Babak Arfaei

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.

Recrystallization and Precipitate Coarsening in Pb-Free Solder Joints During Thermomechanical Fatigue

The recrystallization of β-Sn profoundly affects deformation and failure of Sn-Ag-Cu solder joints in thermomechanical fatigue (TMF) testing. The numerous grain boundaries of recrystallized β-Sn enable grain boundary sliding, which is absent in as-solidified solder joints. Fatigue cracks initiate at, and propagate along, recrystallized grain boundaries, eventually leading to intergranular fracture. The recrystallization behavior of Sn-Ag-Cu solder joints was examined in three different TMF conditions for five different ball grid array component designs. Based on the experimental observations, a TMF damage accumulation model is proposed: (1) strain-enhanced coarsening of secondary precipitates of Ag3Sn and Cu6Sn5 starts at joint corners, eventually allowing recrystallization of the Sn grain there as well; (2) coarsening and recrystallization continue to develop into the interior of the joints, while fatigue crack growth lags behind; (3) fatigue cracks finally progress through the recrystallized region. Independent of the TMF condition, the recrystallization appeared to be essentially complete after somewhat less than 50% of the characteristic life, while it took another 50% to 75% of the lifetime for a fatigue crack to propagate through the recrystallized region.

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.

Improving the thermomechanical behavior of lead free solder joints by controlling the microstructure

Effects of solder alloy, volume and pad finishes on various aspects of microstructure and the corresponding thermomechanical properties of SnAgCu solder joints were investigated. Particular attention was focused on the behavior of solder joints with interlaced Sn grain morphologies. Crossed polarizer microscopy and scanning electron microscopy (SEM) were used to characterize Sn grain structures. Precipitate sizes and distributions were measured using backscattered scanning electron microscopy and quantified using image analysis software. Mechanical properties including hardness and indentation creep were measured. Results show that the amount and frequency of interlacing increased as the joint size decreased, as the amount of Ag in the solder increased, and if the joint was reflowed on ENIG substrates. The interlaced structure was harder and more creep resistant compared to the common beach ball morphology. Image analysis results showed this to be related to much higher densities of secondary precipitates in the interlaced regions. A mechanistic understanding of the microstructure is discussed and recommendations are made as to the design of more reliable solder joints.

The dependence of the Sn grain structure of Pb-free solder joints on composition and geometry

The effects of solder composition, under bump metallurgy and solder joint geometry on Sn grain morphology in Pb free solder joints were examined. SnAgCu solder joints were examined for compositions ranging from Sn, near eutectic SnCu, near eutectic SnAg, to near eutectic SnAgCu. The geometries of solder joints were varied: diameters from 10 to 150 microns, and heights between ten and sixty microns were selected. Solidification temperatures of the samples after various thermal histories were monitored by means of differential scanning calorimetry. Sn grain morphologies were examined by optical microscopy and by scanning electron microscopy and electron backscattered diffraction in order to determine the amount of Sn interlacing. Correlations between the composition, or geometry, and the amount of Sn interlacing are reported. The nucleation rates of Sn from the melt in these Pb solder joints at a number of different temperatures were also measured. An expression for the nucleation rate of Sn in a controlled collapse chip connection solder joint as a function of temperature was thus formulated, providing a predictive capability for Sn solidification temperatures. This study has shown that variations in the temperature of solidification of Sn from the SnAgCu melt result in significant differences in the Sn grain morphology.

Dependence of SnAgCu solder joint properties on solder microstructure

It is well known that variations in the microstructure of lead free solders greatly affect their thermomechanical properties. Sn grain size, orientation and number, as well as secondary Ag3Sn and Cu6Sn5 precipitate sizes and numbers, are all seen to influence the mechanical response of solder joints during isothermal and thermal cycling. The solidification temperature of a SnAgCu solder joint dramatically affects its microstructure. Generally, smaller solder balls (e.g. CSP) undercool more, and thus their microstructure and properties are very different than larger solder balls (e.g. BGA). We report results of a study of the effects of solder joint volume, and pad sizes, on the microstructure and thermomechanical properties of solder joints. Solder joint shapes and dimensions spanned the ranges typical of BGA and CSP assemblies. Temperatures of solidification during cool-down were quantified by differential scanning calorimetry. Sn grain structures were characterized by crossed polarizer microscopy and scanning electron microscopy with electron backscattered diffraction. Precipitate sizes and distributions were measured using backscattered scanning electron microscopy. Corresponding properties, including hardness, strength and fatigue resistance were measured before and after aging for various lengths of times at temperatures up to 125ºC. Smaller solder joints on smaller pads were shown to be harder and stronger than larger ones, but to age faster and eventually end up softer and weaker.

