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)
Regards
Finn