Investigations of electrochemical migration on Sn-Sb alloys

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Supervisor:
Dr. Medgyes Bálint Károly
Department of Electronics Technology

Due to even more environmentally conscious mindset of people and principles of environment friendliness of the European Union the hazardous substances in the electronic industry have been replaced. Just the same way solders used in electronic industry have been also replaced. Lead-free solder alloys were created which contained new materials, the behavior of which we barely know.

In my work I am analyzing the phenomenon of electrochemical migration (ECM), the mechanism of short formation on conductor - dielectric - conductor structures leading to conductive dendrite growth. I analyzed this process in the case of antimony (Sn) used in lead-free solder alloys of the electronic industry. In order to accomplish the inspection, Tin-antimony solder alloys containing different concentration of antimony (SnSb 0.1%, SnSb 1%, SnSb 3%, SnSb 5%) were produced in a foundry. I applied this solder as preform on test PCB in the course of a process I formed. After soldering I put the samples through water drop test (WD test), in which I used 3 VDC and 3.5 wt% NaCl solution equal to the average salt concentrate of sea-water. I observed the process by a microscope which was connected to a computer. Furthermore, I also determined MTTF (Mean Time To Failure) relevant for each sample on the basis of voltage of the measurement arrangement. I averaged the MTTF for each solder group. Then I chose some test PCBs based on the images of the optical microscope. Finally the inspection of the high definition (scanning electron microscopy-SEM) images of dendrites and energy-dispersive spectroscopy (EDS) test were carried out.

It was observable that in the case of 0.1%, 1% and 3% antimony concentrated solders showed similar MTTF data (± 3 sec), while in the case of 5% antimony concentrated samples failure came up approximately 40 seconds earlier. I also observed that presence of antimony could be participating only on these dendrites (formed from SnSb 5% solder alloys). Sn concentrate appearing in dendrites refers to the involvement of antimony to electrochemical migration, which led to a faster failure process in this particular case.

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