The problem you mentioned is due to an imbalance in thermal expansion/shrink between layers (in this case copper vs base material, like laminate). When the copper from one side of PCB is completely etched it tends to warp when the copper on the other side cools. It happens when there is the non-even number of layers.
- Use double-sided PCB (or any even number of layers)
- Instead of solid copper, try to use grid copper, which is named “hatch” in the PCB CAD program.
Read More: Automated Optical Inspection (AOI)