What is underneath ATmega324PB (VQFN/QFN)?

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Heisen
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Posted by Heisen: Fri. Oct 8, 2021 - 02:24 PM
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Hello everyone,

 

I am making a board for ATmega324PB which is in TQFP lead package. Due to shortages and stuff I want to make sure that my PCB design can take ATmega324PB in other package too (QFN/VQFN) with minimal changes (Like I just have to change the footprint).

 

I have many vias underneath the chip for TQFP package for routing and stuff, but unsure about the other packages(VQFN/QFN), does it have a bottom pad? So if there is a bottom pad then it won't work with underneath vias, (it might but I am not comfortable doing it). Then I will be stuck with only TQFP package.

 

On mouser there two images for it.

 

One without the pad.

 

 

and one with the pad.

 

In datasheet I do see something underneath but I am unsure.

 

  • If it is pad, is it connected to ground?

 

Thanks.

This topic has a solution.

“Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?” - Brian W. Kernighan
“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupery

Tags:

AVR Microcontrollers, megaAVR and tinyAVR, solved, microchipDIRECT, AVR DA, AVR DB, ATmega324PB, AVR32DA48, AVR32DB48
Last Edited: Fri. Oct 8, 2021 - 08:48 PM
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Bronze Marktheusch
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Posted by theusch: Fri. Oct 8, 2021 - 05:12 PM
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Perhaps scan through the many prior discussions on this very topic.

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

...

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ron_sutherland
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Posted by ron_sutherland: Fri. Oct 8, 2021 - 06:36 PM
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All I see is "thermally enhanced" when searching the DS for QFN. I would take that to mean a floating metal pad that you can sink heat out of into the board, though I would probably get one and verify it is floating before doing the board. It should probably be connected to a plane to spread the warmth.

 

update: some is in stock both TQFP and VQFN

 

https://www.digikey.com/en/products/filter/embedded-microcontrollers/685?s=N4IgTCBcDaIIIBUC2BTA5gQwMxgCwAUAhEAXQF8g

my projects: https://github.com/epccs

Last Edited: Fri. Oct 8, 2021 - 06:40 PM
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ka7ehk
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Posted by ka7ehk: Fri. Oct 8, 2021 - 08:22 PM
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For compatibility and reliability, you should have NO traces (which also means no vias) within that thermal pad area.

 

Big question: Is this to be a commercial product or is it a hobby project?

 

Jim

 

 

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

 

 

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Heisen
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This reply has been marked as the solution. 
Posted by Heisen: Fri. Oct 8, 2021 - 08:48 PM
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theusch wrote:

Perhaps scan through the many prior discussions on this very topic.

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

https://www.avrfreaks.net/forum/...

 

After reading all of them now and some more, conclusion - it's better to solder the pad if it exists, be it mechanical or thermal reason, it's the right thing to do, connecting the pad to ground or not if it's not mentioned in the datasheet is a different matter. 

 

I am dropping the idea of using QFN/VQFN, vias are more important to me, there is even a power trace going under it. TQFP it is.

                                                                                                                                                                                                          

ron_sutherland wrote:

Thanks ron, I missed that in the datasheet, so the pad do exist. That confirms it.

 

ron_sutherland wrote:
update: some is in stock both TQFP and VQFN

Yes there are few in stock, but the future worries me.

                                                                                                                                                                                                          

ka7ehk wrote:
Big question: Is this to be a commercial product or is it a hobby project?

Started as a hobby, now turning into commercial.

                                                                                                                                                                                                          

 

Going TQFP is the only way for me, I should grab as much as I can right now before they go out of stock but I don't have enough funds to do that big move. I have 10 with me right now. Let's see what happens.

 

Thanks.

 

“Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?” - Brian W. Kernighan
“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupery

Last Edited: Fri. Oct 8, 2021 - 08:51 PM
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gchapman
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Posted by gchapman: Fri. Oct 8, 2021 - 09:09 PM (Reply to #3)
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ron_sutherland wrote:
update: some is in stock both TQFP and VQFN
Similar at Microchip; even the automotive ones are long lead.

Would recommend AVR32DB48 but it's long lead with some stock; some AVR32DA48 have a reasonable lead time.

 

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avrcandies
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Posted by avrcandies: Sat. Oct 9, 2021 - 02:30 AM
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eh?

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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ron_sutherland
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Posted by ron_sutherland: Sat. Oct 9, 2021 - 03:39 AM
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Heisen wrote:
but the future worries me.

 

The future is bright; we will be swimming in Microchip silicon soon (hence I recommend AVR32DB48 also); some may worry that it will be too much, and I do worry about that.  But looking at the EV market and battery backup systems, it's probably going to be some time before silicon gluts again, especially with the power hiccups reported in china; silicon in processing is damaged when power is lost.

my projects: https://github.com/epccs

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