Mengapa Kemasan Nitrida Aluminium Kehilangan Kemampuan Penyebaran Panas Setelah Terpapar Kelembapan
After twelve years working with aluminium nitride substrates in power and RF packages, one persistent problem is a gradual rise in measured thermal resistance after devices have been stored or operated in humid environments. The increase is not explained by solder fatigue or heat-sink contact alone. Aluminium nitride reacts with moisture, and the reaction products can raise the thermal boundary resistance at free surfaces and at interfaces even when the bulk conductivity of the ceramic remains high.
Dense AlN is relatively stable, yet residual open porosity or incompletely sintered grain boundaries allow water vapor to reach reactive surfaces. The reaction produces aluminum hydroxides and ammonia; the hydroxide layer is a poor thermal conductor and can also disrupt subsequent metallization adhesion. On powder or low-density material the reaction is rapid. On fully dense substrates the effect is slower and localized, yet still measurable after weeks of exposure above 60 % relative humidity. Because package thermal resistance is the sum of bulk conduction and interface resistances, even a thin reaction layer of a few micrometers can add several tenths of a degree per watt under high heat flux.
In a controlled exposure trial, 0.5 mm AlN substrates with a measured bulk conductivity of 185 W/m·K were metallized and assembled into power modules. Half the modules were stored for 500 hours at 85 °C and 85 % relative humidity; the other half were held at the same temperature in dry nitrogen. After exposure, junction-to-case thermal resistance under a 40 W load rose by 0.18 °C/W on the humid-stored modules while remaining unchanged on the dry-stored modules. Cross-sections showed a discontinuous surface layer approximately 2–4 µm thick on the humid samples. Laser-flash measurements on companion unmetallized blanks confirmed that bulk conductivity was unaltered, isolating the added resistance to the surface reaction products. Modules that had been sintered to higher density (open porosity below 0.2 %) showed roughly half the resistance increase under the same humid exposure.
The practical controls are therefore densification and surface protection. Residual porosity must be low enough that moisture cannot reach grain interiors; a thin, continuous protective coating or metallization that fully covers free surfaces further limits reaction. For aluminium nitride hydrolysis moisture sensitivity, the relevant process variables are sintering density, residual open porosity, and the integrity of any subsequent coating. AlN substrate thermal cycling reliability in humid conditions is confirmed by measuring package thermal resistance before and after accelerated humidity exposure rather than by bulk conductivity alone. When thermal resistance rises after storage or field exposure in humid air, the distinguishing evidence is an increase in interface resistance with no change in bulk conductivity. Achieving near-full density and protecting free surfaces, as demonstrated by the smaller resistance rise on the higher-density substrates in the 85 °C / 85 % RH trial, limits moisture-induced degradation and preserves the heat-spreading performance of aluminium nitride.