Thermal evolution, phase composition and fracture toughness of electroless Ni-P, Ni-W-P and Ni-Mo-W-P films for solderable surfaces on copper

Electroless Ni-P films (19, 13 and 6 at. % P), Ni95P4W1.1 and Ni91P5Mo4W0.4 deposits on Cu(Zn) substrates were annealed at temperatures up to 450 degrees C. Fracture toughness was evaluated by observing the level of cracking after indentation with a Vickers indenter. The films are mixtures of amorphous a-NiP, fcc-Ni and Ni3P, and their amounts vary with film composition and annealing temperature. The amounts of these phases, as well as film stress, crystallite sizes, preferred orientation and peak shapes were determined with X-ray diffraction (XRD). In the as-made state, all films are ductile and do not crack upon indentation. We show that embrittlement coincides with the transformation of a-NiP to Ni3P, and it is most severe for alloys with high phosphorus content. For Ni91P5Mo4W0.4, there is no Ni3P precipitation after annealing. Annealing increases the hardness of this deposit, and its fracture toughness remains high. Alloyed W and Mo are mostly dissolved in the fcc-Ni phase. Without Mo and W, the shape of fcc XRD peaks changes upon heating, whereas, in the Ni91P5Mo4W0.4, deposit constant peak shapes indicate a constant dislocation density. Independent of composition, film stress changes gradually with increasing annealing temperature from tensile to compressive due to differential thermal expansion of film and substrate.

Automated summary

Electroless Ni-P films with different phosphorus content, as well as Ni95P4W1.1 and Ni91P5Mo4W0.4 deposits on Cu(Zn) substrates, were annealed at temperatures up to 450 degrees C. The films consisted of amorphous a-NiP, fcc-Ni, and Ni3P phases, with their amounts and properties changing with film composition and annealing temperature. The embrittlement of the films is related to the transformation of a-NiP to Ni3P, and is more severe in alloys with high phosphorus content. The presence of W and Mo in Ni91P5Mo4W0.4 promotes a constant dislocation density and maintains high fracture toughness after annealing.