Purpose of exposed ground pads

Question

I have a Beckhoff EL2008 8-channel digital output terminal that I took apart because I'm using an ASIC that is inside the module, Beckhoff's ET1200.

There is essentially a ring of ground pads,some with vias, surrounding the ET1200 inside the module (they all beeped out to the ET1200's ground pins).

ET1200

I can understand if they're just there for bonding the ground planes, but why expose them?

Through past experience, the ET1200 is a tremendously sensitive device, and this leads me to think that they're exposed for ESD protection, increasing the likelihood that they would conduct the discharge instead of the ET1200.

Can anyone conclusively describe their purpose?

Thanks for looking.

Pure speculation...perhaps they are mounting points for a metal shield that covers the sensitive device. If so, they managed to make it work without the shield. — Joe Hass, Sep 06 '13 at 11:02
The pads have thermal reliefs, so they were designed to have something soldered onto them? — RedGrittyBrick, Sep 06 '13 at 11:06
My thought was that the sharp edges of the square pads would increase the strength of the electrical field, and this would maybe attract the ESD. — Rocketmagnet, Sep 06 '13 at 13:12
@JoeHass - But what an odd shield. It would have to jump over so many components. — Rocketmagnet, Sep 06 '13 at 13:14
Have edited the picture to show the pads in question. With red arrow. — placeholder, Sep 06 '13 at 19:48

score 3 · Accepted Answer · answered Sep 06 '13 at 19:54

3

It's likely these exterior (gold bare metal ) pads are to bond the layers together to form a nice faraday cage in situ (as mentioned in OP). Since there are some high speed signals your probes will require adequate grounding during test so they are left open for the probe card to be able to get a clean ground reference. Some have vias in them because they bond through, some don't have vias because the ground is needed close to the pads that are being tested but there are traces underneath. Particularly on the LHS.

answered Sep 06 '13 at 19:54

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So what about the thermals on the gold pads? – Rocketmagnet Sep 08 '13 at 22:31
It is standard practise to make something solderable in case you need to - especially into the ground plane. And design reuse might have dictated why those shapes were used. I'd not read too much into it. – placeholder Sep 09 '13 at 01:26

Purpose of exposed ground pads

1 Answers1