Authored By:
Britta Schafsteller, Mario Rosin, Gustavo Ramos
Atotech Deutschland GmbH& Co KG
Berlin, Germany
Joe McGurran
Atotech USA, LLC
SC, USA
Summary
The ENIG finish is one of the most mature final finishes accepted in the market for decades. ENIG stands for “electroless nickel – immersion gold” where the gold is applied by either fully immersion reaction or mixed (autocatalytic supported) reaction type electrolytes. With the increasing cost pressure in the PCB industry, due to its higher precious metal cost the gold layer in the ENIG finish gets into focus. In this context lowering the target thickness of the gold layer might be considered as one option to cut down the overall process cost. In order to not sacrifice the final finish performance, it needs to be ensured that the solderability and solder joint reliability is not affected by the reduced gold thickness. This is also addressed by industry standards such as the IPC 4552 for ENIG finishes which specifies thickness the ranges for the final finish.
The purpose of this study was to evaluate the impact of the gold layer thickness on the ENIG layer performance in different perspectives. As part of the study it was investigated, how the difference in gold layer thickness can be reflected in layer analysis after thermal curing of the final finish. Therefore, the ENIG layers of different gold layer thickness were subjected to varying curing times and temperatures and the layers were analyzed in regard to their chemical composition. Additionally, the correlation of gold layer thickness and ENIG layer soldering performance was investigated. For the evaluation of the soldering performance, different types of soldering tests were selected which allow a judgement of the solder wetting behavior of the final finish as well as the reliability of the solder joint.
The results collected in this study show that a lower limit value must be recommended for the gold layer thickness in order to ensure sufficient functionality of the final finish. It can be shown that with decreasing gold layer thickness an increased migration of nickel to the surface after thermal curing can be observed. This can lead to the formation of oxides on the surface which may inhibit the solder wetting and formation of the intermetallic compound with the nickel layer. The finding is in line with the soldering results, that indicate a better solder wettability can be achieved with increasing gold layer thickness.
Conclusions
In this study the properties of the gold layer in an ENIG finish were investigated and correlated to the gold layer thickness. It was shown, that with a low thickness of gold the formation of the layer is overall even but consists of small crystals. Only with increasing thickness a dense layer is formed with homogeneous coverage of the nickel surface. Electrochemical measurements confirm that at lower thickness the gold layer exhibits a porosity which does not fully protects the nickel from the corrosive attack of the measurement electrolyte.
This also enable the migration of nickel and nickel oxides to the surface leading to a lower wettability in the soldering process. This can be correlated to the enrichment of nickel oxides at the ENIG surface as confirmed with EDX measurements after thermal aging. To ensure a sufficient solderability and to mitigate the risk of nickel migration to the surface, the gold layer thickness needs to follow the minimum requirements as defined in the IPC 4552.
Initially Published in the SMTA Proceedings
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