the unit difference between sales and picking

Dear all,

   i have set the master data sale unit is "PC",and the WM issue unit is "BOX",and 1 BOX = 3pc;

but when SO has been created ,the unit is "PC",and the outbound unit is "PC",as soon as i try to create wm TO  ref outbound,the TO unit is "PC" not is "BOX".

I want to do that the TO unit is "BOX", and how to do?

Can you help me?

Thanks