Reliability and failure mechanism of solder joints in thermal cycling tests

Previously crack propagation and joint failure in thermal cycling tests were correlated with recrystallization of Sn grains in SnAgCu (SAC) ball grid array (BGA) solder joints. Generally recrystallization of the Sn grains was observed to occur in the high strain region before solder joint failure. In an effort to better understand this failure mechanism in SnAgCu solder joints subjected to mild thermal cycling profiles, and in smaller solder joints that have interlaced Sn grain morphologies, both conventional (-40/125°C, 0/100°C) and mild (20/80°C) accelerated thermal cycling (ATC) tests were performed on various SAC solder joints. Correlations between microstructure and failure mechanism for solder joints on various BGA packages, chip scale packages (CSP), and quad-flat no-lead (QFN) packages were examined. The microstructure of samples was carefully analyzed; selected samples were removed from the chamber after different numbers of cycles in order to investigate the evolution of the SAC solder joint microstructure. Both recrystallization and intergranular crack growth were observed in these SAC solder joints after thermal cycling. Distinct coarsening of precipitates was observed in the recrystallized areas adjacent to cracks, consistent with strain enhanced coarsening. The 20/80°C reliability test results suggested that the failure mechanism of SAC assemblies is similar to that of conventional ATC profiles (0/100°C, -40/125°C) commonly performed in industry. After the same percentage of projected characteristic life, crack lengths were observed to be much smaller for interlaced twinning structures than for larger beach ball structures. This correlation of longer SAC solder joint lifetimes with interlaced Sn grain morphologies suggests that optimized control of Sn grain morphology in SAC solder joints may significantly enhance Pb free solder joint lifetime.

Recrystallization behavior of lead free and lead containing solder in cycling

The present work addresses the effects of thermomechanical history on the recrystallization behavior of lead free and backward compatible solder joints. 30 mil SAC305 balls were reflowed onto BGA pads using either a SAC305 or a eutectic SnPb paste. Systematic variations of the rate of recrystallization with precipitate coarsening as well as with cycling and annealing parameters were characterized and correlated with observations from thermal cycling experiments on lead free BGA assemblies. The introduction of a few percent of Pb has been seen to not only affect the distribution of secondary precipitates but also add minute inclusions of Pb within the individual Sn dendrites. These also act as barriers to dislocation motion but do not coarsen as rapidly as the precipitates, and we find recrystallization in cycling to be strongly delayed.

Controlling the Superior Reliability of Lead Free Assemblies With Short Standoff Height Through Design and Materials Selection

We have recently demonstrated a significantly longer life in accelerated thermal cycling for Land Grid Arrays (LGAs) assembled only with SAC305 solder paste than for the corresponding SAC305 based BGA assemblies. This superior performance was shown to be a direct effect of the solder microstructure. The final Sn solidification temperature strongly affects the initial microstructure of a SnAgCu solder joint, including the Sn grain morphology, and thus the thermomechanical behavior of the joint. Right after reflow, larger BGA joints of SnAgCu alloys, which solidify at higher temperature, reveal either a single β-Sn grain or three large grains with clearly defined boundaries formed by cyclic twinning. The orientations of the highly anisotropic Sn grains are not yet controllable in manufacturing, leading to substantial statistical scatter in the performance of the solder joints. Typical LGA solder joint dimensions, however, tend to facilitate greater undercooling and the formation of an alternative interlaced twinning microstructure.A systematic study was undertaken to identify the parameters that control the interlaced twinning microstructure. Sn grain structures were characterized by crossed polarizer microscopy and electron backscatter diffraction (EBSD). Precipitate sizes and distributions were measured using backscattered scanning electron microscopy and quantified using image analysis software. Systematic effects of solder alloy, dimensions and pad finishes were identified. Recommendations are made as to design and materials selection. The practicality of controlling the desired microstructure, as well as potential disadvantages for certain applications is discussed.Copyright

The effect of nickel microalloying on thermal fatigue reliability and microstructure of SAC105 and SAC205 solders

This study explores the effect of a nickel (Ni) microalloy addition on the thermal fatigue performance and microstructure of two low Ag content, Pb-free solder alloys, Sn-1.0Ag-0.5Cu (SAC105) and Sn-2.0Ag-0.5Cu (SAC205). The alloy performance was evaluated using two different area array component test vehicles, an 84-pin chip scale package (CSP) and a 192-pin fine pitch ball grid array (BGA). The baseline alloy microstructures were characterized using polarized light microscopy and scanning electron microscopy with backscattered electron imaging for phase identification. Thermal fatigue performance was assessed with accelerated thermal cycling (ATC) using four temperature cycling profiles with distinct temperature ranges (ΔT) and temperature extremes. Additionally, each temperature profile used a standard 10 minute dwell time or an extended 60 minute dwell time. A microalloy addition of 0.05% Ni was found to alter the base microstructures of the SAC 105 and SAC205 alloys. Generally, the Ni addition improved the thermal fatigue life but the improvement was not consistent in both alloys, both components, and across all thermal cycling profiles. The most consistent response was with the 84CTBGA component, which showed improved reliability with the Ni addition in all of the thermal cycles.