This invention relates to an apparatus, comprising: a plurality of plates in a stack defining at least one process layer and at least one heat exchange layer, each plate having a peripheral edge, the peripheral edge of each plate being welded to the peripheral edge of the next adjacent plate to provide a perimeter seal for the stack, the ratio of the average surface area of each of the adjacent plates to the average penetration of the weld between the adjacent plates being at least about 100 cm2/mm. The stack may be used as the core assembly for a microchannel processor. The microchannel processor may be used for conducting one or more unit operations, including chemical reactions such as SMR reactions.