Ambiguous PCB Markings

Here’s a little issue we run into now and then. Which reference designator goes with which part? Quick. I need to know. Now. Now. Now!

It’s less of an issue with SMT parts because we machine place them and use your centroid file to do the programming. Still though, It’s always good to have things marked clearly in case rework is needed and for visual inspection.

For through-hole, though, it is definitely an issue because a real human being is putting the parts in and the visual markings are the programming for the human type person.

For best results, take a little extra time and make sure all of your ref designators are clearly associated with the correct part. It’s also always a good idea, when possible, to have all the designators in the same position relative to their part. Consistency is a good thing here. Consistency is a good thing with mashed potatoes too. Who likes lumpy mashed potatoes?

Duane Benson
You say poe-ta-toe, I say ugly brown tuber

3 thoughts on “Ambiguous PCB Markings

  1. I agree, and do take the time on my boards to make the silkscreen look neat and accurate however it’s still easy to make an error or present something that isn’t clear to an operator.

    In my opinion silkscreen is there for reference only for prototyping, servicing etc. and should never be used in this day and age for creating auto-placement programs and to this end I always provide a clear assembly diagram with the component reference inside an outline of the component body plus the x/y centroid file.

    I was surprised however at a couple of our contract assemblers who ignored the centroid file and assembly diagram and reverted to using the silkscreen ident to create placement programs! I never got a satisfactory answer from them on why they still stuck with doing it in this inaccurate, lengthy and old fashioned way but had to put clear instructions on our silkscreen layers “NOT TO BE USED FOR CREATING AUTO-INSERTION PROGRAMS” and write our PCB design standards such that the assembler must use centroid data and the assembly diagram.

  2. I only add silkscreen info for through hole comps or jumpers, ie where human intervention is required, with 0402 chip components, double sided placement and everything crammed in I think silk screens can soon become cluttered and unreadable.
    I also prefer production dept. to use a viewer instead of assembly drawing, much easier to locate things, also can be tailored to thier requirements.
    Still spend time tidying the component ident layers so they are clear and readable using a viewer.

  3. I agree with Duane that reference designators for TH components are most critical. However, I find it very helpful to have polarity markings for SMT components as well.
    While an assembly drawing is a good reference, we’ve seen many designers create their “assembly drawing” by merely overlaying the silkscreen and mask layers…thus an insufficient silk layer results in a poor assembly drawing.
    Finally, while centroid files are critical, we’ve found different designers use different orientations for various componets (SOTs come to mind). It’s important to not rely soley on the centroid file, but compare along side a good assembly drawing. And having polarity markings on all applicable components helps on the manufacturing floor where inspection and rework occur.

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