Electrochemical Migration

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Electrochemical migration (ECM) describes the migration of ions dissolved in water in an electric field from the anode to the cathode after an anode metallization has dissolved. The ions are deposited as a metallic structure with dendrite growth on the cathode. This process is mostly observed on (micro) electronic assemblies. Here, the formation of metallic dendrites between the contacts leads to a reduction in resistance (in the case of thin, fragile dendrites) up to permanent short circuits (in the case of strongly pronounced dendrites) due to metallic bridges.

properties

In addition to the electrical potential difference , the presence of moisture is a driving factor in the ECM. If there is a sufficient moisture film 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 divided 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
  • But there is also bridge formation through interaction with impurities that run from the anode to the cathode

This mechanism affects the reliability and longevity of electronic assemblies. This means that electrochemical migration is often the focus of error-cause analyzes as a possible trigger for malfunctions in the field.

Individual evidence

  1. ↑ Yilin Zhou, Pan Yang, Chengming Yuan, Yujia Huo: Electrochemical Migration Failure of the Copper Trace on Printed Circuit Board Driven by Immersion Silver Finish. (PDF) The Italian Association of Chemical Engineering, 2013, accessed September 28, 2018 .
  2. ↑ a b Helmut Schweigart: ECM as a cause of field failures - Part I: Development and consequences of electrochemical migration . In: Productronic . No. 4 . Hüthig GmbH, Heidelberg 2016, p. 60-63 .
  3. ↑ Helmut Schweigart, Sandra Pilz: Prevention is the best motto - electrochemical migration as the cause of field failures (Part II) . In: Productronic . No. 08-09 . Hüthig GmbH, Heidelberg 2016, p. 18-22 .