Lead-free solder bumping process for high temperature automotive application

In the last years low cost bumping techniques such as electroless Nickel UBM followed by solder stencil printing have become accepted and established production processes for a manifold of applications (Passives, RFID, Memory, LCD, ASICS, etc) capable for wafer sizes up to 300 mm. [1] Bumping processes, initially introduced to provide flip-chip capable devices with the main task of satisfying the increasing demand for cost-effective interconnection methods, and to take into account the ongoing need on further chip miniaturization, increasing I/O count and high frequency signal rate, must deal with the additional aspects on process technology and reliability. Environmental and legal requirements on one side, and marketing aspects on the other side, are strongly demanding lead-free bumping and assembly solutions. Additionally, the higher melting point of the most common lead-free solder alloys is an interesting advantage with regard to high-temperature applications. This paper is introducing based on electroless Ni/Au UBM a very cost competitive, lead-free solder stencil printing process for high temperature automotive applications - as one of the most toughest field of operation and qualification. [2] Initial capability studies are presented based on printing behavior of new solder paste materials of different alloys SnAg4Cu0.5 and SnAg3.5 - with solder particle sizes of type 5 and type 6. Focusing on process qualification under production aspects a comparison of the obtained lead-free bumping results with state-of-the-art experimental data based on detailed process experiences with eutectic Sn37Pb63 is presented. Results of shear test, height distribution and yield data down to fine pitch (250 - 225 mum) pad layouts are shown. The reflowed bump microstructure was examined due to intermetallic formation and UBM integrity by cross sectioning, SEM and EDX analysis. Reliability tests on 1st and 2nd level (T-cycling, T-storage and humidity tests) according automotive requirements have been also performed. Finally stencil printing design guidelines for wafer level stencil printing of lead-free solder pastes have been established.