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 degreesC) was measured in situ by the laser curvature method. Results showed a tensile stress development of similar to85 (similar to26) MPa for the Ni-14 wt.% P (Ni-8.7 wt.% P) film at 360 degreesC. Also, another stress increment of similar to86 MPa was found for the Ni-3 wt.% P films at 420 degreesC. Subsequent X-ray diffraction and differential scanning calorimetry analyses revealed that the former (at 360 degreesC) was related to the transformation of amorphous Ni-P phase to crystalline Ni and Ni3P, and the latter to the precipitation of MY from the Ni crystals.