Investigation of electrochemical migration on immersion silver and lead-free solder alloys in different Na2SO4 solutions

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Supervisor:
Dr. Medgyes Bálint Károly
Department of Electronics Technology

The electrochemical migration occurs on dielectric-conductor-dielectric structures when certain climatic conditions (for example high humidity and temperature) are existed and the structure is under voltage. It could result in the reduction of surface insulation resistance and short circuit formation between the electrodes. Short circuit is formed by the growth of dendrites (metal filaments), with bridging over the insulator layer.

I examined this phenomena, with the use of water drop tests, in sulphuric environment. ’Waterdrops’ were taken from sodium-sulphate solutions with five different concentration (0.1mM/1mM/10mM/500mM/saturated). During the tests voltage charts and real-time optical data were recorded, after that photographs were taken about the printed circuit boards using optical microscopy. Graphs were applied to identificate the value of mean-time-to-failure (MTTF). I used Scanning Electronmicroscopy and Energydispersive Spectroscopy (SEM-EDS) methods to analyse the composition of dendrites, and residues on the boards.

The first type of tests was made on FR4 substrates with copper wiring and immersion silver surface finish. It was found that in the case of solutions between 0.1 and 500 mM with the increase of concentration the MTTF values increased too, however in the case of saturated solution, the short circuit formation mechanism was faster. It can be explained by the formation of white, blue, black residues which have different solubility. According to the SEM-EDS results, the examined boards showed copper dendrites, and the residues were mainly composed by copper and probably by copper hydroxide.

The second type of tests was made on FR4 substrates with copper wiring coated with SAC305 and X solder alloys using stencil printing and reflow methods. The composition of the X solder alloy (wt%) is: Sn=90,95%, Ag=3,8%, Cu=0,7%, Bi=3%, Sb=1,4%, Ni=0,15%. According to the MTTF data it was shown that in the case of 0.1 and 1 mM solutions X alloy has higher migration susceptibility than SAC305 alloy. However, on concentration levels over 10 mM the failures usually happened in the same time on each solder alloy type. Failures were not happened in the case of X covered boards in case of 10 mM concentration. The SEM-EDS analysis proved that the dendrites on the examined boards contained tin, and the residues were composed by tin and probably tin hydroxide. Micro alloys (Bi, Sb, Ni) from X solder were not detected in the composition of dendrites or residues, which means that they probably did not took part in the process of electrochemical migration.

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