Richard R. Chromik

Nanoindentation measurements on Cu–Sn and Ag–Sn intermetallics formed in Pb-free solder joints

Nanoindentation testing has been used to measure the hardness and elastic modulus of Ag 3 Sn, Cu 6 Sn 5 , and Cu 3 Sn intermetallics, as well as Sn-Ag-Cu solder and pure Sn and Cu. The intermetallics were fabricated by solid-state annealing of diffusion couples prepared from a substrate (Cu or Ag) and a solder material (Sn or Sn-Ag-Cu solder), providing geometries and length scales as close as possible to a real solder joint. Nanoindentation results for the intermetallics, representing penetration depths of 20-220 nm and loads from 0.7 to 9.5 mN, reveal elastic/plastic deformation without evidence of fracture. Measured hardness values of Cu 6 Sn 5 (6.5 ′ 0.3 GPa) and Cu 3 Sn (6.2 ′ 0.4 GPa) indicate a potential for brittle behavior, while Ag 3 Sn (2.9 ′ 0.2 GPa) appears much softer and ductile. Using a bulk Cu 6 Sn 5 sample, Vickers hardness testing revealed an indentation size effect for this compound, with a hardness of 4.3 GPa measured at a load of 9.8 N. An energy balance model is used to explain the dependence of hardness with load or depth, where the observation of an increasing amount of fracture with applied load is identified as the primary mechanism. This result explains discrepancies between nanoindentation and Vickers results previously reported.

Thermodynamic and kinetic study of solid state reactions in the Cu–Si system

It has been shown that significant changes in the course of solid state reactions can be realized by decreasing length scale, temperature, or by varying parent microstructures. In the case of the formation of Cu3Si by interdiffusion of Cu and Si, previous research has shown that over a large temperature range reaction rates are determined by the rate of grain boundary diffusion of Cu through the growing Cu3Si phase. We have examined the effect of replacing crystalline Si with amorphous Si (a-Si) on these solid state reactions, as well as the effect of decreasing the temperatures and length scales of the reactions. Multilayered thin film diffusion couples of Cu and a-Si were prepared by sputter deposition, with most average composite stoichiometries close to that of the equilibrium phase Cu3Si. Layer thicknesses of the two materials were changed such that the modulation (sum of the thickness of one layer of Cu and a-Si), λ, varied between 5 and 160 nm. X-ray diffraction analysis and transmission electron m...

Microstructural evolution in lead-free solder alloys: Part I. Cast Sn–Ag–Cu eutectic

Coarsening of the ternary eutectic in cast Sn–Ag–Cu lead-free solder alloys was investigated. The process was found to follow r 3 ∝ t kinetics where r is the rod radius of the dispersed phase and t is time. The effective activation energy for the process is 69 ± 5 kJmol -1 . The two types of intermetallic rods, Cu 6 Sn 5 and Ag 3 Sn, in the eutectic structure coarsen at different rates, with each having a different rate-controlling mechanism. The overall coarsening kinetics for the Sn–Ag–Cu ternary eutectic is significantly slower than that found for the Pb-Sn eutectic, which has implications for long-term reliability of Sn–Ag–Cu solder joints.

Mechanical properties of intermetallic compounds in the Au-Sn system

The mechanical properties of intermetallic compounds in the Au–Sn system were investigated by nanoindentation. Measurements of hardness and elastic modulus were obtained for all of the confirmed room-temperature intermetallics in this system as well as the β phase (8 at.% Sn) and AuSn 4 . Overall, it was found that the Au–Sn compounds have lower hardness and stiffness than common Cu–Sn compounds found in solder joints. This finding is in contrast to common knowledge of “Au embrittlement” due to the formation of either AuSn 4 or (Au,Ni)Sn 4 intermetallic compounds. This difference in understanding of mechanical properties of these phases and the resulting joint strength is discussed in terms of reliability and possible failure mechanisms related to interface strength or microstructural effects. Indentation creep measurements performed on Au 5 Sn, Au–Sn eutectic (29 at.% Sn) and AuSn indicate that these alloys are significantly more creep resistant than common soft solders, in keeping with typical observations of actual joint performance.

Solder metallization interdiffusion in microelectronic interconnects

We investigated the growth of intermetallic compounds in Cu/Ni/Au/PbSn solder joints. The substrates that we investigated had been Au plated by one of two different techniques. The Au finish thicknesses ranged from 0.25 to 2.6 /spl mu/m. After solder renew, structural examinations using optical and electron microscopy of cross-sectioned solder joints revealed the growth of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. Solder joints with thicker layers of Au annealed in Ar gas at a temperature of 150/spl deg/C for up to 450 h, displayed an appreciable growth of Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ at the Ni/sub 3/Sn/sub 4//solder interface. Previous investigators correlated growth of a Au-Sn alloy with the degradation of the mechanical properties of the solder joint. The determination of the stoichiometry of the Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ phase provides some understanding of why this phase grew at the Ni/sub 3/Sn/sub 4//solder interface, as Sn, Au and Ni are all readily available at this interface. The growth of this ternary alloy is also consistent with trends observed in the kinetics of formation of solder alloys.

