Nickel Electroplating

Nickel, gold, and solder electroplating finish the metal features that copper electrochemical deposition builds earlier in the interconnect flow, each adding a specific property copper alone doesn't have: nickel as a diffusion barrier and hardness layer, gold for a solderable, corrosion-resistant, wire-bondable surface, and solder plating to deposit the actual interconnect material that joins a die to its package.

Nickel's barrier function is what prevents copper from migrating straight through a gold finish and ruining it. Copper diffuses readily through many metals at the temperatures a package experiences during assembly and operation; a thin electroplated nickel layer between the copper pad and a gold or solder finish blocks that diffusion path, which is exactly why "electroless nickel / immersion gold" (ENIG) and similar nickel-barrier stacks are standard in package-level finishing rather than plating gold straight onto bare copper.

Solder plating deposits the actual joint material, which makes it functionally different from nickel or gold's protective and finishing roles. Where nickel and gold exist to protect and finish a surface the device doesn't directly rely on electrically beyond conduction, electroplated solder — tin-silver, tin-lead, or similar alloys — becomes the literal mechanical and electrical connection between the die and the next level of packaging once it's reflowed; the composition and thickness uniformity of that plated solder directly determines bump height uniformity and, downstream, how reliably every single bump in an array makes contact during assembly.

Nickel, gold, and solder electroplating complete the journey a copper interconnect takes from "finished on-chip wiring" to "ready to be physically joined into a package," and skipping or getting any one of these layers wrong tends to show up not as a chip-level defect but as an assembly or reliability failure much later in the process.