Identification of Mechanical Properties of Cu6Sn5, Cu3Sn, and Ni3Sn4 Intermetallic Compounds Using Nanoindentation†
Ping-Feng Yang1,2, Yi-Shao Lai 1, Sheng-Rui Jian3, Jiunn Chen1, Rong-Sheng Chen2
1 Central Labs, Advanced Semiconductor Engineering, Inc., Nantze, Kaohsiung, Taiwan, R.O.C.
2 Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan, R.O.C.
3 Department of Materials Science and Engineering, I-Shou University, Kaohsiung, Taiwan, R.O.C.
We report in this paper Young’s moduli and hardness of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds (IMCs) measured by nanoindentation. The samples were prepared by annealing Sn-Cu and Sn-Ni diffusion couples. Indentations performed along the directions lateral and perpendicular to the IMC layers show statistically indistinguishable Young’s moduli and hardness for each of the three IMCs, implying that these polycrystalline IMC aggregates are rather isotropic. Nanomechanical responses of the IMCs were shown to depend greatly on the strain rate during loading while independent of the strain rate during unloading. Multiple pop-in events were observed for Cu6Sn5 during loading at a strain rate lower than about 0.1 s-1 to 0.5 s-1.
Intermetallic compounds (IMCs) developed on the interface between the solder alloy and its bonding pads play a crucial role in the integrity of the solder joint, and hence the reliability of electronic packages. The Cu6Sn5, Cu3Sn, and Ni3Sn4 binary IMCs are major species in a solder system with Cu or Au/Ni/Cu pad metallizations. Ternary or multi-element IMCs such as (Cu1-x,Ni x)6Sn5 and (Ni1-y,Cu y)3Sn4 can also be frequently found because of the adoption of various Pb-free solder alloys as well as the ease of inter-substitution of Cu and Ni due to their similar atomic sizes and lattice structures . It has been identified that fracturing around the interfacial IMC layer is the primary failure induced by drop impacts in particular for Pb-free solder joints [2,3]. Characterizations of mechanical properties and strengths of IMCs in the structural [4-6] or nanomechanical regime [7-14] have therefore attracted great interests.
In this work, we present nanomechanical properties of Cu6Sn5, Cu3Sn, and Ni3Sn4 IMCs obtained by nanoindentation incorporated with the continuous stiffness measurement (CSM) . For nanoindentation of IMCs, difficulties in obtaining consistent and convergent measurement results have been pointed out by Rhee et al. , and data variations were attributed to specimen preparation as well as test methodologies. In most of the cases, Cu-Sn and Ni-Sn IMC samples were obtained by annealing a diffusion couple that placed Sn or a certain solder paste onto a Cu or Ni substrate.
A proper annealing process should be sought in order for the IMC layers to grow uniformly and sufficiently thick to avoid the substrate effect [16,17] during nanoindentation.
In the present work, diffusion couples were Sn-Cu and Sn-Ni. Surfaces of the 10 mm x 10 mm x 5 mm Cu and Ni substrates were rinsed by 30% HNO3 for 5 min before flux was uniformly spread on the surfaces. The substrates were then coated with Sn through hot immersion plating at 250o C for 3 min. A fter reflow with the peak temperature at 240o C for 10 times, the diffusion couples were isothermally annealed at 150o C for 1000 h. Fig. 1 shows cross-sectional images of the interfacial IMC structures after annealing, whose compositions were identified using energy dispersive spectrometry (EDS). Clearly, after the particular annealing condition, either the Cu6Sn5, Cu3Sn, or Ni3Sn4 IMC layer reached a thickness of around 10 µm. Annealed samples were polished along the thickness direction and nanoindentations were
conducted either perpendicular or lateral to the IMC layers.
Figure 1. Interfacial IMC structures after annealing: Cu-Sn (top) and
Nanoindentation was conducted at room temperature using MTS NanoXP® (MTS Corporation, Nano Instruments Innovation Center, Oak Ridge, TN, USA) with force and displacement resolutions of 50 nN and 0.1 nm, respectively. A Berkovich diamond indenter, given