We know most of Al-substrate are single sided boards,but ,yes,there are always some exceptions. Some Al-substrate boards need to be doubles sided.We made a few of such boards,but designer always do not know what they want or what they designed.They do not know the Al-substrate is in the middle or at the bottom side .
Unlike FR4 PCB,any core-substrate boards have two types.One is sandwitch type as the Al-substrate is at the middle of board,like a sandwitch.Another is blind type.
See above for the Al-substrate boards stackup.All of metal-substrate boards like this too.Many double-sided metal-substrate boards used the first type--sandwitch .The metail is at the middle of the board.
Sandwtich Al board process is Al-susbtrate cutting --->drilling-->anodic oxidation--->stackup-->drilling, the last is the same as usual FR4 double layer board process.
Another type--blind type ,process is Al-susbtrate cutting --->drilling-->anodic oxidation
FR4 cutting --->drilling--->plating--->L2 layer---->stackup with above --->L1 layer--> the last is the same as FR4 double sided board process.
From the stackup,the hole is the main difference.The holes are through from TOP to Bottom side on sanswtch type.we can see holes easily. Blind type,the holes are not through,the bottom side is the Alminum.
So,when you design a double sided Al-substrate board,you can indicate it is Sandwitch or blind type or send us the stackup you want .