Corrosion

Corrosion Research and Corrosion Testing at Fraunhofer IISB

Corrosion and corrosion protection are being researched at Fraunhofer IISB. Various environmental tests, for example, damp heat tests, temperature shock tests, and corrosive gas tests are conducted. Within the investment program of the German Federal Ministry for Education and Research (BMBF) for microelectronic research, which is called the “Research Fab Microelectronics Germany” (FMD), new equipment and systems are procured. These include a new temperature shock chamber with a wider temperature range and a higher maximum temperature; a corrosive gas test chamber with a higher possible temperature, humidity, and gas concentration; a HAST (highly accelerated stress test) system, also referred to as pressure cooker, for accelerated testing with high humidity, high temperature and air pressure, and a salt spray test chamber.

Environmental Testing

The investigation of different environmental influences by means of standardized or customized tests serves the simulation of environmental conditions in various fields of application (indoor, outdoor, off-shore, etc.).

Environmental influences such as humidity, temperature, pressure, harmful gas atmosphere and aggressive ions can be simulated and electronic devices and modules can be aged at an accelerated rate.

Precipitates and dendrites in an insulation gap of a direct copper bonded substrate.

Article - "Elektrochemische Korrosion auf keramischen Substraten für leistungselektronische Module" (german) (iisb.fraunhofer.de)

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)

Increasing Requirements and Applications under Extreme Environmental Conditions

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.

Mechanisms of ECM

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

Kurt Fuchs / Fraunhofer IISB (Camera / Editing)

Corrosion Protection

Concepts for corrosion protection
Cleaning, chemical corrosion inhibition and adhesion promotion
Protective coating: potting and parylene
Coating material characterization
→ analyzing methods

Newton‘s rings used to analyze different thicknesses and defects in parylene coatings.

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.

Parylene Coating

An innovative process for the permanent sealing of electronic assemblies and components is Parylene. In this technique, a transparent polymer film is applied to the substrate in a vacuum by condensation from the gas phase.

The Parylene coating promises resistance to acids and alkalis as well as protection against moisture and dust. Due to the gaseous deposition, the Parylene is able to coat areas and structures which are not or only insufficiently coatable with liquid-based processes. Examples for such areas are sharp edges or tops or narrow and deep gaps. A Parylene coating system is also procured within the investment program FMD of the BMBF.