Screaming Circuits: Why Those Castle Walls?


Why Those Castle Walls?

When I pick a subject, I seem to get moderately obsessed with that one topic and I'll end up covering it over and over again. I don't mean to. It just tends to happen that way. Castellated mounting systems seems to be the obsession of the day right now as evidenced by this, this and this very post.

Over on the Circuits Assembly blog, where my posts also show up, Rick posted a question about the form factor:

"I do not completely understand the purpose… would you expound on the functionality?"

It is an odd duck of a form factor and at first glance, it doesn't make a lot of sense. But upon digging into it a bit, I can see where it comes from. Generally, this form factor is used for modules, such as GPS units, RF devices and POL (point of load) power supplies.

  1. First, each of those things are not really chip-able as complete units. They are self-contained functional blocks and really need to be on a PCB.
  2. In the past, most modules of the sort had thru-hole pins for mounting, but SMT allows for less expensive manufacturing when used on a design that is all SMT
  3. The castellations, or half-vias, allow for a fillet on the outside of the module to improve the mechanical reliability of the connection.

It's really not that difficult of a form factor to use. But since it's fairly new, you're unlikely to fined the CAD library footprints to be pre-made. Make sure you read the datasheet carefully when creating the footprint. Make sure you leave pad both under and outside the part.

Duane Benson

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Comments

I think it also makes the part "probe-able" once mounted. Too often, older designs used buried SMD leads and you can't get a test probe under that little PCB...

Murata calls them ILGA - Inspectabel Land Grid Array - or some-such...

Hello Duane,
This is a very interesting topic. I'd like to know more and they say a picture is worth a thousand words. (if you have one)

I'm not for certain what the term "castle Walls" means and sure would like to understand how it relates to highspeed DFM.
How would this practice effect any critical areas/issue. Is it anything like running trenches around the effected area/signal or is it more to the likes of running guard traces?

ps: MANY Thanks! Mainly because you've always been right on target with your topics and I've enjoyed your post for several years now. Your sharing of knowledge is greatly appreciated.
Peace,
Adrian

Good point, Jeff. I was more thinking in terms of mechanics but electrical properties are obviously pretty darned important too. We hear the same advantage you mentioned about QFN and LGA form factors too. And the higher the speeds, the more of an advantage this becomes.

I suspect that the lower electrical parasitics of these modules (which have no inductance-adding leads) are part of what makes them a popular choice for RF modules.

(And they look pretty cool, too.)

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