Evaluation of process parameters for flip chip stencil printing

There is a great deal of interest in solder paste printing in flip chip assembly. The advantages of this process are low cost and high throughput. However, to meet this challenge there is a need to evaluate the stencil printing process parameters. These parameters can be divided into printer parameters, stencil parameters, environmental parameters, and solder paste parameters. The parameters evaluated in this paper are the stencil and solder paste parameters. Several different stencils with various shapes, sizes and pitches were investigated; these dimensions determine the type of solder paste that can be printed. In this paper, we address the need for new paste formulation characterisation, and present a procedure for evaluating solder pastes developed for flip chip application via the stencil printing process. Rheological measurements are used to correlate solder paste behaviour to stencil printing process performance. Fundamental procedures used in industry for characterisation in shear conditions include rheograms (flow curves), thixotropic index and single point viscosity measurements. We show how viscosity changes can be quantified. Viscosity measurements were carried out on a controlled stress-strain Reologica StressTech rheometer with parallel plate geometry. We also report on solder paste creep/recovery properties and how they can be correlated to slump. To correlate solder paste rheological properties to stencil printing behaviour, several printing tests were carried out. These tests can help define the process window for any given flip chip application, in terms of limiting pitch size, pad size and minimum pad gap