a mic package housing that reduces or completely eliminates alumina substrate cracking due to thermal expansion rate differences between the housing and the alumina by using a low - expansion iron - nickel alloy , such as commercially available carpenter 49 , made to astm specification a - 753 - 78 and mil - n - 14411b . such a housing places compressive stresses on glass - to - metal seals used in hermetic feedthroughs and the glass is fused and the stresses relieved by a special process of annealing . manufacturing yields are improved and very large alumina substrates can be used and are attached by hard soldering .