Growth behavior of Cu6Sn5 in Sn–6.5 Cu solders under DC considering trace Al: In situ observation

• Cu6Sn5 IMCs with I-like, X-like, Y-like and bird-like shapes are in situ observed.
• Cu6Sn5 IMCs are refined and X-like shapes form due to the addition of Al.
• When 10 A/cm2 DC is applied, the growth rate and the size of Cu6Sn5 are increased.
• When 100 A/cm2 DC is applied, they are relatively decreased.

High resolution time-resolved X-ray imaging with synchrotron radiation has been used to in situ observe the growth behavior of Cu6Sn5 intermetallic compounds (IMCs) during solidification in Sn–6.5 Cu and Sn–6.5 Cu–0.2 Al (wt. %) solders under applied direct current (DC) field. The morphological evolution of Cu6Sn5 with I-like, X-like, Y-like and bird-like shapes is directly observed. It is shown that trace levels of Al have a marked effect on the solder microstructures and refining the size of the primary Cu6Sn5. The solidification pathway leading to the refinement is observed in real time using synchrotron microradiography. After adding the trace Al, I-like shapes bifurcate into X-like shapes. Furthermore, when DC field with 10 A/cm2 is applied, both the growth rate and the mean size of Cu6Sn5 are increased but decreased when 100 A/cm2 is applied. Meanwhile, the effect of thermodynamic potential barrier caused by DC field on the growth behavior of Cu6Sn5 is discussed.