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Forced convection cooling with direct air impingement on modules mounting LSI hybrid packs is analyzed with one comprehensive thermal mode. A matrix solution for a steady state nodal conductance network is used to predict the temperature rise of the coolant flowing over the module and substrate-to-inlet air temperature differentials. To allow the utilization of a fine nodal mesh to model a hybrid substrate and yet limit the nodal network to a matrix size which can be inexpensively solved by computer, a method is presented to simulate hybrid substrates using equivalent conductances. The problem of determining the forced convection heat transfer coefficient in a module cooling channel where the hermetically sealed hybrid substrates act as protuberances is addressed. Analytical prediction ,of substrate-to-inlet air temperature differences obtained from the thermal model is favorably compared with experimental measurements.