Effect of sulphate ions on the electrochemical migration in case of NiAu surface finish

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Supervisor:
Dr. Medgyes Bálint Károly
Department of Electronics Technology

In our ever-changing world, the electronics industry is going through major development. The distance between tracks is constantly getting smaller on printed wiring boards, which are found in practically all devices and machines around us. Due to the traces becoming finer, Electrochemical Migration (ECM) is an error-causing process which is more and more likely to occur. ECM results in the growth of metal dendrites which can cause short-circuits. It is important to get acquainted with this process to prevent it successfully. In everyday life, our devices can be exposed to external effects which lead to humidity accumulating on the working circuit, thereby making migration possible.

In this paper, the migration behaviour of Electroless Nickel Immersion Gold (ENIG, a contact surface coating commonly used in the industry) was examined, in the presence of sodium sulphate as a contaminant. Circuits with and without solder mask were evaluated. Later, the measurements were expanded with sodium chloride solutions.

The measurements without the solder mask resulted in about the same mean time to failure (MTTF) of around 1 minute, for all concentrations. This meant that the failure was ten times quicker than the reference measurements conducted with distilled water.

In the second series of experiments, with solder mask, the longest failure time (slow dendrite growth) was measured for the solution with a concentration of 0.1 mM, while higher concentrations resulted in ten times smaller MTTF values. Compared with the reference measurement, 0.1 mM showed slower failure, whereas values from 1 mM up to the saturated solution resulted in faster failure. Based on the MTTF data, the solder mask slows down the mechanism of dendrite growth which causes short-circuits.

Examinations involving NaCl solutions showed quicker ECM at lower concentrations (0.1 mM, 1 mM), but at 10 mM the process suddenly slows down. At 500 mM, the MTTF value continued to increase, but by adding more contaminant material up to the point of saturation, the mechanism speeds up again (although the mean time does not fall under the values measured at the lowest concentrations). Compared with the Na2SO4 experiments, the 0.1 mM and 1 mM solutions of NaCl cause faster failure times than sodium sulphate, however at higher concentrations this tendency turns around.

The results show that ECM depends not only on the type, but also the concentration of the contaminant. Energy-dispersive X-ray spectroscopy (EDX) tests showed that, in the case of ENIG coating, it is rather the copper (Cu) that takes part in the process of ECM.

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