how much continuous rms current and maximum peak current for SKKH56/16e, the data sheet is not clear for me.
And also if my current will be 60A rms , how can I choose the heatsink size for this device.
http://shop.semikron.com/out/media/ds/00376645_0.pdf

 

The data sheet seems to say that you will dissipate about 60 watts for a current of 60 amps RMS. It also says you can dissipate heat through the aluminum base plate. So one solution is to mount the device in an aluminum box using some thermal grease to ensure a good thermal bond between the device and the box as heat sink. Alternatively, and likely better, is to find a heat sink that is physically large enough to hold the device, and mount to that, again using thermal grease. Don't hesitate to call the company and get some advice. You're dealing with enough power to hurt you, so be careful.