Why does Rthj-a bigger then Rthj-c

RthJC is the measurement of how well the device can dissipate power through the board and how much the board heats up. RthJA is a measurement of how well the device dissipates power through the environment. Metal and PCB are better conductors of heat, so that dissipating through them will be more efficient.

The actual thermal impedance the device sees is a trickier matter. If you use a heat sink, and or optimal PCB layout techniques, the thermal impedance value can approach RthJC. If not, it will be close to RthJA (either min or max copper, again depending on your specific layout). 

Therefore, it is very difficult to tell, just based on the information you provided, how much the CSD18501Q5A will heat up in the actual application without physically testing it. How much power are you actually dissipating in the device during that 10s on? 

Hi Brett

Thanks your reply,it's very useful to me.

And  the CSD18501Q5A dissipate power during 10s working as follow

Rds(on) at Vgs 7V:2.3mΩ

P=(30A^2)*2.3mΩ=2.07W

How should I calculate estimated value of temperature will rise?

What should I noted when choose the MOSFET for actual application?

Could you offer application note let me know?

Thanks.

Chen, 
The way to calculate rise in temperature is to multiply that power you calculated by the physical Rtheta of your device on the board. This value however, probably has to be measured specific to you board. 

If you believe your device is running too hot and want to play it safe, the CSD18502Q5B is a lower Rdson better performing part. When turned on and running at 30A DC, this part should run cooler. 

This is the most relevant app note on thermals I could find. It should give you a better understanding of what the different parameters mean and how relevant they are  (or are not). 

http://www.ti.com/lit/an/spra953b/spra953b.pdf