Growth and Ripening of (Au,Ni)Sn 4 Phase in Pb-Free and Pb-Containing Solders on Ni/Au Metallization
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K.N. Tu Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095-1595 (Received 5 May 2003; accepted 19 July 2003)
The evolution of an interfacial microstructure between Cu/Ni/Au bond-pad metallization and Pb-free or Pb-containing solder bumps during solid-state aging was investigated. A distinctive difference between the Pb-containing and Pb-free solder bumps was observed. A continuous (Au,Ni)Sn4 intermetallic compound layer was readily detected in the Pb-containing solder bump on the bond-pad. No such layer exists in the Pb-free case; instead a large number of (Au,Ni)Sn4 particles scatter in the bulk of the solder. The different morphologies of the (Au,Ni)Sn4 have been explained on the basis of the contact angle in Young’s equation and of the growth of a flat layer by ripening. A simple kinetic analysis of ripening between a set of monosize spheres and a flat surface is given to describe the layered growth having a time dependence around 0.5.
I. INTRODUCTION
Nickel (Ni)/gold (Au) metallization over a copper (Cu)-conducting trace is commonly used in ball-gridarray packages in the microelectronics industry. The Ni interlayer retards out-diffusion of the underneath Cu.1–3 The topmost Au layer provides surface passivation and promotes solder wetting.4 Numerous works were reported on the interfacial reactions between eutectic Sn–Pb solder and Ni/Au metallization in solid–liquid-state reactions as well as in solid– solid-state aging. Apart from the formation of a Ni3Sn4 intermetallic compound (IMC) at the solder–Ni interface, a homogeneous layer of the (Au,Ni)Sn4 compound on Ni3Sn4 was also observed after aging.5–14 It was reported that the transformation from AuSn4 into (Au,Ni)Sn4 was energy driven, but no explanation was given about the formation of a layer-type (Au,Ni)Sn4 IMC on the Ni3Sn4 IMC.9–11 In the eutectic Sn–Pb systems, solder ball joint strength is significantly degraded by the presence of these two brittle IMC layers after aging, as reported.5–7,14,15 Recently, we observed the absence of a layer-type (Au,Ni)Sn4 IMC in Pb-free solder (Sn–Ag) on Ni/Au metallization after prolonged aging.7,13 Instead, numerous particles of (Au,Ni)Sn4 were found to be dispersed randomly in the bulk of the solder. Many of these particles were close to the solder–Ni interface, and some were sitting on the interface. Interestingly, the solder ball joint strength of Pb-free ball-grid-array packages was not affected by aging even after 1000 h at 150 °C.7,15 2562
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J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 13 Mar 2015
The mechanism of formation of (Au,Ni)Sn4 with different morphologies in Sn–Pb and Pb-free solders is unclear at the moment. It was suggested that the migration of (Au,Ni)Sn4 particles to the solder–Ni3Sn4 interface was hindered by the presence of Ag3Sn particles in the Sn–Ag (Pb-free) system.16 However, a continuous layer of (Au,Ni)Sn4 IMC also was detected in the Sn–Pb–Ag system, which has Pb as well as Ag3Sn particles.5–14,17 On the other hand
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