ENEPIG - How to overcome limitations of pure palladium plating solutions

Electroless nickel/electroless palladium/immersion gold (ENEPIG) is an accepted and well established finish for high performance and high reliability applications in a growing market. With the nickel it provides a reliable diffusion barrier and at the same time is a solderable and bondable surface finish allowing a variety of applications. Nevertheless there are some reasons slowing down the growth of the ENEPIG use in the market in particular in the recent 1.5 years. Due to the recently increasing Pd-price, the process cost became of major interest and cost reduction is one of the main targets in the application of the ENEPIG process. At the same time the electrolytes for electroless palladium plating are well known to be sensitive to wrong handling and contaminations from outside. Target of the development which is presented in this paper was therefore to provide a new Pd-plating solution which can be operated with a low Pd-content and at the same time having a high process robustness. In order to increase the process robustness and to improve the overall bath stability the development focused on the understanding of the complex formation and the reactions in the solution. This paper focuses on the comparison of this new type of Pd electrolyte for pure Pd-deposits with a conventional Pd-plating solution. The new formulation allows to operate the process with half of the original Pd content and with that to reduce the full process costs by almost 50%. At the same time the bath can achieve a far extended bath life compared to the conventional processes in the market without forming any precipitation or plate out. Tests were performed, to evaluate the impact of impurities in the process on the plating performance and the properties of the Pd deposit, which showed, that the bath behaves very robust against contamination. The solder joint reliability and bonding performance of the deposits are compared and evaluated over the full bath life as well as the complete process window. The paper also includes a comparison of the pure Pd deposits versus Pd-P-deposits where both layers are known to have different advantages so that the choice for the one or the other deposit was usually driven by the specific customer needs which shall be fulfilled. The new Pd-finish is compared and rated against the existing Pure Pd and Pd-P finishes in regards to crystal structure, IMC formation, reliability and process handling to provide a fair 1:1 overview over the current solutions for electroless Pd plating.