When a hermetic package encloses more than one substrate, each substrate should be treated as a separate hybrid microcircuit. Each substrate should include its own density factor (KD) and its own function factor (KF), but only the substrate mounted on or serving as the package header should be allocated a package failure rate (λS). For all other substrates, λS=0. The failure rate for the complete hybrid microcircuit package will be the sum of the failure rates for the individual substrates.