Internal stresses on the copper film can cause the board to warp. This is possible even without any heat treatment at room temperature.In processes involving temperature changes, such as reflow soldering, warpage can occur due to the different coefficients of thermal expansion between the copper layer and the substrate.
When individually etched copper-clad boards are stacked together, the difference in copper density of each layer results in a different magnitude of stress on each layer, leading to warpage.
PCBs are often placed in panels to improve PCB assembly efficiency. In turn, the panelling uses rails and legs. After assembly, the legs are removed and the PCB is separated by depanelling. The difference in copper density between the circuit board area and the outrigger area further contributes to warpage.
