Lead-free low-cost flip-chip process chain: layout, process, reliability

The ultimate way for miniaturization is the usage of flip-chip components. Instead of using costly ceramic PCBs, the use of standard FR-4 substrates offers a wide field for reducing costs. Together with the usage of stencil printing, this leads to a low-cost production chain. By using standard materials and processes, the flip-chips can be integrated in the existing SMT-production line. As the use of environmentally friendly lead-free solder alloys has until now not been tested in detail, the processing and reliability still need to be analyzed. This paper aims to show an overview of the lead-free low-cost flip-chip process chain, starting with a suitable layout of the pads and solder resist openings of the substrate and the openings of the stencil. The influence of the process parameters for solder paste printing is shown. The difficulties of placement of flip-chips are discussed. Possible reflow soldering methods are laid down and the aspects of using nitrogen-atmosphere are also taken into account. Additionally, this paper explains the great importance of the underfill process, points out the possible ways of inspecting flip-chip connections and takes a look at the long-term reliability of flip-chip solder joints. As an outlook a basic approach for selective flip-chip soldering is given.