Reliability of Electroless Nickel for High Temperature Applications

Abstract

The electroless Ni/Au plating has gained a high interest for jlip chip technologies because of its high potential for cost reduction of the bumping process. The electroless nickel bumping process pmvides a selective autocatalytic metal deposition directly on the aluminum pads of wafers, no costly equipment for sputtering, photo resist imaging and electroplating is required. The electroless nickel have been quanlified as bumps or under bump metallization (UBW for different assemblies such as soldering, at#esive joining or other bonding processes. For jlip chip soldering a selective solder deposition on the nickel bumps is essential. The highest potential for cost reduction offers the stencil printing of sdder pates. The usual solder is the eutectic lead-tin solder (PbSn37/63) with a melting point of I83 OC. This metallurgical system Ni-UBM / PbSn37/63 shows an excellent reliability. Results of the long-duration aging at I50 OC and other reliability tests will be presented. Eutectic gold-tin (AuSn80/20) or PbSn95/5 are available solders for high temperature application One application field for high temperature awembly is the under-the-hood-automotive application. For operation temperatures (15OoC -250 C) extended requirements for the reliability of the Ni-UBM are given. Due to the plating conditions the electroless Ni bumps show an amorphous structure. At higher temperatures some effects like relmation and cvstallization were observed. During the relaxation (above 230 c) the atoms move over small interatomic distances and get a new arrangement with reduced volume. The change of the amorphous structure could not be observed for this effect. At temperatures above 320 OC a crystallization with a formation of Ni and Nig takes place. The crystallization includes a further reduction of volume of the Ni/P alloy. This eflect can lead to silicon cratering below the Ni if an improper UBM thickness is used. Suitable Ni bump geometries are discussed. In order to investigate the structure of electroless nickel thermal anabsis (diflerential scanning calorimehy, thermomechanical analysis), x-ray difiaction measurements and other structural investigations were cam'ed out. Simplified models for the formation of silicon cracks due to the structure transformation are shown. Some results of the phase formation and phase growth of the Ni-LMB with solders (AuSn 80120 and PbSn 95/5) at 200 c are presented. The infruence of the phosphorous on the phase gmwth and the impact on the reliability at higher temperatures are summarized and discussed.