Are you sure you can get alu boards with 2/4 layers? I thought the vias would short out with the aluminium?
As for shorts: they may be unimportant for many designs for which aluminium substrate is used.
You may have 2-layer boards without vias. A LED PCB may have LEDs on one side, the driver on the other and those are joined by thick wires.
You may have 2-layer boards with vias that short to the aluminium substrate, if they are all the same node. A practical example: a LED PCB may have LEDs on one side, the driver on the other, positive driving voltage going through vias and negative being supplied directly from the flashlight’s metal body to both sides.
(1)This photo is showing a similar configuration. It employs a single layer aluminium PCB, because third-party suppliers were used for both LED PCB and driver PCB, but this is a limitation of what I have at hand, not because technology limits it that way:
This is a stack of three elements:
- A Cree LED PCB, on aluminium;
- A copper disk used to supply negative voltage to the whole stack;
- An AK-47A LED driver, which is coupling to the positive terminal of the battery.
Note that negative wire has just a blob of solder on it, without any regard of shorts to the aluminium substrate or copper disk below it — simply because they can are the same electrical node anyway. The LED PCB and the copper disk act as a heatsink for AK-47A. The only isolated part is the red wire on the other side.
Going to a more general case: as far a I know multilayer aluminium PCBs are actually stacks of thin FE4 boards on bot sides of a thick aluminium board. Not multiple alu boards. And vias through the aluminium are still possible, but that requires covering them with dielectric. I have no slightest idea how that process looks like and how much it costs, but
I imagine they first drill holes and then add dielectric, instead of using boards with dielectric on them.
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(1) Typically the battery positive terminal is easy to couple with the middle of the PCB, while the negative one goes to the body’s tail and the return path is through the body.