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My PCB manufacturer just informed me that he is facing PCB warpage issue in my simple, single sided, copper poured PCB. He says to avoid this issue he suggests to add a dummy copper patch by keeping 3mm copper clearance from all sides. I am not very experienced with PCB issues. Can somebody tell me what leads to this issue and what are possible solutions to this problem ?

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The problem you mentioned is due to imbalance in thermal expand/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 non-even number of layers.

Solution? Use double sided PCB (or any even number of layers) or instead of solid copper try to use grid copper (this is named "hatch" in PCB CAD program).

Please see https://www.multi-circuit-boards.eu/en/pcb-design-aid/copper-balance.html

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