Eric J. Cotts

The microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue

During the solidification of solder joints composed of near-eutectic Sn-Ag-Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints

The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.

Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys

The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 μm). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.

Influence of Sn grain size and orientation on the thermomechanical response and reliability of Pb-free solder joints

The size and orientation of Sn grains in Pb-free, near eutectic SAC solder joints were examined. A clear dependence of the thermomechanical response of these solder joints on Sn grain orientation was observed. Solder balls with Sn grains of particular orientation (a-axis perpendicular to the substrate) were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn along the a-axis is half the value along the c-axis; joints observed to be damaged had maximum mismatch in the coefficient of thermal expansion between solder and substrate at the joint interface, as well as tensile stress modes during the hot part of the cycle

The enthalpy of formation of thin film titanium disilicide

Solid state reactions between thin films of titanium and amorphous silicon (a-Si) were studied using differential scanning calorimetry. Multilayered diffusion couples were heated from just above room temperature to 900 K while monitoring the heat flow from the sample during the reaction. From analysis of our data we have calculated the enthalpy of formation to be ΔHf=−62±5 kJ/mol for the reaction, a-Si+Ti→C49−TiSi2 and have subsequently estimated the heat evolved during the reaction c-Si+Ti→C54−TiSi2 as ΔHf=−56±5 kJ/mol. Based upon this estimate, we find the enthalpy as determined from thin film samples for this system agrees with previous studies which made use of bulk sampling techniques.

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...

Dependence of Sn Grain Morphology of Sn-Ag-Cu Solder on Solidification Temperature

Near-eutectic Sn-Ag-Cu samples were produced in different sizes and geometries, with different solidification temperatures. The Sn grain morphologies of samples were characterized and found to be correlated with the sample solidification temperature; the lower the solidification temperature, the higher the degree of interlacing observed. These Sn grain morphologies were observed to be consistent with a simple model that envisions the nucleus in an undercooled Sn-Ag-Cu liquid to be Sn atoms clustered around a Ag atom in a hexagonal configuration that allows Sn to grow epitaxially on each of its surfaces. At intermediate degrees of undercooling, a mixed Sn grain morphology is observed, with the interlaced portion associated with the region closer to Sn nucleation in these samples.

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.

The effect of Sn grain number and orientation on the shear fatigue life of SnAgCu solder joints

A study of the dependence of room temperature shear fatigue lifetime of SnAgCu solder joints on Sn grain number and orientation was conducted. Both essentially single Sn grain and multi (two or three) Sn grain samples are found in many SnAgCu solder joints in the field, and these Sn grain morphologies were examined here. The mean fatigue lifetime was found to be significantly longer for samples with multiple Sn grains than for samples with single Sn grains. For single grain samples, correlations between Sn grain orientation (with respect to the loading direction) and lifetime were observed, providing insight on early failures in SnAgCu solder joints.

Microstructure and Damage Evolution in Sn-Ag-Cu Solder Joints

The implementation of no-Pb soldering is progressing rapidly, often without immediately obvious problems. However, our current understanding of pertinent materials issues is far from sufficient to prevent surprises, and potentially serious miscalculations. The fundamentally different nature of no-Pb from SnPb solder joints has serious consequences for the design and interpretation of accelerated tests. The present work addresses the behavior of ball grid array (BGA), wafer level chip scale package (WL-CSP) and flip chip assemblies with SnAgCu solder joints in long term thermal cycling. Components were assembled onto either Cu or Ni pads on high-Tg printed circuit boards and cycled between 0 o C and 100 o C. Samples were removed at various stages for cross sectioning and microstructural characterization at various stages. Careful examination of the formation of intermetallic compounds at the pad surfaces, the size and orientation of the Sn grains, the morphology and number of precipitates, and the growth of cracks, revealed the sensitivity of the damage evolution to the solder microstructure. The consequences of the observed pattern of damage evolution are discussed.