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We will be using a 2 layer board, meaning there are 2 conductive copper layers where we can route electrical signals. You can load a preset for the stackup we use by going to Design → Layer Stack Manager and then File → Load Stackup from Server and navigate to Managed Content → Templates → Layer Stacks and select JLC 2L 1oz 1.6mm. 1.6mm refers to the approximate thickness of the finished board, and 1oz refers to the thickness of the copper layers. Why thickness is measured in ounces is beyond me (these Americans amirite), but 1oz copper represents the thickness of 1oz worth of copper spread evenly over 1 square foot. That equates to about 35um.
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