Analysis of the Redeposition of AuSn 4 on Ni/Au Contact Pads when Using SnPbAg, SnAg, and SnAgCu Solders T. LAURILA, 1,2 V. VUORINEN, 1 T. MATTILA, 1 and J.K. KIVILAHTI 1 1.—Laboratory of Electronics Production Technology Helsinki University of Technology, FIN-02015 TKK, Finland. 2.—tomi.laurila@hut.fi Interfacial reactions between SnPbAg, SnAg, and SnAgCu solders and Ni/Au surface finish on printed wiring board and especially the redeposition of AuSn 4 intermetallic compound have been investigated. The following major results were obtained. The first phase to form during soldering in the (SnPbAg)/Ni/Au and the (SnAg)/Ni/Au systems was Ni 3 Sn 4 . During the subsequent solid-state annealing, the redeposition of AuSn 4 as (Au,Ni)Sn 4 occurred in both systems. This was explained with the help of the concept of local equilibrium and the corresponding ternary phase diagrams. It was concluded that the stabilizing effect of Ni on the (Au,Ni)Sn 4 provided the driving force for the redeposition. Contrarily, when the solder alloy contained some Cu, the first intermetallic to form was (Cu,Ni,Au) 6 Sn 5 and no redeposition of AuSn 4 was observed. Thus, a very small addition of Cu to the Sn-rich solder alloys changed the behavior of the interconnection system completely.This behavior was explained thermody- namically by using Cu-Ni-Sn and Au-Cu-Sn ternary phase diagrams. The growth kinetics of the interfacial reaction products in the three systems was observed to be somewhat different. The reasons for the observed differences are also discussed. Key words: AuSn 4 redeposition, intermetallic reactions, lead-free, phase diagrams Journal of ELECTRONIC MATERIALS, Vol. 34, No. 1, 2005 Regular Issue Paper 103 (Received May 25, 2004; accepted August 12, 2004) INTRODUCTION Gold is generally used in electronics as a thin pro- tective surface-finishing layer to ensure the solder- ability. The amount of Au present in the soldering systems is therefore usually quite small. However, the behavior of these small amounts of Au with other metals is theoretically interesting and is of great importance in soldering applications. One of the most important interactions is without doubt the interplay between Ni and Au, since Ni/Au metal- lizations are widely used in electronic devices for well-known reasons. Recently, Mei et al. 1 revealed a problem that is peculiar to the Ni/Au metallization with SnPb solders. They discovered that after pro- longed aging (150°C for 2 weeks), the AuSn 4 inter- metallic compound, which had formed during the soldering in the bulk solder, redeposited at the sol- der/substrate interface. The reconstituted interface was significantly weakened and failed by brittle fracture along the surface between the redeposited AuSn 4 and the Ni 3 Sn 4 layer formed during the reflow. Minor and Morris 2 studied the mechanism of the redeposition of the AuSn 4 intermetallic compound. They used Cu substrates with electrodeposited Ni (7 μm) followed by Au deposition (1.5 μm). After sol- dering (peak temperature about 220°C), the samples were aged at 150°C for various times ranging from 3 h to 504 h. The as-solidified solder interconnections contained dense distributions of small needlelike AuSn 4 particles evenly distributed throughout the bulk solder. The interface between Ni and the bulk solder consisted of a layer of Ni 3 Sn 4 that contained a very small amount of Au. A coarse intermetallic layer developed on top of the Ni 3 Sn 4 during aging. It thickened roughly as t 0.5 , i.e., indicating that the growth was diffusion controlled. The simultane- ous depletion of AuSn 4 needles from the bulk occurred. The redeposited intermetallic compound in