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 125°C. 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.