Electrochemical migration

The ever-increasing requirements in terms of packing density, power density and price-value necessitate measures that represent a balance between requirements profile and property profile. This is particularly true for equipment in the telecommunications, automotive and aerospace industries, since these assemblies are frequently exposed to highly variable climatic conditions. They must be highly reliable and highly available, but still remain affordable as a "mass product". This leads to a conflict of objectives, which can increase the probability of failure due to electrochemical migration if the wrong, supposedly inexpensive materials are selected. It is therefore important to be aware of the cause and consequences of this cause of failure.

Particularly in the case of day-night differences or intermittent operation, the temperature fluctuations can cause water to condense on the surfaces. Even if no direct water film forms, plastics (circuit carriers, potting compounds, protective coatings) can also absorb moisture below the dew point. Depending on the material, this occurs at around 70% relative humidity. Metal surfaces in particular can cause dew formation even at lower relative humidity due to their greater thermal inertia.

A frequently described reason for failure of insulating materials in the presence of moisture is the formation of leakage current paths. In the presence of moisture (electrolyte), the surface resistance is reduced by ionic impurities and solution of carbon dioxide. The electrochemical decomposition of the insulation material and the formation of partial discharges lead to an increasing carbonization of the insulation path. If the distance through these conductive paths is sufficiently short, flashover can occur. The so-called tracking index makes a statement about how sensitive an insulation material is to this electrochemical degradation process.

In contrast, electrochemical migration forms a conductive path through metal ions and salts. They are formed either as a direct result of dew formation or by adsorption/absorption on/in insulating materials or on impurities and in defects. In particular, solder residues that have not been cleaned off can represent real moisture buffers that retain moisture even after the surface has dried. Dust and other residues act as crystallization nuclei for condensation.

As a result of chemical (metals go into alkaline solution) or electrochemical degradation (different voltage potential present), water-soluble metal ions are formed which move along the potential gradient and concentration gradient. The potential grad ient is largely determined by the voltage applied and the distance between the potentials. The concentration grad ient, on the other hand, is determined by, for example, the position of the impurity relative to the metal ion source. Re-precipitation occurs with drying and concentration increases with repeated dew formation.

If there is a continuous potential difference, metallic deposits are formed by electrolysis at the cathode (ground), which appear as dendrites, fiber bundles or bands. The formation of such conductive deposits occurs most intensively at points of high field line concentration, i.e. edges and peaks. The field concentration also increases leaching effects and leads to improved ionic conductivity. Electrochemical migration within materials (e.g. along glass fibers) is also referred to as Conductive Anodic Filament Growth (CAF) . A prerequisite for this is the diffusion of moisture into the substrate material.

In particular, the high DC voltages (300-450 VDC and 750-1050 VDC) that have been increasingly used for a few years now, coupled with further advances in miniaturization, increase the risk of electrochemical migration of metal ions. In the simplest case, a fault is detected by the fault currents and the module is switched off. In the worst case, the conductivity increases until the unwanted current path becomes an ignition source.

There are several standardized procedures (e.g. IEC 60068, environmental influences) on how to check the climatic safety of assemblies. However, since there are other geometric (distances, delaminations, field line concentrations) or environmental (aggressive gases, impurities) and mechanical (vibration, thermal expansion, etc) influencing variables in addition to the two main influencing variables "heat" and "humidity", it is hardly possible to make a general statement on the appropriate test methodology. Accelerated aging due to the use of excessive voltage, particularly high humidity or deliberate introduction of contamination is only of limited significance, depending on the application. In contrast, accelerated temperature cycling tests at different degrees of humidity often represent the real operating conditions quite well.