Author Topic: Copper pours and QFN chips  (Read 366 times)

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Offline gautam023

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Copper pours and QFN chips
« on: June 10, 2021, 01:50:30 pm »
Hello everyone,

I am a newbie to PCB design. I have a QFN chip, a power converter IC (LT3958 - datasheet here).

Polygon pour is not surrounding the bottom pins unless I remove thermals from it. Is it recommended to remove thermals in this case? I have attached screenshots with and without thermals.

Thank you in advance for any help.


PS: I am using Eagle software.
 

Offline Galaxyrise

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Re: Copper pours and QFN chips
« Reply #1 on: June 12, 2021, 02:00:04 pm »
the datasheet wants low impedance loops, so you shouldn’t have skinny thermal relief. thermals are to make localized soldering easier, but you already have large pads you can’t reach with an iron. If you’re doing stencil + reflow, then you don’t need thermals. If using hot air, plan to solder that entire loop at the same time.

The data sheet uses a solid fill, too, and I don’t think I would stray from their recommendation in that respect either
I am but an egg
 

Offline JustMeHere

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Re: Copper pours and QFN chips
« Reply #2 on: June 12, 2021, 05:55:32 pm »
Agree with the post above.  Follow the datasheet
  Get rid of those holes in the pour.  You will have problems with them anyway. The solder will likely bridge over them, so you're not getting any benefit.  It just makes less contact space for your pad to contact and transfer heat.
 

Offline Unixon

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Re: Copper pours and QFN chips
« Reply #3 on: June 12, 2021, 06:40:48 pm »
Leave thermals on original polygon, you're probably need them for other parts.
Add another polygon with solid fill and without thermals on top of the first polygon just for a small area under and near IC (cover thermals + polygon wire width).
« Last Edit: June 12, 2021, 06:43:55 pm by Unixon »
 

Offline gautam023

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Re: Copper pours and QFN chips
« Reply #4 on: June 15, 2021, 07:37:12 am »
Thank you for the help everyone.

Using another polygon for just under the IC with thermals off and Isolate of 6 does the job. Now the copper pour is surrounding the pads.
 


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