First, that is when the copper is copper mesh good or good, big board, copper mesh is good, because when the copper will remain warped by external circumstances, the grid will not be restored to maintain the ~ the original formation, the general process of copper left side also aim is to protect the warpage of the board.
IPC standard warpage is less than 0.75%, is qualified products!
How to measure, first put in the marble platform PCB board
Or greater than 5mm thick glass plate
Use plug gauge measuring the size of the largest corner
Then take the diagonal length of the PCB using a measuring tape
Divides two numbers, if more than seven thousandths on substandard
