I think Sophokles answered your question.
<-- this was posted elsewhere, maybe in this thread IIRC.
do the chips really transfer more heat into the PCB than to the upper side?
Yes, they are designed this way. And you shouldnt put a heatsink on top. Yifu said this will have a negative effect instead.
Can anybody verify this?
On computer mainboards. (NB/SB , MOSFETS) or RAM you can cool with out problem from the top.
The differential temperature in one chip should be less than 1°C ... i can not image that this would have a negativ effect.
The only can explain the negativ effect throuhg wrong/to high mounting force. (so that the chip will break)