In-Situ Stress Determination of Electroless Cu on PCB-Relevant Substrates

Internal stress evolution of electroless Cu was investigated on Ajinomoto Build up films (ABF), Polyimide (PI) and on Ni-Fe substrates ex-situ via X-ray diffraction (XRD) and in-situ via substrate curvature. The applicability of ABF -and PI-foils for dual-leg measurements was evaluated. The substrate material influences the Cu-film stress during deposition, during relaxation at room temperature and after annealing for Ni containing and Ni-free electroless Cu-baths. For Ni containing baths, the amount of co-deposited Ni dominates the internal Cu-film stress during deposition. The substrate type has no impact in this case. During relaxation at room temperature, the progression of stress is substantially affected by the substrate material. The Cu-film stress after annealing is governed by the type of substrate and does not depend on the electrolyte or the incorporated Ni-content of the Cu film. For Ni-free Cu-baths, the stress depends on the substrate type during deposition as well as during relaxation at room temperature. Due to the large impact of the polymer substrate on the Cu-film stress during relaxation, no correlation was found between the stress state directly after plating dominated by the type of Cu-electrolyte and the stress state after relaxation. On the contrary, the Cu-film stress on metal substrates is generally shifted by about 50 MPa towards tensile stress during relaxation. It is proposed that the Cu-film stress and crystal structure after relaxation and annealing is governed by strain hardening of the polymer substrate material at the interface.