In-situ characterization of the microstructure transition of electroplating Cu during self-annealing and its effect on the substrate warpage

The warpage of printed circuit boards (PCBs) has been a serious reliability concern in microelectronic packaging applications because of the flatness requirement of electronic assemblies. Electroplating Cu might play a key role in the PCB warpage. The objective of the present study was to systematically investigate the microstructure/stress evolutions of electroplating Cu during self-annealing and the dependence of the PCB warpage on the Cu microstructure transition. Cu crystallographic evolution, including the grain size and orientation, was in-situ characterized by means of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Additionally, the phase-shifting shadow moiré and cantilever methods were employed to conduct non-destructive, real-time measurements of the PCB warpage. These characterizations showed that the plating current density was an important factor for the Cu self-annealing behavior and that a remarkable crystallographic transition upon Cu self-annealing induced a stress relaxation and PCB warpage without thermal processes. The high-temperature annealing pretreatment accelerated Cu recrystallization and stabilized the Cu microstructure, providing a possible mitigation strategy for the PCB warpage. •Crystallographic transition from nanoscale to microscale upon Cu self-annealing.•Cu transition can be accelerated by increasing current density and temperature.•Cu crystallographic transition induced a remarkable stress relaxation.•Strong dependence of the substrate warpage on the stress relaxation.•High-temperature annealing prevented the PCB warpage induced by Cu self-annealing.