Microstructural evolution in lead-free solder alloys: Part II. Directionally solidified Sn-Ag-Cu, Sn-Cu and Sn-Ag

The tin-silver-copper eutectic is a three-phase eutectic consisting of Ag 3 Sn plates and Cu 6 Sn 5 rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu 6 Sn 5 rods in the binary and ternary eutectics had activation energies of 73 ′ 3 and 82 ′ 4 kJmol - 1 , respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag 3 Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ′ 3 and 38 ′ 3 kJmol - 1 for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag 3 Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu 6 Sn 5 coarsening and Ag 3 Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic.

Evaluation of strain rate sensitivity by constant load nanoindentation

Constant load measurements by nanoindentation offer the potential for measuring strain rate sensitivity from individual features and defects on a submicron scale. However, recent reports reveal a conflicting load dependence (both increasing and decreasing strain rate sensitivity with load) which has yet to be fully explained. In this study, constant load measurements on five materials (Zn, Al, Cu, Ti, and SiO2) were conducted over a range of peak loads, and then compared with both constant strain rate results and conventional values in the literature. The load dependence was found to be caused by the increasing contribution of drift errors throughout the test. A proposed framework, involving higher loads, shorter hold and loading times, and a physically sound fitting method, was found to produce unambiguous results free from load dependencies, with improved correlations to conventional values and reduced standard deviations.

Calorimetric study of the energetics and kinetics of interdiffusion in Cu/Cu6Sn5 thin‐film diffusion couples

Differential scanning calorimetry was used to characterize the energetics and kinetics of interdiffusion in solder/metal diffusion couples. The heat of formation of Cu3Sn from Cu6Sn5 and Cu thin films was found to be ΔHr=−4.3±0.3 kJ/mol, similar to the results of previous measurements on bulk samples. We have seen that the nucleation of Cu3Sn begins at temperatures near 360 K, but that the nucleation and initial growth of Cu3Sn is not a well‐defined Arrhenius process in these diffusion couples. Later portions of our differential scanning calorimetry scans were identified with diffusion‐limited growth of Cu3Sn. From these calorimetry data we have estimated the averaged interdiffusion coefficient, D(cm2/s)=D0 exp (−E/kbT), where kb is Boltzmann’s constant and D0=3.2×10−2 cm2/s and E=0.87 eV/atom.

Third Body Behavior During Dry Sliding of Cold-Sprayed Al-Al2O3 Composites: In Situ Tribometry and Microanalysis

Abstract Reciprocating sliding wear experiments were conducted on cold-sprayed pure aluminum and Al–22.6 wt% Al2O3 coatings using a custom-built in situ tribometer. Using a transparent sapphire counterface for the wear tests, the dynamic behavior of third body material in the contact was optically observed. The presence of Al2O3 particles led to greater stability of the transfer films adhering to the sapphire counterface, as well as greater stability of the friction coefficient and lower wear rates. Ex situ microanalysis of material in the wear tracks and transfer films suggests that the presence of Al2O3 particles promoted strain localization during sliding. This produced more uniform third body microstructures and protected the underlying aluminum matrix from deformation, which slowed the rate of transfer to the counterface.

Crystal structure of Au1-xNixSn4 intermetallic alloys

Abstract Ni coated with a thin layer of Au is commonly used as a metallization in electronics packaging. Such Ni/Au metallizations are alloyed with Sn at temperatures above the Pb–Sn solder liquidus. Thermal treatment of these joints in the solid state, for instance at 150°C, has been shown to result in the formation of Au 1− x Ni x Sn 4 compounds at the solder interface. These compounds are found to be deleterious to joint integrity. In this work, synthesis, microstructural analysis and crystal structure determination of Au 1− x Ni x Sn 4 alloys were performed. The alloys were examined by powder and single crystal X-ray diffraction, metallography, electron microprobe analysis, and thermal analysis. It was found that Ni atoms substitute for Au atoms in the AuSn 4 structure giving compounds with general composition Au 1− x Ni x Sn 4 (with x ≤0.5). The structure of the title compounds is considered isotypical with the AuSn 4 phase (the PdSn 4 structure type), with a symmetry increment from Aba2 to Ccca . Crystal structure refinement from powder data using the Rietveld method was performed for two alloys with x =0.25 and 0.50 yielding Au 0.73 Ni 0.27 Sn 4 (sp. group Ccca , a =6.448 A, b =11.606 A, c =6.441 A) and Au 0.51 Ni 0.49 Sn 4 (sp. group Ccca , a =6.424 A, b =11.522 A, c =6.384 A). A single crystal experiment was performed for compound Au 0.91 Ni 0.09 Sn 4 (sp. group Ccca , a =6.523 A, b =11.676 A, c =6.485 A).