Development of Quality Control (QC) Tools for Solder Pastes used for Flip Chip Assembly based on Oscillatory Tests

As the trend towards further miniaturisation of pocket and hand held consumer electronic products continue, the electronics manufacturing industry is very keen to develop materials and processes which will facilitate the implementation of board level interconnections at the sub 100mum geometry. The development of solder pastes for ultra-fine pitch applications is critical for achieving this process. This requires the formulation and development of better performance paste products: in terms of printability, good adhesion, tack, reflow soldering, and the long-term solder joint reliability. The other challenge for solder paste manufacturers is that they have to develop and manufactures these higher performance pastes at increasingly lower costs. The implementation of an effective materials characterisation process at the paste development stage and the use of quality control tools during the paste production stage can help paste manufacturers to meet these challenges. Rheology is the study of flow and deformation of materials which can be used to investigate the flow behaviour of solder pastes and to predict its performance in the printing process. Rheological instrumentation and measurements is increasingly being used in R & D laboratories for characterising paste materials, monitoring manufacturing process parameters and predicting product performance. The most basic and widely used instrument for predicting solder paste behaviour is the steady shear viscometer. A wide variety of existing devices have been developed for the measurement of steady shear viscosity, many of which are specific to a particular industry or materials. In principle, however, all of these devices share a common goal: to measure the bulk viscosity of a material as it flows in a steady or continuous. Unfortunately the traditional viscometer is ineffective as a material characterisation tool when compared to the modern rheometer. This is because the rheometer can measure the bulk viscosity - and also the visco-elastic behaviour of a material. The aim of this study is to develop quality control (QA) tools for the various aspects of solder pastes process, for example, product formulation, paste mixing and packaging, as well as tools for defect diagnosis on the production line. The oscillatory test was used to study the solid characteristics and cohesiveness of the solder pastes and flux medium. In the oscillatory test the stress is applied in a sinusoidal form to determine the structural changes within the sample and to characterise its visco-elastic behaviour. This oscillatory test case study shows how rheological QA tools can be implemented for solder pastes and flux medium. In addition, it also demonstrates how the QA tool can be utilised for predicting the printing performance of new development pastes by correlating the paste´s rheology to specific paste deposition (printing and/or dispensing) process performance in a real production environment