Reliability of electroless nickel for high temperature applications [flip chip packaging]

Electroless Ni/Au plating is interesting for flip chip due to its bumping cost reduction potential. Electroless Ni bumping provides selective autocatalytic deposition on Al wafer pads. For flip chip soldering, selective solder deposition on Ni bumps is essential. The greatest cost reduction potential is in solder paste stencil printing. The Ni under-bump metallization (UBM)/PbSn eutectic solder system shows excellent reliability. Results of long-term aging at 150°C and reliability tests are presented. AuSn or PbSn eutectic solders are used for high temperature applications. For 150°C-250°C operation, extended Ni-UBM reliability requirements are given. Electroless Ni bumps show an amorphous structure. At high temperatures, relaxation and crystallization effects were noted. During relaxation (>230°C), atoms move over small interatomic distances to a new reduced volume arrangement. Amorphous structure change could not be observed for this effect. At temperatures >320°C, crystallization with Ni and Ni ₃P formation occurs. Crystallization includes further NiP alloy volume reduction, which can lead to Si cratering below Ni if an improper UBM thickness is used. Ni bump geometries are discussed. To study the electroless Ni structure, thermal analysis (DSC, thermomechanical analysis), X-ray diffraction and other measurements were carried out. Simplified Si crack formation models due to structure transformation are shown. Results of Ni-UBM phase formation and growth with AuSn/PbSn solders at 200°C are given. The influence of P on phase growth and impact on reliability at high temperatures are discussed