What To Avoid: Bad Land Patterns, Part 1

No, not that kind of land pattern (although it is bad - drought is no laughing matter). Nope, we're talking component land patterns - the pattern of the pads on your PCB. If you get them wrong, you'll end up with like the farmer that had crops in pictured dry locale.

OK, pedantic time. Here's a land pattern for a design that we recently received. It's for a 10mm x 10mm QFN-88. We made an image by transmogrifying straight from the original gerber files:

OK, great - looks like a QFN. What's the problem? Well, this is the package outline, from the component datasheet:

Hmmm. There's definitely a problem here. Can't spot it? Let's combine the user's land pattern with the datasheet, and see what we see:

Wow, that lines up pretty much exactly. Perfect. And that's the problem - it isn't supposed to.

Land patterns are not the same as package outlines. Now, normally, datasheets give both, but in this case, the component datasheet only had the package outline. And that was probably what was used in designing the PCB.

So, you ask... what do? What is the right way to proceed if the component manufacturer doesn't provide that information?

Well, you could ask them. I'm guessing that an email to their design support folks will give you what you need right quick. Or, perhaps, you might check out if the component manufacturer provides a reference design - in this case, buried under a ton of web, you can find some Altium-format suggested land patterns at a reference design page. Which, if you have the right conversion tools, might be useful. (Of course, if you're already using Altium or some other high-spec software, they give you tools just for this purpose, and you can stop reading now).

But what if you're just stuck? What if you're not using hi-test software? What if you're writing a blog post on this topic during a holiday weekend?

Well, there are standards for this type of thing. For land patterns, IPC-7351B is your friend. But it's an expensive friend, sadly - while information may want to be free, (some) standards bodies like to make money. The official document is available if buying horribly DRM'd PDF files is your thing. Or you can buy an overpriced paper copy. Yes, the electronics industry is suggesting that you use ink pressed into pulped wood, to keep current with cutting-edge technology.

No wonder why no one follows IPC-7351B.

Of course, information still has a way of getting out. One trick is to just Google for other documents that reference the IPC standard. Here's an excerpt from a PDF that I found on Linear's site:

Hey, even second-hand information can be useful.

But wait - IPC (which stands for, somehow, "Association Connecting Electronics Industries"), has a drizzle of generosity for us. Kind of like a brief downpour on that miserable landscape, above - it isn't going to make your garden grow, but it's better than more dust. It's called the IPC-7351B IPC Calculator (that's some Redundancy Dept. of Redundancy type redundancy right there), and it's freely downloadable.

But... it's Windows only. And it's doesn't do all that much, unless you upgrade to a paid version. Like I said, better than dust. But even as limited as it is, it doesn't mean it isn't useful at all. Here's an illegible screenshot of the IPC-7351B IPC Calculator in action:

Of course, you can't save anything out of it (unless you pay). And there's no ruler (unless you pay). But they can't take away my unalienable right to screenshots, which I can paste into another program, and then do a rough overlay that sums things up:

So, IPC thinks that the right pad size should be the same width as the package outline, but each pad should be about twice as long as the outline - and it should extend a bit "under" the part, and a whole lot outside the part outline. And hey, that'll work a treat when getting your electronics assembled.

There are other implementations of the IPC standards, so look around, and find what works for you. And even a standard isn't the law - sometimes you just need to do your own thing. But keeping it standard is a good idea, and one that we recommend for best results.