Don Ball, Process Engineer, Chemcut Corporation, State College, PA, USA
Etchant puddling on the top surface of large panels is an effect that has been known for many years. Because the etchant is generally sprayed on the surface of a panel faster than it can run off it tends to build up or puddle on the center of the panel. The result is a differential etch rate across the top surface of the panel; the rate being the slowest in the center of the panel where the puddle is deepest and gradually increasing towards the edge of the panel where the etchant pool is thinnest due to runoff.
The difference in feature sizes between the center of the panel and the edges, whether line widths on a circuit board or hole diameters in a chemically machined panel can be quite dramatic and have a very detrimental affect on yields.