Developing a dual-layer system for the mitigation of tin whiskers
thesisposted on 05.11.2018, 11:25 by Dan Haspel
There are very few studies that have investigated directly the effect of an oxide film on tin whisker growth, since the cracked oxide theory was proposed by Tu in 1994. The current work has investigated the effect of using an electrochemically formed oxide and both a molybdate conversion coating and a tungstate conversion coating on tin whisker growth from Sn-Cu electrodeposits on Cu, and compared it with that from a native air-formed oxide. X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of coating parameters on the thickness and composition of the oxide film. The XPS studies show that the oxide film formed using either of the conversion coating baths was significantly thicker than that produced from the potassium bicarbonate-potassium carbonate bath. Initial observations suggest that both the tungstate-based conversion coatings and the molybdate-based conversion coatings significantly reduced whisker growth by over 80 %, compared with a native air-formed oxide, and provide improved whisker mitigation compared with the electrochemically formed oxides. The current work has also investigated the potential of using a dual-layer system, comprised of both an electrochemically formed oxide bottom layer and an acrylic conformal coating top layer, for the mitigation of tin whisker growth. The electrochemically formed oxide used in the dual-layer system was produced at 2 V vs. Ag/AgCl while passing a charge of 60 mC cm-2 and the thickness of the conformal coating was aimed to be between ~5 μm to ~6 μm. This thickness was chosen to enable the study of whisker growth on a shorter time scale and to study the effect the electrochemically formed oxide had when used in conjunction. Initial observations showed that the dual-layer system provided improved whisker mitigation compared with both the electrochemically formed oxides and acrylic conformal coatings when used singularly. As part of the self-healing work, nanocapsules filled with the reactive agent were needed to be synthesised and the compatibility of them with different solvents needed to be studied. Capsules filled with the reactive agent were successfully synthesised, however, it was found that the capsules agglomerated and the size of the capsules, in some instances, were too large to be incorporated into a thin conformal coating. Regardless, the capsules were still analysed to check the compatibility with different solvents, to identify a suitable conformal coating mixture that would not dissolve the polymer shell of the capsules. It was observed that the capsules were stable in three out of the five solvents that were analysed, them being isopropanol (IPA), butanone and methylcyclohexane.
Loughborough University Departmental Fund
- Aeronautical, Automotive, Chemical and Materials Engineering