Microchannel heatsink reliability can be affected by thermal stresses that arise due to temperature gradients between the base and fin and along the fin length. These stresses are combined with the bonding stresses that arise in attaching components at elevated temperatures to the silicon heatsink and then cooling the structure to the cryogenic operating temperatures. These bonding stresses are potentially large because of the differences in the values of the coefficients of thermal expansion in silicon heatsink material, and the attached component materials. The stress results shown are for a 17:1 aspect ratio heatsink cooled in liquid nitrogen. The temperature gradients are a result of a surface heat flux of 1.3 kW/cm2, approximating the heat dissipation of an rf power chip. The chip is connected to an aluminum nitride substrate, then the chip and substrate module are attached to the heatsink at a bonding temperature of 600 degree(s)K, as for a gold-tin eutectic bond. The stresses are shown to be within the allowables of the materials involved.