in nature. Of the two pastes, paste "B" demonstrated a lower viscosity than paste "A", which was possibly be due to the higher metal content of paste "A" sample. The results also allowed for the identification of the rate of structural breakdown, which was predominantly higher within paste "A". These results are of high value when attempting to characterise the suitability of a paste for printing, as firstly; they demonstrate that the metal content may affect its viscosity. This has obviously influence the printing process at both the paste roll and aperture emptying stages. Secondly, the rate of structural breakdown was also of high importance, as it shows that the strength of the microstructure within paste "A" was greater, as the viscosity remained higher than paste "B ". This could then lead to possible identification as to the possibility of slump occurring, as the rate of structural breakdown was high at low shear rates