Authored By:
Britta Schafsteller
Sandra Nelle, Gustavo Ramos, Dirk Tews, Mario Rosin,
Atotech Deutschland GmbH, Berlin, Germany
Summary
Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) is a well-accepted and established finish for high performance applications. The properties of the coating are determined mainly by the characteristics of the palladium layer and the type of the gold electrolyte used and have been investigated in various studies.
In general, PdP or pure Pd coatings are available, which differ not only in their coating properties but also in their plating behavior and in the properties of the electrolytes used. The subsequent gold layer might be deposited by either an immersion type electrolyte or alternatively a reduction assisted gold bath which also will impact the final layer properties.
Looking back on a long experience of ENEPIG plating with pure Pd as well as with PdP, this paper will focus on the comparison of pure Pd and PdP deposits and the interaction with the gold electrolyte type in use. The initial start reaction of the Pd bath is studied and the difference in the behavior of the PdP and pure Pd electrolyte was compared. Knowing that the crystallinity of the deposited layer is different for a pure Pd deposit compared to a PdP layer, the porosity was determined with the help of electrochemical as well as microscopical techniques to identify and judge the probability of a corrosive attack of the gold electrolyte to the underlying nickel.
To be able to reliably judge the corrosive attack of the gold electrolyte, the corrosion was statistically evaluated by rating the number of corrosion events as well as the depth in the nickel deposit.
When comparing the performance of the finishes, considering the properties of electrolytes and the plating conditions, it becomes obvious that there is no single solution for all requirements. Rather, that the process allows manifold options to define the best conditions to the customer needs.
Conclusions
The main finding of this study is that there is no “right” or “wrong” combination of Pd and Au electrolyte for an ENEPIG finish. Comparing the pure palladium versus the PdP coating in terms of the reliability they perform statistically comparable even though their crystallinity is different. In fact, it was observed that the structure of the palladium layer may impact the type and the intensity of the corrosive attack of the gold electrolyte to the underlying nickel. For the PdP layers a higher tendency for nickel surface corrosion was observed while the corrosion in ENEPIG coatings with pure Pd more often showed penetration (a “spike” type corrosion) into the nickel. Nevertheless, no significant difference in solderability and wire bonding performance was detected. The impact of the gold electrolyte type on the overall corrosion was found to be rather low. Depending if immersion or autocatalytic gold was used, the type of corrosion slightly varied, but the overall amount of corrosion attacks stays comparable.
In fact, the main difference appears in the plating behavior of the two different palladium electrolytes as the PdP electrolyte seems to be more sensitive to a poor controlled process like contamination in the rinse after nickel plating.
In such cases the pure Pd electrolyte showed more robustness against variations in the complete process flow.
Therefore, the conclusion of this paper is, that the decision for a pure Pd or PdP bath and an immersion or autocatalytic gold bath type should not be driven by the technical performance only, as this appears to be very comparable for all the different combinations. Rather, that additional factors like achievable bath lifetime, bath stability, required bath maintenance and the targeted plating thickness should be considered to find a reliable combination to fulfill all requested requirements.
Initially Published in the SMTA Proceedings
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