The Silent Interface Degrader: Managing Intermetallic Growth in Solder and Metal-Based TIMs
When the Thermal Interface Material (TIM) itself is a metal—such as solder, indium foil, or sintered silver—a different class of long-term reliability challenge emerges: the formation of Intermetallic Compounds (IMCs). These are brittle, ordered alloys that form at the interface where two dissimilar metals meet and diffuse into each other over time, especially under heat.
The IMC Reliability Paradox:
- Initial Benefit: A thin, continuous IMC layer is often necessary for forming a good metallurgical bond (e.g., in soldering).
- Long-Term Risk: As diffusion continues, the IMC layer thickens and becomes brittle. It can develop voids (Kirkendall voids) due to unequal diffusion rates of the two metals. This brittle, porous layer increases thermal resistance and becomes the preferential site for crack initiation under thermal mechanical stress, leading to joint failure.
High-Risk TIM Scenarios:
- Solder TIMs (Sn-Ag, In-Ag): Used in die-attach for power modules. The interface between the solder and the component backside metallization (e.g., silver, nickel, gold) is a prime site for IMC growth (e.g., forming Ag3Sn, Ni3Sn4).
- Indium Foils: Used in cryogenics or for soft, conformable joints. Indium readily forms IMCs with many metals (copper, gold, silver), which can drastically alter the mechanical properties of the joint over time.
- Sintered Silver: While pure silver-to-silver sintering minimizes IMC issues, the interfaces with surface finishes (e.g., on a DBC substrate) must be managed.
Managing the IMC Threat:
- Diffusion Barriers: Employ thin, stable metal layers as diffusion barriers. For example, a nickel barrier layer is often used under gold or silver finishes to slow the reaction with tin or indium from the TIM.
- Material Pair Selection: Choose metal pairs with slow, predictable IMC growth kinetics for the intended operating temperature range.
- Aging Studies & Modeling: Conduct high-temperature storage tests on test vehicles and measure the growth of the IMC layer over time. This data feeds into lifetime prediction models (like the Arrhenius equation) to ensure the joint remains reliable for the product’s lifespan.
For metal-based TIMs, thermal performance is inextricably linked to interfacial chemistry. Our expertise extends to recommending compatible material stacks and surface finishes to control IMC growth, ensuring your high-performance metallic joints remain robust from the first power cycle to the last.
