I'm currently designing a PCB with a couple of fast electronic elements (600MHz ARM, DDR3 ram etc.) and I'm quite puzzled which layer should those elements be placed upon.
The product will be closed in not-so-airtight plastic enclosure, but airflow will probably be rather constrained.
One might imagine that air coming in contact with warm IC package should be directed upwards, which would help the heat to be dissipated by convection. If such IC would be placed upside down, on the bottom layer, the heated air would have "nowhere to go", and the element would become considerably hotter.
The question is, does this effect really exist? Even so, is it noticeable enough to considerably affect working temperature of such IC packages?