Corrosion

Testing methods (performed at Fraunhofer IISB)

• Salt spray (e.g. DIN EN 60068-2-52)
  
• damp heat testing (e.g. DIN EN 60068-2-67)
  
• thermal shock testing and temperature cycling (e.g. DIN EN 60068-2-14)
• Corrosive gas (H₂S, NO₂, Cl₂, SO₂ and mixed gas) (e.g. DIN EN 60068-2-42)
• Highly accelerated stress test (HAST), pressure cooker test (PCT) (e.g. DIN EN 60068-2-67)
• Combined with voltage treatment (power cycling) (e.g. DIN EN 60068-2-67)

Due to steadily increasing requirements in terms of higher packaging densities or miniaturization, the problem of electrochemical corrosion is coming more and more to the fore. Furthermore, the demand for using power electronic modules under extreme environmental conditions is rising.

Examples of such applications are inverters in wind turbines, especially offshore ones, or photovoltaic systems. Power electronics installed in trains, for example, need to withstand fast climate changes when driving through a tunnel. This also applies to automotive and telecommunications industries, which serve a worldwide sales market and must ensure the functionality of the electronic components in different climate zones, with high temperature fluctuations and extreme humidity. Most extreme environmental conditions are to be found in the field of aerospace with extreme climate changes, radiation, and mechanical stress. Under the mentioned conditions, condensation resulting from a temperature drop can lead to moisture in the module, for example.

ECM occurs immediately in electronic packages if there is a gap due to delaminated insulating potting material, metals and metal combinations that tend to corrode, humidity and a sufficient voltage load. Given these conditions, metal ions dissolve and the positive metal ions migrate from the anode to the cathode, where the ions are captured. In this way dendrites grow from the cathode to the anode.

Dendrite growth:

• Potential and humidity between metallic structures is present
• Metal ions dissolve
• Positive metal ions migrate from the anode to the cathode
• Ions are captured at the cathode
• Dendrites grow from the cathode to the anode

As corrosion or rather corrosion resistance is not purely a material property but a system characteristic, the module concept must be considered as a whole. Corrosion behavior and corrosion resistance equally depend on three spheres of influence – the metal, the environment, and the design of the technical system, ranging from the microscopic surface quality and the material combination to the overall concept.

The most obvious measure is to prevent humidity in the module as well as delamination of the coating material. If this cannot be inhibited, it has to be at least delayed. Metal, solder, or other residuals from the production process have to be avoided in or around the insulation gaps. The metals and metal combinations used have to be well chosen, and it is possible to use metallic coatings to prevent corrosion.

The adhesion of the potting has to be long-lasting, at least ensured by an appropriate surface quality of the interface. To achieve this, bonding agents can be used. Polymeric coatings of the metallization (for example polyimide or Parylene) are insulating and can prevent the diffusion of humidity. The whole case of the module can be encapsulated by adhesives and sealed with additional polymer materials to avoid any humidity in the module. Other possibilities are using potting materials with corrosion inhibitors, semipermeable membranes on the interface to the environment, or even hermetic housings with a dry interior.