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How accurate should I draw package outline in silkscreen? (WARNING: LONG POST)

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luisr:
I've seen some video tutorials and read some info but seens like no body pay much attention about accuracy while drawing the silkscreen... I mean... the tutorials mention that silk screen must be drawn on its respective layer but they don't mention anything about taking into account the trace width used to draw the package outline...

I wish I could explain better but can find the words so i attached two images:
- In one image I didn't take into account the trace/line width so the component will overlap the silkscreen when placed on the board (silkscreen2.png)
- I the other image I took into account the trace width so the component will not overlap the silkscreen so it can be used to accurately align the component while manually placing the components on the board (for automatic component placement we need fiducials, right?) (silkscreen1.png)

Anyway, I'm probably asking a non sense since components are not manufactured perfectly and the final measurements of the package outline will vary according to the tolerances of the manufacturer... also probably the silkscreen tolerances and accuracy will vary from one pcb manufacturer to another, also some components will align themselves due to surface tension but... Do you think is worthy to accuratelly define the package outline as I show in the "silkscreen1.png" image?

EDIT: Forgot to mention that I was asking if is worthy to accurately draw the package outline in the silkscreen(tplace) and tdocu for SMD packages like QFN, BGA, etc... obviously TH packages will align itself according to their pins

c4757p:
I always try to draw silkscreen outlines to the maximum package size specified in the datasheet (yes, taking line thickness into account) so I can use them as guides when arranging the components. I don't usually worry about shape if it's not perfectly square, as long as the entire package fits in the outline.

I don't think it has anything to do with manufacturing, it's for design. Hell, I could have just left out the silkscreen Gerbers in all of the PCBs I have ever had made with no problem, and just printed it out myself as a guide for assembly. If they're being machine assembled it's even more unnecessary. But I want it to show me how close I can get the components when I am laying them out - I even use a "silkscreen" on homemade PCBs for this purpose.

EEVblog:
It all depends on whether you want to see the silkscreen as an identifier after the component is loaded, or as a lazy man's "actual size" measure that helps with layout.
Usually I like the use the silkscreen as the actual size measure, so I'll use the centre line of the silkscreen as the component size market. So I'm left with half the silkscreen hanging from under the component depending on line width.
Requirement vary a lot though.

Dave.

luisr:
Using the silkscreen as a layout helper is not my intention... that's what "tdocu" (eagle) and "top assy"(diptrace) is used for... in on those layers I draw the component shape making sure that the outer border of the trace matches the real measures of the package outline and any other clearance requirement the package may have.

In the other hand my intention with a accurate silkscreen is to help me aligning components while assembling the board... what i have seen is that normally the center line of the trace in the silkscreen is used as indicator of the components' real measure (as Dave mentioned).

I was thinking about this and came with another question: Is there any standard or "best practice" about this matter?
I mean... let's say that I use the outer border of silkscreen trace as a reference of he real measure of the package outline and then share my footprint with a friend... if he/she is more used to use the center line then he/she might think that my silkscreen is wrong and no accurate...

Thanks for anwsering  :-+

EEVblog:

--- Quote from: luisr on March 07, 2013, 12:58:02 am ---I was thinking about this and came with another question: Is there any standard or "best practice" about this matter?
I mean... let's say that I use the outer border of silkscreen trace as a reference of he real measure of the package outline and then share my footprint with a friend... if he/she is more used to use the center line then he/she might think that my silkscreen is wrong and no accurate...

--- End quote ---

Well, in the PCB manufacturing industry, the center of the line is always the reference point. So if you only provide a board outline using silkscreen, then they will take the center of the line as the actual dimension of board to provide you. This was common back in the old days, but is still the defacto standard today for all things like that. Like if you use say a thick 20mil trace on the mechanical layout to define your board outline (so it looks good in printouts), then they take the center.

Dave.

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