Qualification of the system electroless nickel bumps and PbSn5 for high temperature applications

Electronic components are applied more and more at higher operation temperatures. For reliability the most difficult problem is to find a suitable metallurgical systems, especially for temperatures above 200°C. Typical difficulties which limit the life time of electronic components include the altered electric behavior due to interdiffusion of solder and metallization materials or the brittleness caused by intermetallic phase formation. So, the choice of the solder joint metallurgy is a key issue for the reliability of the whole assembly. The electroless Ni/Au plating process offers a possibility for cost reduction for advanced packages. The process provides a selective autocatalytic metal deposition directly on the pre-treated aluminum pads of wafers without any costly equipment for electroplating and photo lithography. Combined with stencil printing of solder paste such as PbSn63, PbSn5 or ternary solders it is available for mass production. Due to the nature of the chemical deposition process used at Technical University of Berlin/Fraunhofer IZM the chemical nickel deposits consists of nickel and phosphorous with a phosphorous content of about 9 wt-%. This high phosphorous content leads to an amorphous structure. The absence of grain boundaries can influence the phase growth of intermetallic phases between Ni metallization and solder. Additional the phosphorous seems to inhibit the Ni diffusion into the solder by formation of a phosphorous-rich layer between nickel and solder. The focus of the paper is to summarize the results from the qualification of the metallurgical system PbSn₅ and electroless nickel used as underbump metallization for temperatures around 200°C. The phase formation and the phase growth will be described for different temperatures up to 250°C. Two different Ni bump heights will be used. The influence of the amorphous structure will be shown. The role of the phosphorous for the phase growth will be discussed