Final finishes can be subdivided into metallic and organic finishes. For the purpose of this article, the focus will be on the metallic finishes using the combinations of nickel (Ni) and/or palladium (Pd) and/or gold (Au). Variations on this theme are used extensively in the electronics market of today. The Ni/Pd/Au mutations are the inevitable result of technical requirement changes coupled with true and perceived acceptance within the industry. One such optimization is the phosphorus contents in the Ni and Pd layers. This subtlety will not be focused on in this article as the impact on the key topics is negligible.
This subgroup of metallic final finishes can also be further divided by their application bias. Traditional ENIG processes are biased towards using a protection layer to ensure extended lifetime reliability by protecting the base copper.
- Electroless nickel/immersion gold (ENIG)—the workhorse
- Electroless nickel/electroless palladium (pure palladium and phosphor containing palladium)/immersion gold (ENEPIG)—the all-purpose solution
The next-generation surface finishes need to be biased towards satisfying lifetime requirements in combination with enhanced technical performance.
- Electroless palladium/autocatalytic gold (EPAG)—fine-line, high-frequency, solder and bonding application
This broad segregation implies the inclusion or exclusion of Ni. This Ni protection layer (4–7 µm) has a physical impact on line and space capability whilst simultaneously having a negative impact on high-frequency applications.
The symbiotic relationship between technology influences and the resultant requirements for the final finish is the driving force for this article. It is also the intention of this article to highlight the superiority of the direct palladium processes in achieving the expected requirements of the future.