Characterization and Estimation of Solder-on-Pad Process for Fine-Pitch Applications

Popular solder-bumping mechanisms such as electroplating and stencil printing suffer from either high process costs or technical limitations. A low-cost solder-on-pad (SoP) process has been developed to meet the requirements of fine-pitch solder bumping. This paper focuses on the characterization and estimation of the SoP process. To form a solder bump without soldering defects, optimum process conditions should be carefully designed. A model to estimate the bump volume and predict the bump height is suggested. By optimizing the composition of solder paste material called solder-bump-maker, and by adjusting the process conditions, Sn-Ag-Cu solder bumps with different heights are obtained. The experiments and analysis to understand the impact of parameters were based on test vehicles with 80 μm pitch size. Then, the measured heights of solder bumps are compared with the model to see how they fit the estimation. Finally, a similar process has been conducted to test vehicles with pitch sizes of 150 and 40 μm, to confirm the scalability of the SoP process in different pitch sizes.