I'm working on a PCB motor, the board needs to be thicker for mechanical strength (the board is 10"x12"), but I don't want the coil layers getting too far away from the working surface, as it decreases the strength of the magnetic fields.
I have an 8 layer board, with 5 layers of coils on inner layers (my last prototype put coils on the surface, but the plating process increases the thickness of the copper and causes that phase to have a lower resistance). All 8 layers are nearly entirely filled with copper. Before hearing about warping, I made a stackup that used minimal thickness on all dielectrics, as well as minimally thick cores for the first 2 of 3 cores. This gives minimal thickness between the coil layers and the top surface:
(using PCBWay's available materials):
Top 1oz copper
3.5mil dielectric
Inner 1oz copper
8 mil core
Inner 1oz copper
3.5mil dielectric
Inner 1oz copper
8 mil core
Inner 1oz copper
3.5mil dielectric
Inner 1oz copper
80mil core
Inner 1oz copper
3.5mil dielectric
Bottom 1oz copper
For a total thickness ~3.0mm
PCBWay has said of this stackup that "this will cause problems with warping", but I believe they're still offering to build it, so it can't be too bad?
* Am I likely to run into issues with this?
* Is there a better compromise to be had?
* Making the board thicker should help reduce warping, even if it's mainly in a core that's off-center, right?
* Would increasing the last dielectric thickness from 3.5mil to 7.5mil help much?
* What about changing the copper thickness on the bottom? The thicker copper could balance out the larger number of layers that are above the big core