Metal Information Center

Controlling the Microstructures from the Gold-Tin Reaction

July 21, 2007 – 5:51 am

The microstructures from the reaction between Au and Sn under different conditions were studied. A Sn/Au/Ni sandwich structure (2.5/3.752 µm) was deposited over the Si wafer. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic (wt.%). When the reaction condition was 290°C for 2 min, the microstructure produced was a typical two-phase (Au^sub 5^Sn and AuSn) eutectic microstructure over Ni. In contrast, when the reaction condition was 240°C for 2 min, a AuSn/Au^sub 5^Sn/Ni layered microstructure was produced. In both microstructures, a small amount of Ni was dissolved in Au^sub 5^Sn and AuSn. When the AuSn/Au^sub 5^Sn/Ni layered structure was subjected to aging at 240°C, the AuSn layer gradually exchanged its position with the Au^sub 5^Sn layer and eventually formed an Au^sub 5^Sn/AuSn/Ni three-layer structure in less than 9 h. The driving force for Au^sub 5^Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. The dominant diffusing species for the AuSn and Au^sub 5^Sn has also been identified to be Au and Sn, respectively.
The bonding materials in electronic/optoelectronic packaging serve one or all of the following three major functions: electrical connection, mechanical support, and heat dissipation. According to their melting temperatures, solders for bonding applications in electronic/optoelectronic packages are classified as soft solders and hard solders. Soft solders, such as Sn and In alloys, have low melting temperatures, but exhibit lower yield strengths, which lead to lower creep resistance.1 Solder creep reduces the reliability of optoelectronic packages because the alignment of devices cannot be maintained over time. The growth of Sn or In whiskers in soft solders is also known to cause problems in electronic/optoelectronic packages. The growths of Sn and In whiskers have been observed between laser and submount as a result of solder surface migration and electromigration.2′3 Hard solders, including Au-rich Au-Sn, Au-Si, and Au-Ge alloys, have higher melting temperatures and higher yield strengths. Therefore, they are more resistant to creep. Additionally, whisker growth has never been observed in hard solders. One drawback of the hard solders is their higher melting temperatures.

The Au-rich Au-Sn eutectic solder (Au20Sn, wt.%) has a lower melting temperature (278°C) compared to other hard solders, such as Au3.15Si (363°C) and Aul2Ge (356°C). This property makes Au20Sn useful for bonding devices that are sensitive to high processing temperature but need good creep resistance, such as GaAs4-6 or large Si die on alumina.7 In addition, the high thermal conductivity of Au20Sn (57 W/m°C) makes it particularly useful for bonding higher power devices that demand good heat dissipation.

The Au-Sn binary system is a complicated equilibrium phase diagram.8 There are two eutectic compositions, Au20Sn and Au90Sn. The former is widely used for soldering because of its favorable mechanical properties. The latter is not of much interest because it forms brittle phases. The reaction between Au and Sn has been previously studied.9-13 Four of the compounds in the Au-Sn system, Au5Sn, AuSn, AuSn2, and AuSn4, have been observed in Au-Sn thin-film couples of various compositions.9-12 It was found that Au could diffuse very rapidly into a tin-rich matrix.9-12 It has also been reported that the microstructure of the Au20Sn solder on a Cu substrate was strongly affected by the amount of Cu dissolution during the reflow process.13 The objective of this research is to study the reaction of Au and Sn, which can produce Au20Sn solder joints on a Ni substrate. Specifically, we investigate the possibility of producing Au20Sn solder joints on a Ni substrate with different microstructures by changing the bonding condition.

EXPERIMENTAL

The samples used in this study, illustrated schematically in Fig. 1, were formed by depositing an Sn/Au/Ni three-layer structure onto the Si wafer (300-µm thick) through evaporation. The thicknesses of the Sn, Au, and Ni layers were 2.5 µm, 3.75 µm, and 2.0 µ, respectively. This amount of Au and Sn, if uniformly mixed, will produce an alloy with the Au20Sn composition.

The samples were reacted for 2 min either at 240°C or at 290°C. Then, all the samples were aged at 240°C for up to 72 h. Subsequently, the samples were mounted in epoxy and metallurgically polished in preparation for characterization. The reaction zone for each sample was examined using scanning electron microscopy (SEM). The compositions of each phase were determined using an electron probe microanalyzer (EPMA) operated at 20 keV. In microprobe analysis, the concentration of each element was measured independently, and the total weight percentage of all elements was within 100 ± 1% in each case. The average value from at least three measurements was then reported.

RESULTS

Figure 2a shows the resulting Au20Sn microstructure after reaction at 290°C for 2 min. The two reaction products formed over Ni are (Au,Ni)^sub 5^Sn and (Au,Ni)Sn according to the EPMA measurements. These two compounds have the Au^sub 5^Sn and AuSn crystal structures, respectively, but have small amounts of Ni dissolved in the Au sublattice. From the EPMA measurements (summarized in Table I), the Ni concentration is 1.1 at.% in (Au,Ni)^sub 5^Sn, and 8.6 at.% in (Au1Ni)Sn. As can be seen in Fig. 2a, the reaction products have a two-phase, aggregate-type microstructure. This is quite reasonable because the reaction temperature (290°C) was higher than the Au^sub 5^Sn + AuSn eutectic temperature of 278°C. This aggregate microstructure was partially due to the solidified eutectic microstructure. At least part of the Sn and Au layers became molten during the reaction. It is worth noting that the (Au,Ni)Sn phase is always in direct contact with the Ni layer in all of our 290°C samples.