What is the point of component courtyards ?

Hi guys,
I'm strugling through the IPC-7351 standard and there is one thing (well, many actually) that bugs me. The component courtyard is defined as: "Courtyard – The smallest rectangular area that provides a minimum electrical and mechanical clearance (courtyard excess) around the combined component body and land pattern boundaries."
OK, great, but what is it used for? To me it would seem that this is the area in which no other component (or part of it) can be placed. But this is not true. Take a look at a TQFP 48 footprint. You can easily put a 0402 resistor into one of it's corners (I've done that plenty of times)



So can anyone explain it to me, why exactly are courtyards used for?

George

The IC itself takes up a certain amount of space, then besides that it has those small terminals extending outward, then besides that it needs the lands on the board.

Perhaps the courtyard idea is an easy way to keep track of total square inches devoted to the IC, at times such as when calculating how much room is available on the board?

Consider the SMT placement machines, if you have no space around the component then when placing one component it can knock the other.
Also consider the heat flow in a reflow oven, if 2 components are right next to each other then you restrict the flow of heat between them.
Also some optical inspection needs a little space to be able to see at an angle.

Add it all together and you end up with the need for a little space around the component for optimum assembly, this space in the IPC document is defined as the courtyard. Some people ignore the IPC one and have a very small courtyard and get away with it, others may not be able to.

I can recall an old technical document I had that told me that there should be 1mm between different component pads, this was achieved by having a 0.5mm courtyard on each. I think IPC7351 says something similar - I cannot remember.
These days to have 1mm between pads is a luxury.

You will note that there are 3 sets of dimensions in IPC, Least, Nominal & most (or something like that) getting bigger to make for a more easily asembled board with least manufacturing defects.

As for placing the components in the corners, this is also something that I always say too. Components are not always rectangular but the courtyards/placement outlines are often just drawn as a rectangle/square.
If you follow the shape of the component itself you are more likely to be able to get more components in closer however the std would have been far more complicated in dimensioning etc.

The purpose of the courtyard is to provide a clear criterion for correct component placement. You can e.g. utilize it in the way, that when placing a number same package components like resistor or capacitor chips, you simply align the courtyard rectangles.

In special cases, you may want to ignore courtyards, e.g. in the said TQFP example. You should be sure, that the assembly service provider actually doesn't need the room for his tools. It would be at least over-engineering to modify the TQFP courtyard for this special case.

Cant understand what bugs you in IPC-7351, its one of the best standards to be developed for PCB designers and for footprint standardisation EVER, and I strongly reccomend PCB designers use and follow it...

One very important point designer miss is the ability to rework parts. How will you be able to remove a part with everything packed in tightly?
If you build a one time use product this is not a problem. But for very expensive custom build items, you will be happy having all that room around components.

A valid point Loosemoose, this makes the component courtyard even more important, and its calculation then depends on the method and equipement that will be used to remove a device, bottom terminated components being the most critical.
On the subject of courtyards, I have always used them as I deem them critical... I once employed a contracter who didn't, when I checked his board there were over 400 comp to comp errors. When asked how and why, he said he never used the courtyards and just placed components by eye... Guess what, he was out of the building faster than the speed of sound.
My point being, these things are there for a reason, in this case it is to make the board easier to assemble, there are to many PCB designers coming through who have not had experience of PCB assembly, or dont seem to think it matters, in my view this is wrong, we are designing a product that has to be assembled efficiently.