Al/stainless-invar composites with tailored anisotropy for thermal management in light weight electronic packaging

• Property anisotropy can be optimised using an “invar” honeycomb as reinforcement.
• Reaction with liquid Al can be controlled using Cr-rich alloy “stainless-invar”.
• The absence of strain hysteresis during cycling precludes global matrix yielding.
• Low composite CTE results from localised plastic flow around interfacial voids.
• An invar fraction of 38 vol.% brings about performances suitable for targeted application.

Composite plates in which the low CTE phase has the shape of a honeycomb are anticipated to present optimum anisotropy of thermal expansion and thermal conductivity for baseplates in electronic packaging. This design is explored by choosing an invar alloy for the low CTE phase. In order to allow the formation of a passivation layer protecting from reaction with liquid Al during squeeze casting, the honeycomb is made of the Cr-rich alloy commonly called “stainless-invar”. Composite plates containing 20 vol.% and 38 vol.% stainless invar were processed using honeycombs with the same thickness over cell side ratio. Experimental CTE values are significantly lower than the predictions of three different thermo-elastic models. The very limited amplitude of the strain hysteresis precludes the occurrence of global plastic yielding in the matrix. It appears that, owing to the high contiguity of the low CTE phase, the low value of the experimental CTE results from void closing and opening by localised plastic flow. A honeycomb volume fraction of 38% is necessary for bringing the average CTE down to the level suitable for packaging applications. The ratio of transverse thermal conductivity to density then amounts to about half of the performance of the best Al/SiC composites.

Figure optionsDownload as PowerPoint slide