Do tables of XY coordinates tarnish the souls of engineers?

[Berni]

I don't really mind too much the goofy ways parts are notated.  But what I really hate is when the drawings were taken from a CAD package that thinks lines are infinitely thin.  Seems the "official" PDF rendering is to draw them as 1 pixel wide no matter how much you blow them up.  The result is often lines that are so faint they can barely be seen. 

Some PDF viewers allow you to set a minimum line thickness in some way.  Others feel this is a problem with the author of the data sheet.  Obviously the people making the data sheets don't use the same PDF viewer I do.

Maybe that is an issue with people that have 4K monitors. Single pixel lines on a 1080p screen show up just fine.

When they get annoying is when the lines retain color from a CAD tool that uses a black background. Resulting a PDF with lines being bright green #00FF00 or bright cyan #FFFF00 on a white background, making it impossible to see anything. Seams like Chinese datasheets like to do this the most.

And on the topic of Chinese datasheets, there are sometimes mechanical drawings that are under constrained with dimensions. At first i thought i am just not reading it right so i went to ask the mechanical engineer how far 'this thing' is from 'that thing' and yeah the response was "well this was drawn by idiots so good luck". My fix for those cases is to run the PDF trough CorelDraw to extract the vector graphics and import the whole thing into Altium or similar so i can just trace over it. Or sometimes if i have the physical part il just go measure it myself with some calipers. But i should not have to do that!

[Siwastaja]

Compare this:

A mechanical engineer draws a traditional mechanical drawing of a metal sheet with 42 holes in it. The drawing is aimed for the person who will mark the midpoints of the holes on the workpiece then drill the holes. The mechanical engineer will not even consider anything else than drawing the measurements of the hole centers; even though they are "imaginary" points in the final piece, they are what the workers need.

Now consider component footprint drawing.

A mechanical engineer draws a component footprint with 42 square pads in it. The drawing is aimed for the person who will enter the midpoints of the pads into their CAD. The mechanical engineer will not even consider anything else than drawing the measurements of the pad centers---- oh wait!

So it's definitely not a rule or tradition of "mechanical engineering". It's a tradition of footprint drawings, and completely arbitrary.

[penfold]

Compare this:

A mechanical engineer draws a traditional mechanical drawing of a metal sheet with 42 holes in it. The drawing is aimed for the person who will mark the midpoints of the holes on the workpiece then drill the holes. The mechanical engineer will not even consider anything else than drawing the measurements of the hole centers; even though they are "imaginary" points in the final piece, they are what the workers need.

Now consider component footprint drawing.

A mechanical engineer draws a component footprint with 42 square pads in it. The drawing is aimed for the person who will enter the midpoints of the pads into their CAD. The mechanical engineer will not even consider anything else than drawing the measurements of the pad centers---- oh wait!

So it's definitely not a rule or tradition of "mechanical engineering". It's a tradition of footprint drawings, and completely arbitrary.

One simply cannot compare round holes with square pads. Their shape alone is quite different let alone conventions for dimensioning them.

There are many ways of placing a pad and defining its location, sometimes having an offset from centre is the best option, sometimes not, maybe even its an arbitrary shape... a table of recommended xy values then becomes a less intuitive way of presenting the information. Its obviously a bigger concern for irregular components, but nonetheless, it happens, and until we have one EDA package to rule them all, there remains many different options for "entering" the footprint information, yet theres far fewer ways of measuring and verifying it.

[Medved]

"Is there any reason manufacturers prefer this way rather than giving you a table with the XY coordinates of the centers of the pads?".

Yes there is, but you need to know a bit about mechanical engineering to understand why:
Dimensions need to refer to measurable points. An imaginary "center" is not one. You also need a reference point for the dimensions (there are several ways to define this)
Edges, corners, diameters, angles etc. can all be measured physically for tolerance and quality control. A "centre coordinate" can not. You can derive it, but that's not the same.

And to add: Mechanical design usually need tight tolerances between some features and loose between others. This way you define where the tight tolerances matter so are subject to QC.
Agree, for the purpose of redrawing into an electronic EDA tool, where all the subsequent manufacturing uses to be more than 10x more accurate than the footprint design really need, the accuracy argument becomes irrelevant, it is still where the tradition came from.

[Bassman59]

I tend to avoid vendors who provide crap datasheets or don't think you are worthy enough to have a 3D step file for their component. 3D models help me verify footprints and mitigate risk from mistakes. You can get a hint of the bad brands when Digikey has no link to the 3D models. For some 3D models, you have to register or even email some sales department. I won't use a part (especially connectors!) if there is no 3D model, whether it be from the OEM,  SnapEDA, 3D Content Central, Octopart or elsewhere.

I agree -- I look elsewhere if there is no proper 3D model of a part, especially a connector or anything that has to fit in with the mechanical design.

The thing about DigiKey and models -- sometimes those SnapEDA models are worthless. Oh, great! You have a 3D model of this thing! Oh, shit, it's actually just a fucking cube that's the outside dimension of the part!

[gnuarm]

One simply cannot compare round holes with square pads. Their shape alone is quite different let alone conventions for dimensioning them.

Square pads, round holds... don't care.  I see plenty of sheets that show dimensions from centers.  In fact I recall years ago asking a mech eng why they did drawings that way (you had to "assume" the lines were in the center) and he said that's the standard way of doing it. 

[Berni]

One simply cannot compare round holes with square pads. Their shape alone is quite different let alone conventions for dimensioning them.

Square pads, round holds... don't care.  I see plenty of sheets that show dimensions from centers.  In fact I recall years ago asking a mech eng why they did drawings that way (you had to "assume" the lines were in the center) and he said that's the standard way of doing it.

Most datasheets from the big brands do it properly so that it makes sense when drawing in a PCB tool. For chips the measurements are even given standard designations, like pin pitch is always "e". When it comes to these devices i just run Altiums footprint wizard, type in the numbers and it spits out the correct footprint complete with a proceduraly generated 3D model and all.

It's more about those people that give you the footprint for some weird non symmetrical part that has SMD pins all over the place and they are all dimensioned between edges of each other, so you have to punch a bunch of numbers into a calculator to actually figure out where the center of a pad goes, usually to top it off the centers also don't fit onto any neat sensible mm or mills grid.