Evaluation of rheological properties of lead-free solder pastes and their relationship with transfer efficiency during stencil printing process

Although stencil printing is widely used in surface mount technology, it is believed to be the main source of majority of defects in the final assembly. Such defects, which could lead to major reliability issues, can be controlled and/or minimised through proper understanding and control of the flow and deformation behaviour of solder pastes. This study concerns the characterisation of three different Pb-free solder pastes used for ultra-fine pitch assembly applications. We also investigate the paste transfer efficiency through linearly varying stencil apertures sizes, and correlate the paste rheological properties with their transfer efficiency – to provide further understanding of the effects and interactions of stencil printing process variables on the paste transfer efficiency. Three rheological tests, namely: the viscosity, thixotropic and oscillatory amplitude sweep were employed in the characterisation. The paste samples were printed on Cu substrates using stencil printing (with varying aperture cavity dimensions in the range 0.79 mm3–1.62 mm3). A three-level design on two factors experiment, 32, was used to determine significant level of parameters in terms of main effect and interactions. Our results show that the paste type and volume of stencil aperture interact during printing and that pastes with unique rheological properties produced distinctive transfer pattern. The results also show that the printing process variables and their interactions were significant on transfer efficiency of pastes. These results will be of interest to R&D staff involved in formulation of new Pb-free pastes and the design of stencils for ultra-fine pitch assembly applications.

► Pb-free solder paste transfer efficiency depends on paste rheological properties.
► Paste type is more critical than aperture size in influencing paste deposit amount.
► Paste type and aperture size interact with effect more than individuals combined.
► Solder paste formulation should be aperture size range specific.
► Aperture overfill is predominant at very small cavity and impacts miniaturisation.