The effect of NaCl on water condensation and electrochemical migration

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Supervisor:
Dr. Medgyes Bálint Károly
Department of Electronics Technology

The effect of chloride ion contamination (that is common in electrical tehnology) on water condensation and on electrochemical migration (ECM) was investigated in my thesis. Due to the ECM, electrical short circuits can occur between two conductive surfaces, resulting in capital failures. Its actuality is provided by the propagation of large scale integration of electronic circuits and the increasing utilization of lead free solders.

The examinations can be separated into three main parts.

The point of reference was the examination of water condensation and ECM on bare copper (Cu) and galvanic tin (gSn) patterns prepared on FR4 substrate, using a special Thermal-humidity Bias (THB) test, called dew point test without any contaminant presence. We managed to confirm the previous results, that the mechanism of ECM failures consists of the water condensation (condensation time), dendrite formation (migration time), and is terminated by shorts. When the dew point was reached, small droplets occured on the metal surface first, than on the insulator layer as well. During the nucleation of water droplets, water bridges were formed locally between the conductor lines. These water bridges played the role of the electrolite, enabling the growth of dendrites. Finally, a continous water layer was formed on the surface, causing short circuit.

Than we carried out the dew point test on different solders simulating the presence of ionic contaminators. Compared with the model, in which we used destillated water, the order of the basic processes changed. The reason of this is that the NaCl crystals play role as condensation cores, and as they touch the electrodes, intensive reactions happen, and dendrites can start growing. Thus the presence of NaCl crystals accelerated the condensation intensity and catalisated ECM processes.

Finally the effect of solutions with different NaCl concentrations was investigated on gSn and iAg surface finishes during water drop tests. The results of the measurements in case of gSn surface finishes showed that higher concentration of NaCl increased the mean time to failure (MTTF). Dendrite production and precipitations can be observed in all cases among the investigated concentrations. The precipitations were probably Sn(OH)2 and Sn(OH)4. In case of medium concentration (10mM) the precipitation created a „wall” along the negative pole, so it hindered the dendrite growth. However, shorts were formed later. Energy dispersive spectroscopy (EDS) investigations were carried out in case of iAg surface finishes. The Cu conductor under iAg played dominant role. According to MTTF the following ranking was established: 0,1mM ≤ 1mM < telített NaCl << 10mM << 500mM. In cases of low concentrations significant precipitation can not be seen. In cases of medium and high concentrations probably different types of copper-oxide and Cu(OH)2 were formed.

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