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Non-Toxic Stabilization for Mixed Reaction Gold
In this paper a novel gold electrolyte with autocatalytic properties is introduced. It is capable of depositing high layer thicknesses and defect-free ENIG deposits.
Technical Paper
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Authored By:
Sandra Nelle, Britta Schafsteller, Robert Spreemann, Dirk Tews
Atotech Deutschland GmbH & Co.KG
Berlin, Germany
Summary
In the field of printed circuit board (PCB) manufacturing, surface finishes play a crucial role, serving both as a protective layer and as a facilitator for various interconnecting techniques. Among the commonly used high-end processes Ni/Au, Ni/Pd/Au, and Pd/Au gold deposition is employed. Different types of gold electrolytes are available in the market, catering to varying target thicknesses. These electrolytes exhibit different plating mechanisms, ranging from fully immersion reaction types to mixed reaction types. Mixed reaction type gold electrolytes employ a combination of immersion and autocatalytic deposition behavior.
The latest generation of mixed reaction gold electrolytes, characterized by a heightened level of autocatalytic reaction, offer significant technical advantages by enabling the deposition of thick gold layers onto nickel and palladium surfaces without compromising the integrity of the nickel layer during plating. However, due to their increased autocatalytic properties, these electrolytes necessitate continuous dosing of stabilizer components, typically based on potassium cyanide, to maintain stability. Therefore, the plating mechanism of the gold electrolyte was thoroughly examined, elucidating the roles of cyanide and the reducing components. Given the associated health risks with handling potassium cyanide, alternative stabilizer components were explored and identified to effectively stabilize the gold in the plating electrolyte.
As a result, in this paper a novel gold electrolyte with autocatalytic properties is introduced. It is capable of depositing high layer thicknesses and defect-free ENIG (Electroless Nickel Immersion Gold) deposits. The stabilizer used in this electrolyte is non-toxic and does not contain free cyanide, ensuring an environmentally friendly approach. Extensive testing was conducted for Ni/Au, Ni/Pd/Au, and Pd/Au deposits, comparing the new electrolyte’s performance results with the current industry-standard processes. The evaluation focused on solder joint reliability, nickel layer corrosion, thickness distribution, and characterization of the gold deposit. An easy, safe, and stable processing, with bath lifetimes up to 20 MTO at a gold concentration of 0.6 g/l could be proven with this new electrolyte.
Conclusions
This paper introduces a novel gold electrolyte with KCN-free stabilization and mixed-reaction behavior. The differences in reaction mechanism between immersion and mixed reaction were explained and a hint to caution for the naming was given. Ni/Au layers in this paper were referred to as ENIG and Ni/Pd/Au as ENEPIG. It has to be kept in mind that for a mixed reaction bathes also the naming ENAG and ENEPAG could be potentially used. As the terms IG/AG in the process name do not necessarily reflect the exact properties of the plating bath, a clear guideline for naming in the industry could be a way in order to better clarify the differences in the reaction mechanisms on first sight.
The first chapter of the paper was dedicated to the process of elaborating a KCN-free alternative for mixed reaction gold bathes and deeper understanding of the reaction mechanism from an electrochemical point of view. A mixed-reaction gold bath of 0,6 g/l gold with nontoxic stabilization and therefore no cyanide handling for replenishment was invented. The so designed gold bath could be applied for various finishes: ENIG, ENEPIG and EPAG.
Next, this paper focused on checking the plating reliability of that new bath in terms of bath stability, ENIG corrosion performance and thickness distribution. A consistent behavior over bath life with thickness distribution of < 5% COV was found. In terms of ENIG Ni corrosion an outstanding result of corrosion rating of 0 at all 480 checked locations could be found.
In the last section reliability in terms of wetting, solder joint and bond performance was checked. Besides fresh bath make ups, also aged bath conditions were used. Further, the produced layers were tempered and/or (multiple) reflowed to examine the performance. Wetting tests unveiled an excellent wetting result of the gold surface crated by the new bath electrolyte. On both, pads and more critical through holes an excellent solder adhesion to the surfaces being joined was found.
Ball shear testing revealed very good and consistent results over bath age and multiple, up to 5 times reflow for ENIG and ENEPIG. Even in harsher cold ball pull testing ENEPIG samples of 100 hours bath age at one and five times reflow exhibited only ductile fracture modes 2. This indicates a very good quality of the solder joint and therefore high reliability. In this chapter, further implications for manufacturers and board designers were given in selecting an appropriate final finish for high reliability applications.
Initially Published in the SMTA Proceedings
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