iNEMI Solder Paste Deposition Project report — Optimizing solder paste printing for large and small components

As the density of board design increasing fast, the distance between the adjacent components becomes much smaller. When the miniature chip components and the fine pitch components which require smaller volumes of solder paste are close to the castle-like components, connectors with poor pin co-planarity and CCGAs which require more solder paste, only one single thickness stencil could not satisfy all of the components at the same printing process. At present, the step stencil is the cheapest and most popular solution, but the layout density could not increase more because of the keep-out distances. The objective of this iNEMI Solder Paste Deposition Project is to understand the major factors to step stencil printing quality and where are the limitations. The widely recognized industry standard IPC-7525 has been used as the starting point for an experimental program that explores the effect of varying the keep out distance for 0201 and 0402 chip components, CSP and SOP with pitches down to 0.4mm, and larger components represented by CCGA. Other variables included in the experiments to determine if they had an effect on the sensitivity of paste transfer to keep-out distance included stencil type, step height and solder type. In the first stage of the project the printing to each pad was measured with automated 3D SPI systems and optimum combinations of parameters were also identified by statistical analysis. In the second stage, a confirming printing and an assembly validation were done and optimum combinations were carefully checked up. Results indicate that the minimum keep-out distance for each kind of components applied in this project can be obtained following the optimum design. In this paper the authors will explain the methodology chosen to achieve the project objectives and recommend future work directions.