ATmega64M1 in QFN32 - connect bottom pad to ground?

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I'm designing a new PCB using the Automotive grade ATmega64M1 chip in the 7mmX7mm QFN32 package. It has a large solder pad on the bottom.

Nowhere in the datasheet can I find where I'm supposed to connect this pad to ground. It seems like every other QFN device specifically calls this out.

Also, which ground should be connected? Digital ground (GND) or analog ground (AGND)? Or should I tie them together under the package?

Since I'm not suspecting that the microcontroller is going to generate much heat at all, can I omit the thermal feed-throughs to the bottom side of the PCB?

This device is going to go into a pretty harsh environment as far as temperature extremes, pressure and vibration, so I'd like to think that I'm doing everything I can to bolt the chip down properly to the PCB. Any other harsh environment design tips would also be appreciated.

Thanks!

Dale Wheat

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Indeed, I don't see anything in that datasheet either.

Other models have a note like

Quote:
2. The large center pad underneath the QFN/MLF package should be soldered to ground on the board to ensure good mechanical stability.

which is re-enforced here:
http://support.atmel.com/bin/cus...
Quote:
...
If you do not solder the ground paddle to the PCB , and run a drop test of the product, it is likely that the package will fall off. ...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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So, its mechanical rather than thermal or electrical?

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Quote:

So, its mechanical rather than thermal or electrical?

If you click through to the "white paper" off the Atmel support link above, they go into thermal extensively. Electrical/grounding is mentioned but I don't see much of anything about mechanical in that app note.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Yes, agreed, it should be soldered to the PCB. I plan on doing that.

And yes, I agree, it should be soldered to ground. All the QFN documentation says this is one of the main advantages of the package.

I guess my specific questions are:

1) Why isn't this indicted in the data sheet? I understand it's a relatively new package, compared to TQFPs, SOICs and DIPs. It would just be nice in a "warm fuzzy" way to have this design criterion spelled out for me. I'd rather not guess. My guesser is trending badly these days.

2) Which ground? Digital ground (GND)? Analog ground (AGND)? Or both?

Pending a substantive response, I will measure the resistance from the ground "paddle" as it's called to both the GND and AGND pins on the sample parts I have at home.

I appreciate the contributions so far to this question. I'm going to send these boards to a Bad Place and I'd like to know they have a decent chance of operating.

Thanks!

Dale Wheat

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dalewheat wrote:
1) Why isn't this indicted in the data sheet? I understand it's a relatively new package, compared to TQFPs, SOICs and DIPs. It would just be nice in a "warm fuzzy" way to have this design criterion spelled out for me. I'd rather not guess. My guesser is trending badly these days.
Have you raised the question with Atmel? It's possible that they forgot to cut and paste that part from the datasheets for the other packages. Or maybe the answer is "connect it to a pad that doesn't connect to anything else". Either way, they are the people that know.
Quote:
2) Which ground? Digital ground (GND)? Analog ground (AGND)? Or both?
My GUESS would be digital ground. Normally you try not to dump random things into the analog ground, and this pad might be picking up a little noise from the operations of the chip. Digital ground is noisy anyway, so that's a better place to put it IMHO.

Quote:
I'm going to send these boards to a Bad Place and I'd like to know they have a decent chance of operating.
Disclaimer: I am a software guy. Having said that, things I have seen done to boards that had to live out in the rain include...

Conformal coat. Sometimes you have to tape over things like a switch or a sensor, conformal coat, and then pull the tape off.

Mechanical support of big parts. This can be gluing them down, or adding a bracket or clamp or something like that. Surface mount makes this less necessary than previously.

Connectors. At least use ones with a good latching mechanism. If you skip the connectors and solder wires direct to the board, maybe use stranded wire, don't get too much solder on the wire (that turns it into a solid wire which likes to break), and organize some kind of strain relief for the wire.

More mechanical support of the PCB. Instead of just four holes in the corners, a hole every few inches on the interior of the PCB. Or, put some kind of insulating material down (Sil-Pad or similar?) and bolt the whole PCB to an aluminum/steel/whatever slab.

If you have room and money, put two of everything on the board, plus some way to switch between them. Okay, unless your name is Boeing or NASA, you usually don't do this.

I hope this helps!

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connecting thermal pad to ground is very common so it can be done even if the datasheet doesn't tell it.

btw, you connect it to the digital ground and I believe you have an inductor between the analog and digital ground.