Residual stress measurements in electroless plated Ni–P films

Electroless plated Ni–P films are widely used as under-bump-metallurgy in microelectronic packaging, however high tensile stresses in the films often cause reliability concerns. Here, Ni–P films with varying P content were made by changing the pH of the plating solution, and the film stress during the thermal cycling tests (up to 480 °C) was measured in situ by the laser curvature method. Results showed a tensile stress development of ∼85 (∼26) MPa for the Ni-14 wt.% P (Ni-8.7 wt.% P) film at 360 °C. Also, another stress increment of ∼86 MPa was found for the Ni-3 wt.% P films at 420 °C. Subsequent X-ray diffraction and differential scanning calorimetry analyses revealed that the former (at 360 °C) was related to the transformation of amorphous Ni–P phase to crystalline Ni and Ni 3 P, and the latter to the precipitation of Ni 3 P from the Ni crystals.