First, I think the problem you are experiencing is "etching" not "etchback". Etchback refers to process which cleans the drilled holes on a PCB prior to plating. Etching is the process which defines the features (pads, circuits, planes, etc.) on the surface of a PCB, on either the innerlayers or outerlayers. Yes, the idea that all features on a panel etch at the same rate is false. The etching process is influenced by many variables; chemistry type, concentration, specific gravity, temperature, thickness of surface copper, conveyor speed, nozzle type, spray pressure, spray impingement, among others can all affect the etch "uniformity". Most etching equipment manufacturers have made significant strides in recent years to reduce the impact of these variables, but good etch uniformity across a panel is still difficult to achieve. From a design viewpoint (and manufacturing), one important issue is feature spacing. When the etching solution makes contact with the copper it begins to dissolve that copper. Once that solution is saturated with dissolved copper, it must be removed and replaced by fresh solution for the etching to continue. In areas of dense circuitry, this "fluid transfer" is harder to achieve. In areas with isolated traces, fluid transfer or turnover takes place quickly; therefor these isolated areas etch faster than the dense areas. The manufacturer must pay close attention to the areas that are densely populated, in order to eliminate shorts. The isolated areas will then sometimes be "over-etched". This is a brief answer to a complex question. If I can be of further help, let me know. Mark Dowding Multek, Irvine [log in to unmask] ______________________________ Reply Separator _________________________________ Subject: FAB: Etchback Author: [log in to unmask] at INTERNET_GATEWAY Date: 7/26/96 8:10 AM We are having a significant problem with quickturn cards coming in overetched, and we're trying to understand some things about the process of etching. One question that has come up is: Is it possible for etching to be greater in one area of the card over another? If so, how is this possible, and what are the design considerations for this? Some here contend that every feature should etch down by the same nominal amount, say .002 inch, whether the feature is an .008 trace or a large ground plane, since the whole PCB is exposed to etchant at the same time. Is there fault in this thinking? Thanks in advance. Jim Ennis Adtran Inc [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************