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QFN footprint trade-off - isolation or pad size ?

Printed From: PCB Libraries Forum
Category: Libraries
Forum Name: Footprints / Land Patterns
Forum Description: [General or a CAD specific issues / discussions]
URL: http://www.PCBLibraries.com/forum/forum_posts.asp?TID=3214
Printed Date: 29 May 2024 at 5:51am


Topic: QFN footprint trade-off - isolation or pad size ?
Posted By: sot23
Subject: QFN footprint trade-off - isolation or pad size ?
Date Posted: 23 Jan 2023 at 6:54am
Hello,

There is a question that I often have to ask myself when designing footprints for fine pitch QFN or SON package.

For exemple, here we have a VQFN package from TI :  https://www.ti.com/lit/ml/qfnd364b/qfnd364b.pdf?keyMatch=RSH%252056&tisearch=Search-EN-everything" rel="nofollow - https://www.ti.com/lit/ml/qfnd364b/qfnd364b.pdf?keyMatch=RSH%252056&tisearch=Search-EN-everything  (drawing : RSH0056D if the link doesn't work).
It has a pitch of 0.4mm. The pin size on the device is 0.15mm-0.25mm.

In order to be able to solder correctly the QFN to the board, I would have drawn padstacks with a width of 0.25mm to accomodate the biggest that the pin can get. But if I do that, I will have a pin to pin spacing of 0.15mm, and I see a lot of people recommending to not go under 0.2mm.

The only solution would be to create padstacks with a width of 0.2mm. It would allow the pin to pin spacing to be 0.2mm. But now the padstack is smaller that the biggest the pin can get...

What would you do in this situation? What is the worst, risking to bridge the padstacks with a pin to pin spacing that is to small ? Or risking to have bad contact with a padstack that is to small ? 
TI decided to avoid the bridging risk and recommend 0.2mm wide pads. My boss tell me 0.15 pin to pin spacing is just fine and to design a 0.25mm padstack...

More generally, how do you deal with this kind of tradeoffs ? What are the considerations to prioritize ? 

Thank you ! I wish you a great day !



Replies:
Posted By: Tom H
Date Posted: 23 Jan 2023 at 8:32am
Can you please post what the TI Part Number is? 

The Case Code is RSH0056D, but we need the Part Number. 

The pad width should always be equal or greater than the terminal lead width. 

But you also have a minimum 0.15 gap between pads. 



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Posted By: sot23
Date Posted: 24 Jan 2023 at 1:48am
Hello and thank you for the answer. The part is a TMS320 microcontroller, F280049RSHSR version. Datasheet available here :  https://www.ti.com/product/TMS320F280049" rel="nofollow - https://www.ti.com/product/TMS320F280049 .

As you mention, it seems logical that the pad width should be equal or greater than the lead terminal width. But why does TI suggest to use 0.2mm pad when they also state that the terminal lead width range from 0.15mm to 0.25mm ?


Posted By: Tom H
Date Posted: 24 Jan 2023 at 10:40am
The reason why the component manufacturer recommended a 0.20 pad width is to help center the package on the pads. 

You cannot run a signal trace or copper pour between the pads. They're too close and would violate the design rules. 

I would say that any pad width between 0.20 and 0.25 is acceptable. 



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Posted By: sot23
Date Posted: 25 Jan 2023 at 1:03am
Hello,

Thank you for the reply !
Also, i saw your video on youtube with Fedevel Academy, very informative !! Thanks for your work.



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