Methods of forming semiconductor device assemblies and interconnect structures, and related semiconductor device assemblies and interconnect structures

A method of forming an interconnect structure, comprising:

• contacting a conductive structure with another conductive structure to form a combined structure comprising a copper material directly on a nickel material, a solder-wetting material directly on the copper material, a tin-containing solder material directly on the solder-wetting material, another copper material directly on the tin-containing solder material, and another nickel material directly on the another copper material;
• and
• heating the combined structure to diffuse all of the copper of the copper material and the another copper material into the tin-containing solder material and form a homogeneous distribution of Cu6Sn5 intermetallic directly on and extending completely between the nickel material and the another nickel material.

An interconnect structure comprising a homogeneous distribution of an intermetallic compound comprising copper and tin directly on and extending completely between a nickel material and another nickel material.