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Fine pitch packages are becoming more important as a way to increase circuit densities. Fine line solder paste application is critical to the successful assembly of fine pitch packages on Printed Circuit Boards (PCBs). Solder paste is a homogenous, suspension of solder powder particles in a flux binder. To choose the right paste for the intended application, careful attention needs to be paid to several attributes, including the percentage of metal/flux, particle size distribution and shape, rheology, slump, spread, cohesion, tackiness and solvent evaporation rate of the solder paste.

To ensure the behavior of solder paste prior to actual production, it should be analyzed as soon as it is received from the vendor and during actual use in manufacturing. The appropriate temperature, humidity, storage conditions, handling procedures, etc. should be identified so that the paste does not deteriorate. This research conducted statistically valid experiments to identify correlations between the rheological characteristics of the solder paste and the quality of the print obtained. The shear stress versus the shear rate diagram, known as the rheogram, was studied for the presence of any correlation with the print quality obtained. The shear rates involved during stencil printing were determined using available empirical models. This model was used in conjunction with the equilibrium flow curve to determine the applicable viscosities for the different print conditions. Extensive stencil printing experiments were conducted to determine the performance of the solder paste. The quantitative indicators of print quality were solder deposit volume, height and area. Apart from this visual inspection was also carried out to judge the quality of the print in terms of sharpness of the print, rolling of solder paste, release of paste during stencil lift-off, and the occurrence of bridging.

The above stencil printing experiment, along with regression analysis with the static viscosities obtained from the Brookfield viscometer helped explore the validity of using single point viscosity values for determining the behavior of the solder paste on the stencil printer. Similar regression analysis with the absolute viscosities helped determine the ability of the absolute viscosities to predict the behavior of the solder paste during stencil printing. Printing parameters such as the squeegee speed, squeegee durometer, squeegee pressure and their interrelationship with the rheology of the paste were also determined through extensive testing and statistical analysis. There seems to be some correlation between the Brookfield viscosities as well as the absolute viscosities derived from the equilibrium flow curve to the print quality. Correlation analysis determined the absolute viscosities of the solder paste to be positively correlated to the Brookfield viscosities. The regression models for the average volume as well as the standard deviation of the average volume for the different aperture shapes for both the polyurethane and the metal squeegee experiments are derived.


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