Causes
- Copper imbalance between layers or between the two sides
- Asymmetric dielectric build
- Lamination and reflow thermal stress
- Large unsupported areas and thin cores
Prevention
Balance copper by layer, keep the stackup symmetric around the centre, add thieving where a plane is missing, and agree the panel orientation with the fabricator.
Acceptance
Measure against the IPC limits for the board size and thickness. A board that passes at room temperature can still fail after reflow, so warpage is often checked after assembly simulation.
