Intentional Metal Whisker growth for Accelerated failure testing

Metal Whiskers (MW) are needle shaped protrusions unpredictably growing from the surfaces of many technologically important metals, including such and ubiquitous base of soldering alloys as tin (Sn) (used extensively in electronic boards). Whiskers lead to current leakage and short circuits in sensitive electronic equipment and electric packages causing significant reliability problems and billion-dollar losses to various industries varying from aerospace, auto industry, military and customer electronics, and medical devices.

Part of our research contributed to understanding the nature and the mechanism of formation of metal whiskers and demonstrated the ability to accelerate its growth for reliability testing.

We reported on the modification of tin (Sn) film surfaces under a laser beam irradiation (resulting in a field-induced nucleation) that triggers surface plasmon polariton (SPP) excitations. Our findings will potentially lead to the development of accelerated failure testing methods of electronic components, which is
currently not possible, or very difficult, due to the random and
unpredictable nature of whiskers formation.

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Whisker mitigation using NiO sublayer

Our experimental work also includes developing whisker prevention using semiconductor layer. Using nickel oxide (NiO) sublayer, to prevent the growth of metal whiskers, was successfully patented (US Pat. No. 10,967,463) recently.

A thin NiO film was applied on a Cu-coated substrate before the deposition of a thicker Sn layer. The growth of Sn whiskers was then followed by optical and scanning electron microscopy and was compared with the whisker growth on a control sample without the NiO sublayer. No whiskers were observed on the sample with the NiO layer even after 12 months, whereas the control sample developed whiskers

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Microscopic Structure of Metal Whiskers

Although MW were discovered almost 70 years ago, there is little agreement on the basic mechanisms that cause for whiskers growth. We studied the nanostructure of metal whiskers using the material characterization tools to reveal the underlying structure was accomplished. Several questions pertaining to the Metal Whiskers’ diameters, diameter stochasticity, and the origins of the well-known striation structures were answered in this project.

Our observations focus on a number of questions, such as:

why MWs’ diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids)

why the diameters remain practically unchanged in the course of MW growth

what is the nature of MW diameter stochasticity

what is the nature and the origin of the well-known striation structure of MW side surfaces.

Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters.

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Laser-based fabrication of microstructures

On-chip inductors often consume large area for moderate inductance (L) values and have relatively low-quality factor (Q). Besides reducing the physical circuitry of on-chip inductors, enhanced L and Q are also required in radio-frequency (RF) applications.

Part of our project involves fabrication of microbump structures on nickel (Ni) films by single-pulse, localized laser irradiation. Conditions for the reproducible formation of such microstructures have been identified in terms of laser-irradiation and film parameters. Ni films deposited by vacuum evaporation were found to be better suited for reproducible laser microstructuring because they lack the typical cracks and voids morphology of sputtered films. An improvement in the inductance and the quality factor of on-chip spiral inductors, incorporating such laser-microstructured ferromagnetic nickel thin films was observed, which demonstrates the potential of such a laser-based method for fabrication or fine tuning of various micro /nano-electric/electronic sensor and other components and systems.

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