All of our kitchen carcasses are 18mm, sides and back. Some are dowled and glued, some are  cammed and screwed. Despite what others may say, there is no differnce in the rigidity of the carcass between 'rigid' and 'flat pack' - the former are assembed at the factory, the latter are assembled on site. Rigid are harder to transport but quicker to fit, crewed carcasses are more flexible to fit because they can be modified by the fitter.