This question looks confused?But it really is a good question for Metal core PCB (MCPCB),as High Technology and High Value,metal core PCB (MCPCB) is not as popular as FR4 PCB,so lots of PCB designer think double sided PCB is same as double layers PCB,it is right for FR4 PCB,but not right for MCPCB and Ceramic PCB.You may understand the different between Metal core PCB and FR4 PCB if you can understand this question.Here we just talk about the PCB substrate,pls see the stackup as below for more information:

A double sided MCPCB means metal core in the middle, top and bottom each side has one copper layer, between metal core and copper layer there is a dielectric layer. See below stack up for better understanding.

A double layer MCPCB means metal core on the bottom, a 2 layer FR4 PCB on the top, between FR4 PCB and the metal core, there is a dielectric layer. See below stack up for better understanding

If you would like to know more for MCPCB or Ceramic PCB,pls Contact us at any time.