# Which is more important XY plane thermal conductivity or z direction thermal conductivity?

Im seeking an alternative to thermal grease/ pastes. I was researching graphite pads when I stumbled across the soft PGS line from Panasonic.

I need to choose a suitable thermal interface material to connect the CPU die to a heat sink. I do not wish to use thermal paste ( it is greasy). I was wondering which material would be most suitable.

PGS :Excellent thermal conductivity : 700 to 1950 W/(m·K) ( no data for z direction; I found some sources stating that the z direction was about 15W/m-K but I can not verify this).

“GraphiteTIM(Compressible Type) PGS with thermal conductivity : X-Y direction 400W/m∙K, Z direction (28W/m∙K).

The heat sink will be made of aluminium ( roughly 200-250W/m-k) or copper (roughly 350-386W/m-k).

Which is more important? XY thermal conductivity or Z direction thermal conductivity?

Since the heat-sink has a defined thermal conductivity, is there any advantage in choosing a conductivity greater than the heat sink's value? (Example, two materials A and B with a thermal conductivity of 700 and 1000W/m-k respectively for xy plane; assume both have similar z direction conductivity. Does choosing the 1000W/m-k over the 700W/m-k material provide any benefit to the overall system, considering that the heat sink is limited to 386W/m-k? Thanks.