Galvanic Corrosion is an electrochemical corrosion mechanism that occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte — the more reactive metal (anode) preferentially dissolves while the less reactive metal (cathode) is protected, creating a "battery effect" that accelerates corrosion of the anodic metal far beyond its normal corrosion rate, with the classic semiconductor example being gold wire bonds on aluminum bond pads where the aluminum dissolves while the gold remains pristine.
What Is Galvanic Corrosion?
- Definition: Corrosion that occurs when two metals with different electrochemical potentials (positions in the galvanic series) are electrically connected and exposed to a common electrolyte — the potential difference drives current flow that dissolves the more active (anodic) metal and protects the more noble (cathodic) metal.
- Galvanic Series: Metals are ranked by their electrochemical potential — gold (most noble/cathodic) > platinum > silver > copper > nickel > tin > lead > aluminum > zinc > magnesium (most active/anodic). The further apart two metals are in this series, the stronger the galvanic driving force.
- Three Requirements: Galvanic corrosion requires: (1) two dissimilar metals, (2) electrical contact between them, and (3) an electrolyte (moisture film with dissolved ions) bridging both metals — removing any one of these three prevents galvanic corrosion.
- Area Ratio Effect: The corrosion rate depends on the cathode-to-anode area ratio — a large cathode (gold) connected to a small anode (aluminum bond pad) creates intense localized corrosion of the aluminum, which is exactly the geometry in wire bonding.
Why Galvanic Corrosion Matters in Semiconductors
- Gold-Aluminum Interface: The most critical galvanic couple in semiconductor packaging — gold wire bonds on aluminum bond pads create a galvanic cell where aluminum dissolves preferentially, weakening the bond and eventually causing open circuits.
- Copper-Aluminum Contact: Copper redistribution layers or bumps on aluminum pads create another galvanic couple — copper is more noble than aluminum, causing the aluminum to corrode at the interface.
- Accelerated by Moisture: Even thin moisture films (nanometers thick) from humidity provide sufficient electrolyte for galvanic corrosion — the corrosion rate increases dramatically above 60% RH as continuous moisture films form on surfaces.
- Intermetallic Complication: At gold-aluminum interfaces, intermetallic compounds (Au₄Al, Au₅Al₂, AuAl₂) form during thermal aging — these intermetallics have different electrochemical potentials, creating additional galvanic couples that accelerate degradation.
Galvanic Corrosion Prevention
| Strategy | Implementation | Effectiveness |
|---|---|---|
| Avoid dissimilar metals | Use Cu-Cu or Au-Au bonds | Eliminates galvanic couple |
| Barrier layer | Ni/Pd between Au and Al | Blocks direct contact |
| Passivation | SiN/SiO₂ over bond pad | Prevents electrolyte access |
| Moisture exclusion | Hermetic package, dry environment | Removes electrolyte |
| Conformal coating | Parylene, silicone over bonds | Moisture barrier |
| Minimize area ratio | Large anode, small cathode | Reduces corrosion intensity |
Galvanic corrosion is the electrochemical attack that threatens every dissimilar metal junction in semiconductor packages — driven by the potential difference between noble and active metals in the presence of moisture, requiring careful material selection, barrier layers, and moisture exclusion to protect the gold-aluminum, copper-aluminum, and other bimetallic interfaces critical to package electrical connectivity.
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