For the accuracy of the copy board, it depends on two aspects, one is the accuracy of the software and the other is the accuracy of the original image.

For software accuracy using 32-bit floating point can be said to not have any precision limits, so the main thing depends on the original scan image accuracy, let’s say, if the photo taken with 1 million pixels can be washed 5-inch photo, but if you want to wash it into a 20-inch photo that is not clear at all, the same reason, so for the high precision requirements of the circuit board, to copy out Very high precision PCB diagrams, in the scan to choose a higher DPI.

DPI is the meaning of how many points per inch. That is, the distance between each two points on the scanned image is 1000/DPI, in mil.
If the DPI is 400, then the distance between the two points on the image is 1000/400 = 2.5 mil, which means that the accuracy is 2.5 mil.
This is the most scientific basis, so some people say that the accuracy can reach 1mil or less, that is the premise. In fact, the accuracy of the copied board mainly depends on the original scanning accuracy.

To sum up, when scanning the board set the DPI should be based on the actual board requirements of the precision, if the precision requirements such as cell phone board line spacing below 1mil, then the need to scan the DPI should be set at more than 1000DPI. Scanners on the market today can meet this condition.
The higher the DPI, the clearer the picture, the higher the accuracy, but the disadvantage is that the picture is too large, the hardware requirements are high, so according to the specific circumstances of the specific settings. For the general accuracy of the board generally use 400DPI is very good, cell phone board and so on can be set at 1000DPI or more.