Thermal Humidity Bias test on lead-free solder alloys in case of NaCl contamination

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Dr. Medgyes Bálint Károly
Department of Electronics Technology

Electrochemical migration (ECM) is a type of short formation mechanism, which can be formed in the presence of surface moisture (e.g. condensation) on given conductor – insulator- conductor (e.g. pectinate) structures. Under voltage bias due to the presence of moisture, dissolution of metals begins in the conductor - insulator pattern, which causes the growth of conductive filaments (dendrites) which will result in short circuit formation. During the migration studies, I used NaCl solution as an electrolyte. Concentration of the solution was the same as the average NaCl content of seawater (3,5 wt%). For my method of examination, I chose dew point test using a climate chamber followed by SEM-EDS (scanning electron microscope - with energy disperse spectroscopy) analysis on various lead-free solder alloys (CVP-390, OM338T, Innolot). I have investigated the mechanism of electrochemical migration through the examination of the samples' surface using optical microscopy. According to results of the dew point test, A migration susceptibility order was determined. To increase the reliability of the measurements I made at least 10 tests with each type, to ensure that incidental mistakes have a minimal effect.

After the investigations of the migration susceptibility of solder alloys under DC voltage, I repeated the tests under AC voltage (with different frequency square wave signals). Tests were performed on fewer samples, since the limited time, but I was able to approximately determine MTTF values at each frequency. Analysis of the results showed the behaviour of the solder alloys from the aspect of electrochemical migration, inspected under DC, AC voltage. According to the results MTTF was the shortest in the case of Alpha CVP-390, failures happened after closely the same time by Innolot and OM338T solder alloys.


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