Hongqun Dong

Role of different factors affecting interdiffusion in Cu(Ga) and Cu(Si) solid solutions

A detailed diffusion study was carried out on Cu(Ga) and Cu(Si) solid solutions in order to assess the role of different factors in the behaviour of the diffusing components. The faster diffusing species in the two systems, interdiffusion, intrinsic and impurity diffusion coefficients, are determined to facilitate the discussion. It was found that Cu was more mobile in the Cu–Si system, whereas Ga was the faster diffusing species in the Cu–Ga system. In both systems, the interdiffusion coefficients increased with increasing amount of solute (e.g. Si or Ga) in the matrix (Cu). Impurity diffusion coefficients for Si and Ga in Cu, found out by extrapolating interdiffusion coefficient data to zero composition of the solute, were both higher than the Cu tracer diffusion coefficient. These observed trends in diffusion behaviour could be rationalized by considering: (i) formation energies and concentration of vacancies, (ii) elastic moduli (indicating bond strengths) of the elements and (iii) the interaction parameters and the related thermodynamic factors. In summary, we have shown here that all the factors introduced in this paper should be considered simultaneously to understand interdiffusion in solid solutions. Otherwise, some of the aspects may look unusual or even impossible to explain.

Microstructural Evolution and Mechanical Properties of Au-20wt.%Sn|Ni Interconnection

In this paper, the microstructural evolution and properties of Au-20wt.%Sn|Ni reaction couples were investigated from two perspectives: (1) by analyzing the microstructure of the as-soldered and aged samples, as well as (2) by measuring the mechanical properties of the intermetallic compounds formed within the reaction zone. The evolution of interfacial reaction products for both the as-soldered and aged interconnections was rationalized by using the experimental results in combination with assessed thermodynamic data from the Au-Ni-Sn system. Moreover, nanoindentation tests were implemented to measure the indentation modulus and hardness of the compounds formed at the interface. It was found that aging had a negligible influence on the elastic modulus and hardness of AuSn and Au5Sn, while the solubility of the third element significantly changed the indentation modulus and hardness of the intermetallic compounds.

Microstructural Evolution and Mechanical Properties in (AuSn)eut-Cu Interconnections

The interfacial reactions between the widely employed solder Au-20wt.%Sn and the common contact metallizations (e.g. Ni, Cu and Pt) are normally complex and not well determined. In order to identify the proper contactor for Au-20wt.%Sn solder, the present study focuses on (1) rationalizing the interfacial reaction mechanisms of Au-20wt.%Sn|Cu as well as (2) measuring the mechanical properties of individual intermetallics formed at the interface. The evolution of interfacial reaction products were rationalized by using the experimental results in combination with the calculated Au-Cu-Sn phase diagram information. It was found that the growth of the AuCu interfacial intermetallic layer was diffusion-controlled. The diffusion path of Au-20wt.%Sn|Cu at 150°C was proposed. The hardness and indentation modulus of the interfacial reaction products were measured using nanoindentation tests. The results revealed a significant influence of the Cu solubility on the mechanical properties of (Au,Cu)Sn and (Au,Cu)5Sn, i.e. their hardness and contact modulus increased with the increase in the amount of Cu. Furthermore, results obtained here for the Au-20wt.%Sn|Cu joints were compared to those from Au-20wt.%Sn|Ni in order to assess the similarities and differences between these widely used interconnection metallization systems.

Effect of Ti on the interfacial reaction between Sn and Cu

The effect of Ti on the solid state reactions between Sn and Cu has been investigated in this work. Based on the experimental results the following statements about the effect of Ti can be made: Firstly, the presence of Ti does not have measurable effect on the thickness of either Cu6Sn5 or Cu3Sn during solid state annealing. However, the unevenness of both Cu6Sn5 and Cu3Sn layers is increased by the addition of Ti. Secondly, there is no marked solubility of Ti to either Cu6Sn5 or Cu3Sn. Rather Ti reacts with Sn to form large Ti2Sn3 platelets inside the solder matrix. These findings were subsequently rationalized with the help of the assessed Cu–Sn–Ti phase diagram. By utilizing this phase diagram information, the absence of any marked effects of Ti on the growth of Cu–Sn intermetallic compound (IMC) formation was rationalized. As there is a very low solubility of Ti to SnAg solder and to Cu–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Cu–Sn IMC layer thickness or quality by Ti alloying.

Analysis of microstructural evolution in SLID-bonding used for hermetic encapsulation of MEMS devices

The need for reliable hermetic sealing of electronic components has arisen along the increasing popularity of silicon based Micro Electro Mechanical Systems (MEMS). In this paper we have analyzed the formation and evolution interconnections made with Solid Liquid Interdiffusion (SLID) bonding by utilizing thermodynamic-kinetic method. The analysis of the phase transformations and consequent formation of stresses in Au-Sn system during bonding as well as the remelting temperatures of the Au-In-Sn interconnections can be carried out with the help of thermodynamic equilibrium diagrams. In addition, by combining qualitative thermodynamic calculations with qualitative kinetic considerations the evolution of interfacial microstructures between Ni contact metallization and the Au-Sn bonding alloy was predicted.

Thermodynamic-Kinetic method on Microstructural Evolutions in Electronics

Thermodynamics and diffusion kinetics determine the microstructure of a given reaction couple between dissimilar materials under specific conditions. The microstructure, in turn, affects strongly the performance of multimaterial assemblies under different stress and environmental states. Hence it is shown here how the combination of thermodynamics and kinetics (namely the thermodynamic-kinetic method) can be used to rationalize the evolution of microstructure between dissimilar materials. The relevant thermodynamic considerations are given first, before moving to the foundations of diffusion kinetic considerations most relevant for the thermodynamic-kinetic method. The method itself is then briefly discussed, before finalizing this chapter with a couple of case studies from the field of electronics.

Improved methods for development of high reliability electronics

To meet the critical reliability requirements of aerospace electronics, reliability design needs to be integrated into the early stage of product design process. In this paper the role of finite element analysis, thermodynamic calculation, and microstructural simulation in reliability design are introduced, followed by the discussion of failure oriented accelerated tests with emphasis on failure mechanisms. The typical failure mechanisms of three single loading tests as well as two combined thermal and mechanical loading tests are covered. Lastly, the challenges in reliability assessment and failure analysis of MEMS devices are discussed through two case studies, MEMS microphone and gyroscope.