Have you ever wondered exactly what happens to your CAM files once they go to a fabricator? In today's column, I will attempt to explain a few of the standard edits made by a fabricator and what they mean to the end-user. So, what types of edits get done to the artwork files before processing?

One of the first edits done in a typical CAM department prior to fabrication of the PCB is an etch compensation. This is an increase in feature size of the artwork that includes all lines, pads and other features in copper grown to account for the known loss at an etcher. The general rule of thumb is for every half ounce of starting copper, a fabricator will do a half mil of etch compensation to the artwork so that after etch the lines' features are at the original size. For example, a 0.005 line with a 0.005 space on 1 ounce copper start would then have a 0.001 mil etch comp on the artwork so the lines PRIOR to etch would then be 0.006 lines with 0.004 spaces. 

Does imposing an etch compensation ever cause a problem?

Certainly it could. If the etch compensation cuts into the available airgap/space value to the point when you cannot produce the part based upon your process minimums, you may receive a phone call from the fabricator asking to start on a lighter copper weight (and, therefore, have less etch comp imposed so that available air gap or space value is not compromised).

Some of the other edits performed at a CAM stage would be removal of non-functional pads on inners. Some customers do not want this done, as it can take away from the rigidity of the layer and create lamination registration issues all its own. Removal of non-functional pads on the outers would only be applicable if the pads were either undersized or 1:1 with the intended hole size. In many CAD systems this is how a non-plated hole is defined, However, let's say a 0.125 non-plated hole exists in a land area with vias or a large pad on the surface (like a 0.5 pad for a 0.125 NPT hole). Here, we understand there is to be a connection by the screw head to the chassis so only a minimum clip over the hole size is done to the pad or land. 

Some CAD systems have limitations on what pad sizes and thermals sizes can be used and are not always appropriate for the device. An example of this would be very large clearances on internal planes that cut off spoke ties of nearby thermals and create a disconnect. Here, we typically propose to reduce the relief/clearance sizes if adequate distance to adjacent planes can be maintained. The opposite of that approach would be very dense surface-mount devices where the thermal size defined overlaps and creates disconnects or isolations to adjacent thermals. If the thermals can be reduced, we will offer that. In extremely tight devices, we may suggest removing the thermal ties entirely in favor of direct connected vias.

Other edits include things like the addition of bus bars for gold tips or additional metal outside the part edge to break the tent in dry film for edge-plated type features. This is done for the processing of edge-plated type features and is typically transparent to the end-user.