The Hygroscopic Hazard: How Moisture Absorption in TIMs Leads to Corrosion and Electrochemical Migration
Beyond thermal and electrical failures, a more clandestine threat exists: electrochemical failure. Certain Thermal Interface Materials can act as a reservoir for moisture and ionic contaminants, creating the perfect conditions for corrosion and conductive filament growth between biased PCB features, leading to intermittent shorts and catastrophic failures after months in the field.
The Failure Pathway:
- Moisture Ingress: Some polymer matrices, especially non-silicone ones like certain polyurethanes or acrylics, are hygroscopic—they absorb water vapor from the air during storage or assembly. Even silicones can trap ambient moisture at interfaces.
- Ionic Mobilization: Many TIMs contain trace ionic residues from their manufacturing process (catalysts, initiators). Absorbed moisture dissolves these ions, creating a weak electrolyte within the TIM or at its interface.
- Electrochemical Attack: When voltage is applied across nearby conductors (e.g., adjacent PCB traces or a component lead to a grounded heatsink), this electrolyte enables electrochemical reactions. This can lead to:
- Corrosion: Dissolution of metal (e.g., copper) from an anode.
- Conductive Anodic Filament (CAF): The dissolved metal ions migrate through the TIM or along the interface and plate out as a metallic dendrite, eventually bridging the gap and causing a short circuit.
Identifying and Preventing the Hazard:
- Material Screening: Require suppliers to provide data on moisture absorption (per ASTM D570) and ionic contamination levels (e.g., via Ion Chromatography testing for chloride, bromide, sodium ions).
- Bias-Humidity-Life Testing: Subject assembled units with the TIM to accelerated life testing (e.g., 85°C/85% RH with bias) and monitor for increased leakage current or electrical shorts.
- Barrier Strategies: In high-reliability applications, consider using TIMs with integrated moisture barriers or apply conformal coatings to adjacent circuitry to isolate them from potential TIM outgassing or moisture release.
A TIM should be chemically benign. Specifying materials with low hygroscopicity and certified low ionic content is a critical step in building electronics that are robust not just to heat, but to the chemical challenges of real-world environments.
