Characteristics of Cu6Sn5 grains formed on different Cu substrates under temperature gradient

With the trend of miniaturization of advanced packaging technology, the number of Cu
₆
Sn
₅
intermetallic compound (IMC) grains is decreasing and then resulting anisotropic properties in the solder joints. Therefore, how to control the orientation of Cu
₆
Sn
₅
grains in micro solder joints has become a key issue for the reliability of 3D packaging. (001) and (011) single crystal Cu and polycrystalline Cu were specially designed as substrates to characterize the orientation evolution of Cu
₆
Sn
₅
grains reflowing by imposing temperature gradient (TG). There exists jagged interface in Cu/Sn/Cu and (011)Cu/Sn/Cu solder joints at the cold end between Cu
₆
Sn
₅
and solder comparing with the flat interface of (001)Cu/Sn/Cu solder joint. Rapid and directional growth of Cu
₆
Sn
₅
grains at the cold end with preferred orientation is observed on both single Cu substrates by imposing TG, which is completely distinct from the random orientation and scallop-type Cu
₆
Sn
₅
grains without TG. And the Cu
₆
Sn
₅
grains grow along the c-axis lying on the (001)Cu, i.e., the <;0001> directions of Cu
₆
Sn
₅
are parallel to the interface between Cu and IMC. And the $ <; 11\bar 20 > $ directions of Cu
₆
Sn
₅
are parallel to RD, which also is the direction of the temperature gradient, i.e., ${\left\{ {11\bar 20} \right\}_{{\eta }}}$ //{001}
_Cu
. The Cu
₆
Sn
₅
grains in the (011)Cu/Sn/Cu solder joint at the cold end presents an about 30° angle with the Cu substrate. And there exists an intersecting angle about 90° between the two dominant directions. The perpendicular pairs prism-type Cu
₆
Sn
₅
grains grow along one direction on the (001)Cu surface due to the grain boundary migration driven by the large misorientation.