During an evaluation of thermally actuated
micro-machined relays, Dr. Alexander Teverovsky (QSS Group, Inc. at NASA
Goddard) observed Gold Whiskers and micro-crystallites on
various gold-plated nickel structures (e.g., contacts, springs, actuators)
internal to the relay package. See Dr. Teverovsky's report "GOLD
WHISKERS: Introducing a New Member to the Family" for a
discussion of his observations and additional images.
During a NASA-sponsored evaluation of commercially available
micro-machined relays, internal examination via Scanning Electron Microscopy (SEM) revealed Gold Whiskers on many of the
gold-plated surfaces inside of the devices. In the manufacture of
the subject micro-relays, several internal elements were fabricated using
a nickel surface micro-machining process, in which high aspect ratio
structures (the thickness of nickel is 20 um)
were manufactured by electroplating nickel onto lithographically defined
copper plating stencils. Gold
was then electroplated over the nickel to a thickness of approximately 2 um.
The tips of the stationary and movable contacts were additionally
plated with hard gold to a thickness of ~ 5 um.
Internal examination revealed Gold Whiskers of
various size, shape, diameter, and density.
Typical whiskers had lengths of several micrometers and diameters of
approximately 200 nm (see Figure 1). Small, micrometer-sized
whiskers with diameters from 100 nm to 400 nm were found in most of the
inspected parts (see Figure 2).
Figure 3 illustrates needle-like
whiskers with diameters of 100 nm to 200 nm and lengths from 2 um
to 4 um. Large,
grass-root-like whiskers, of approximately 10 um
in length and ~1 um in diameter,
which were observed on several parts, are shown in Figure
irregularly shaped, toothpaste-like whiskers of more than 20 um
in length were observed in two out of 22 inspected parts (see Figure
As is conventional with most
kinds of metal whiskers, the reasons and conditions of their growth are NOT
CLEAR. In this specific example microanalysis of the gold
plating found the presence of Rubidium (Rb) in areas with high whisker
density. It is possible that Rb may have played an important role in formation of the
micro-crystallites and whiskers. In
any case, product designers, part engineers, failure analysts, and quality
assurance specialists should be aware of the possibility of formation of
whiskers on gold platings.
Additional References: For background information regarding the
micro-relays discussed above, consult the following publications.
These references DO NOT discuss the observed gold whiskers since those
observations are beyond the scope of the subject papers.
- A. Teverovsky, A. Sharma, "Analysis of Failure Modes and
Mechanisms in Thermally Actuated Micromachined Relays for Harsh
Environments Space Applications", Symposium on Design, Test, Integration,
and Packaging of MEMS and MOEMS, 2003, Napoule, France
- A. Teverovsky, A. Sharma, "Reliability Evaluation of Thermally
Actuated Micromachined Relays for Space Applications", SPIE:
Micromachining and Microfabrication, 2003, San Jose, California
USA, p. 12-21.