Assembly drawing definition - Page 1
Forum section: electronics / general

author: zphaze
author_rank: Contributor
timestamp: May 03, 2020, 11:38:34 am
content: Hi,I'm doing a PCB for a company (college project). I got most of my footprints online, and I'm being asked to change some things.Notably I must use their layer assignment convention.The company has (among others) the following conventions :QuoteI wonder now... given this footprint here :There is an outer purple frame, an inner purple frame, and a 3x3D body definitions.The thing is... I'm not too sure what should be considered assembly drawing (M01) and what should be considered 3D body (M05). The hatched rectangles areactual 3D body definitions (so they go in M05). I'm refering to the inner rectangle as the frames surrounding each 3D body.Does the assembly drawing refer to the components "actual physical placement of the component" or is it more of a safe region that "contains/surrounds" the component?In other words, would it be the "outer" rectangle that would refer to as the assembly drawing, or the inner one? Or is it both? Or are the inner rectangles just extra information (andthus should go on another free layer like M13).I understand the assembly drawing is useful to show the component placement, but I'm confused about what it really tells i.e. if it's informations about precise component dimensionsor just a set of areas that contain the components themselves.Thanks!Thanks!

author: PlainName
author_rank: Super Contributor
timestamp: May 03, 2020, 02:21:44 pm
content: I would think the M01 would be the inner purple rectangle, which is the physical size of the component. That's based your statement that the drawing will be used to show component placement.The outer purple rectangle would be the courtyard, aka M03, which defines the keep-other-stuff-out area for the component. That isn't necessarily an area that the component fills itself - it might define the area needed to get things like tweezers in there if you're hand-placing, for instance. I don't see that as being useful to show where a component is placed.

author: zphaze
author_rank: Contributor
timestamp: May 03, 2020, 02:44:52 pm
content: Well, my statement is vague, in the sense I don't really understand what an assembly drawing actually does (I never really had to care, but I'm asking because I want to do things right), basically what is it's real purpose.I was saying an assembly drawing, *from what I understand*, seems (from googling the term) to be something to indicate component placement, but I'm not sure if it needs to be superprecise or not (i.e. if it has to "stick around closely" to the edges of the 3D model so that, assuming the 3D model has the right dimensions, the assembly area contains almost exactly the physical component area).Or if there's some kind of high tolerance regarding the component position (i.e. if a rectangle that is bigger than the actual component is suitable, to indicate the component lies "somewhere in that area").The component footprint came with a courtyard as well (green layer, it's hidden in my view configuration for the purpose of showing only the purple, which I assume is about assembly drawing), hence why I'm confused which purple rectangle would belong to M01

author: PlainName
author_rank: Super Contributor
timestamp: May 03, 2020, 04:44:06 pm
content: An assembly drawing would show precise component positioning via dimensions, but the actual representation shouldn't be treated as accurate. That's why they would normally say "DO NOT SCALE" or similar. Indeed, they might even be exaggerated in places for clarity, but the dimensions will still be accurate (in other words, a gap may be made large enough to see without a microscope but be dimensioned correctly as '0.01mm').So, given that context, I wouldn't think that the outer rectangle is the one needed. But... it depends on what the drawing is to be used for, and I can also think of a case where it would be appropriate. No chance of acquiring a sample drawing that you could work it out from? Or even just ask?Regarding the existing courtyard, it's common for externally produced footprints to have stuff on the wrong (for you) layers which can confuse what they are. Which is, presumably, why you are needing to do this! I often find that the supplied courtyard for a component is on the bottom layer despite the thing being surface mount on the top layer. The courtyard is also more about processes than the physical dimensions, a bit like the silkscreen, so the supplied one may not be appropriate either.I think in your position I would ask for more detail about what they want, in the sense of how stuff will be used. If they have specified layer definitions it's likely they have a house style document which should spell out this kind of thing.