PCB Assembly Services - Screaming Circuits: Castellated Mounting Holes


Castellated Mounting Holes

Before the trade show, I wrote a post about an LGA module. I understand, that this technically isn't an LGA (Land Grid Array) form factor. It is similar in many ways, but it's not quite the same. I think the proper terminology is a "surface mountable module with castellated mounting holes".

Castellated It basically looks like the board has smt pads on the bottom side with vias in them and it's been cut right through the middle of the vias. I don't know how the little board is produced, but that might just be the case. Anyway, it should be treated similarly to an LGA with a few additional considerations.

First, an LGA usually will have pads at that are all the same size. This has pads of different size and your PCB footprint should consider that.

Second, these may have a trace and via keep out area - an area where you don't want to put any traces or vias due to the risk of a short.

In the example shown in the original post, at first glance, the PCB doesn't appear to follow good keep-out practices. It's probably okay because everything on the module is coated with soldermask. But, if there are any defects in the module soldermask or if registration is off a bit, the board could see some problems.

Duane Benson


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Why is the lga going to have pads of the same size usually? Is it just because that's the standard, or is there actual logic behind it?

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