Interpreting package outline dimensions

[ThermalRunaway]

Hi everyone,

I've been busy on my latest project, and I'm doing my own PCB design. Naturally, most of the parts I need aren't in the Eagle library so I'm creating my own footprints. So far I've been okay (apart from the regular annoyance of inferred dimensions - just bloody state them all!). But I've just come across a package outline document for a schottky diode, which I thought would be easy-peasy, but I'm really confused by it.

Please see the image attached.

What I want to know is, why are there two sets of numbers for the dimensions on this diagram? For example, the body width has a figure of 6.22mm and a figure of 5.59mm underneath. So... which one is it? I've tried both and actually neither of them look quite right.

Am I missing something obvious here?

Thanks in advance for the advice,

Brian

 

[ThermalRunaway]

Could these actually be tolerance extremes? So they've quoted maximum and minimum values?

I dismissed this idea earlier because I thought it was too large a difference to be max/min values. But I've been looking at some other diode datasheets and some of them quote tolerances for the measurements that are in the same ball-park as this. Usually they do quote a nominal value as well though, which this one doesn't.

I must admit, this is my least favourite part of project development. Drawing footprints and interpreting package diagrams. Especially when most of the measurements you need are inferred. Grrr.

Brian

 

[Sceadwian]

It's stating that it's not a square package... That's it's wider on the top than the bottom... What's so hard to understand? There is no inferring, the declarations of dimension are VERY clear.

 

[ThermalRunaway]

It's stating that it's not a square package... That's it's wider on the top than the bottom... What's so hard to understand?

...are you sure? I don't think that's right. I think it's just stating max/min values. The top of the package is not going to be wider than the bottom, surely. And if so, what about the sides? Nah - that's just ridiculous, I'm not buying that.

There is no inferring, the declarations of dimension are VERY clear.

There is no inferring in this document, you're quite correct. I was actually having a moan about the larger task in general. In my limited experience, it seems to me that these kinds of drawings will often state lots of unimportant dimensions and then leave it to you to deduce the important ones from it. For example, sometimes the width of a pad will not be explicitly stated but it is possible to work it out from some other less important dimensions on the drawing. I've actually been finding it easier to draw my own rough drawing and then put my own dimensions on it as I work them out from the drawing - less confusing that way. Perhaps it's just me!

But as you said, all the measurements on this drawing are explicitly stated. I just wasn't sure what the top and bottom numbers were supposed to represent. Having checked out some other land patterns for SMT diodes, I think they're max/min values.

Brian

 

[MRCecil]

Hi Thermal,

Actually, I think the body dimensions are the min and max manufacturing tolerances placed there perhaps for the folks that set up the pick & place tools at a board production facility. For your footprint, that body will just be part of the silkscreen, and is not that critical unless grossly over or undersized. The important parts, for your case, are the pad dimensions.

Hope this helps some.

 

[ThermalRunaway]

Thanks for the response,

Yes I fully understand that the pads are the only important part of the footprint. I like to include the full body in a documents layer which is ignored when I output to gerber but is useful during component placement on the PCB design because I can see the physical sizes of my components. And also I like to put a bit of silkscreen around the outer edges for clearer PCB assembly later on.

My first thought was that these could be max/min tolerances but they seemed to be quite big differences to me and also I would have expected them to quote typical values as well (of course, you can work them out from the max/min). But having looked into it further I think you're right, they are indeed max/min tolerances.

Thanks again for your input.

Brian.

 

[Sceadwian]

The top of the package is not going to be wider than the bottom, surely.

Why is that sure? Perhaps they mounted it upside down? Stranger things happen... It can reverse a pinout incompatibility
.

 

[dougy83]

It's stating that it's not a square package... That's it's wider on the top than the bottom... What's so hard to understand? There is no inferring, the declarations of dimension are VERY clear.

Why is that sure? Perhaps they mounted it upside down? Stranger things happen... It can reverse a pinout incompatibility
.

All dimensions in that drawing are min/max values (except of course the one marked REF). If you still believe otherwise, then please explain the dimension between the bottom of the lead and the bottom of the case (i.e. 0.008 / 0)