The following questions are sometimes posted on the forum or to our support team:
- What are "Drill Backoff" errors in the Design Rules Check and can they be ignored?
- Why does the PDF print have some holes filled?
- Why can't I produce a negative PDF image?
Before answering these, if you require a positive or negative PDF as artwork for exposing and processing your own PCB, then please see the following article which discusses using the Windows PDF printer option to achieve what is required.
Photo-plotters and history.
An area that has changed dramatically is the manufacturing processes used to produce the PCB. Producing the photographic film used to expose the photosensitive layers is still done by photo-plotters but the later generations are very different from the early versions. A web search will reveal the history and current models and methods used.
The differences between the generation of these plotters leads to the questions that are asked about "drill backoff errors". Just to clarify the situation if you are supplying your Gerber files to a manufacturer, these errors can be ignored as also can any dangling track error that are within a pad area; the reason is that the manufacture will reprocess your Gerber files to fill all the holes (to prevent holes being etched) as they etch the copper before the drilling stage.
So why do we have a "drill backoff" check?
Some users with advanced in-house processes still have versions of the photo-plotters which 'draw' or plot each line and shape by optical means. The line width and shape are controlled by the optics and aperture and the path determines the shape.
The process still does the copper etch before the drilling stage, but this generation of plotter will reproduce any tracks as drawn by the designer into the pad 'hole' area. The consequence of this is that the high speed small diameter drills will break if they meet a target hole with a partial copper content. Drill Backoff is a process built into DesignSpark PCB to remove and clear the pad hole of tracks.
Let's now explore the Drill Backoff error and how it is defined.
Drill Backoff Errors
The 'check' is selectable in the Design Rules Check window.
But what is it?
DesignSpark PCB "Help" provides a good description of this in the index under "Drill Backoff Check". Below is an extract that illustrates how the error is checked.
From the image above the copper track (grey) must not encroach into the drill hole. Also the rounded end of the track must not meet the round copper edge of the pad to cause an undercut as illustrated in "Help".
DesignSpark PCB 'backs off' the track from the hole to meet the above requirements where this is possible. However if it cannot be achieved help states:
The above explains why your holes are filled!
If you process with the uncorrected Drill Backoff Errors the filled holes will allow the holes to be drilled through a continuous copper area hence removing the risk of drill breakage and possible PCB damage.
The PDF option in the Manufacturing Plots simulates this type of photo-plotter such that the user knows what will be plotted.
Note: Most recent generation photo-plotters now use a laser raster scanning pixel based technology so the requirements to perform the drill backoff is removed.
Drill backoff checks do not need to be performed for normal manufacture as they use raster scan pixel based techniques.
If required it is available and the PDF option will simulate this output.
Artwork produced for 'home' production must use the 'Windows' output option, either direct to the printer or to a PDF file.