Evaluation of process parameters for flip chip stencil printing

As industry moves towards successful implementation of volume production of flip chip assembly, there is a lot of interest in achieving solder paste printing at the flip chip geometry. The advantages of this process are low cost and high throughput. However to meet this challenge there is a key need to evaluate the stencil printing process parameters. These process parameters can be divided into four groups: printer parameters, stencil parameters, environmental parameters, and solder paste parameters. The process parameters evaluated in this paper are the stencil and solder paste parameters. A variety of different stencils with a range of shapes, sizes and pitches were investigated. These dimensions determines the type of solder paste that can be printed. One approach currently adopted by the industry is to reduce the solder alloy particle size to facilitate paste flow through the very small stencil apertures. However, reducing the particle size in this way has been shown to radically affect the paste rheology and consequently the printing behaviour. In this paper we address the need for characterising new paste formulations, and present a procedure for evaluating solder pastes now being developed for flip chip application using the stencil printing process. Rheology is one means of characterising solder pastes. Rheological measurements are needed to correlate the behaviour of solder pastes to their performance in the stencil printing process. Fundamental procedures used in the industry for characterisation in shear conditions include rheograms (flow curves), thixotropic index and single point viscosity measurements. In particular, we show how viscosity changes can be quantified. Typical viscosity measurements were carried out on a controlled stress-strain Reologica StressTech rheometer with a parallel plate geometry. We also report on the characterisation of the creep/recovery properties of solder paste and how it can be correlated to slump. To correlate the measured solder rheological properties to the stencil printing behaviour, several printing trials were carried out. These trials can help in defining the process window for any given flip chip application, in terms of the limiting pitch size, pad size and the minimum gap between pads.