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

• Published in: Surface & coatings technology 2019-04, Vol.364, p.383-391
• Main Authors: Yang, C.H., Yang, S.P., Huang, B.C., Lee, C.Y., Liu, H.C., Ho, C.E.
• Format: Article
• Language: English
• Subjects: Annealing; Circuit boards; Circuit reliability; Crystallographic transition; Crystallography; Dependence; Destructive testing; Electron backscatter diffraction; Electronic assemblies; Electronic packaging; Electroplating; Electroplating Cu; High temperature; Internal stress; Microstructure; PCB warpage; Pretreatment; Printed circuits; Recrystallization; Self-annealing; Stress relaxation; Substrates; Warpage; X-ray diffraction
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2019.02.079

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.