Electrochemical migration (ECM) is the dissolution and movement of metal
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s in presence of electric potential, which results in the growth of
dendritic
Dendrite derives from the Greek word "dendron" meaning ( "tree-like"), and may refer to:
Biology
*Dendrite, a branched projection of a neuron
*Dendrite (non-neuronal), branching projections of certain skin cells and immune cells
Physical
* Dendr ...
structures between
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. The process is most commonly observed in
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s where it may significantly decrease the insulation between conductors.
The main factor facilitating the ECM is humidity. In the presence of water, ECM can go very quickly. Usually the process involves several stages: water adsorption, anode metal dissolution, ion accumulation, ion migration to cathode, and dendritic growth. The growth of the dendrite takes fraction of a second, during which time the
resistance between anode and cathode drops almost to zero.
Characteristics
In addition to the electrical potential difference, the presence of moisture is a driving factor in the ECM. If there is a sufficient film of moisture with condensation and even at low electrical voltage, the ECM can form a bridging structure between the contacts after just a few minutes.
In general, the process can be broken down into the following steps:
* Adsorption of water through condensation on the surface between the contacts (often promoted by hygroscopic ionic impurities
)
* Alkalization of the water due to the applied potential difference and thus lowering of the pH value in the water film (initiates the corrosion of (contact) metallizations e.g. silver, copper, tin)
* Dissolution of the anode material (silver, copper, tin etc.)
* Migration of the metal cations to the cathode
* Reduction of the migrated cations and deposition on the cathode with the formation of a metallic dendrite
* Dendrite growth in the opposite direction, towards the anode
* Reduction of the resistance between the contacts up to a permanent short circuit
* There is also bridge formation through interaction with impurities that run from the anode to the cathode
This mechanism impairs the reliability and longevity of electronic assemblies. This means that electrochemical migration is often the focus of failure root cause analyzes as a possible trigger for malfunctions in the field.
See also
*
Whisker (metallurgy)
Metal whiskering is a phenomenon which occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in the vacuum tube era of electronics early in the 20th century in equipment th ...
References
Electrochemistry
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