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.

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. IntroductionThe thermomechanical response of near eutectic, SnAgCu solder joints depends upon solder microstructure. Sn is the major component in many Pb free alloys. Sn displays large anisotropies in its mechanical properties, so that the mechanical response of large grained, near eutectic, Sn-AgCu alloys depends upon Sn grain morphology. The distributions of Ag3Sn and Cu6Sn5 precipitates in this Pb free solder also affect its mechanical properties. To understand the mechanical behavior of Sn-Ag-Cu solder joints, the evolution of Sn grain morphologies, Sn grain orientations, and precipitate (Ag3Sn and Cu6Sn5) microstructures must be characterized.Near eutectic compositions (Ag and Cu concentrations near 3.5 and 0.9 weight percent respectively) of SnAgCu reveal β-Sn dendrites surrounded by a network of Ag3Sn and Cu6Sn5. The β-Sn dendrites are generally formed by growth from one of three Sn grains formed in a six-fold, cyclic twinning solidification event. Large differences in the numbers and sizes of the secondary Ag3Sn and Cu6Sn5 precipitates can exist from sample to sample and within given