QFN 16 layout question

Thread Starter

captoro

Joined Jun 21, 2009
207
Hello,

I am making a PCB with a QFN-16 pads, I am having issue with heat so I modified the PCB so I can add a heat sink and fan.
I created a small hole on the PCB in the center of the QFN chip. Can this help with the heat issue ? could it be bad?

ken
 

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MrChips

Joined Oct 2, 2009
30,720
Putting a hole alone under the package does not help to dissipate the heat.
Instead, I would leave the hole but connect it to a solid ground plane on the bottom side of the board.
This will act as a heat sink. Note that the QFN package has EP = exposed thermal pad. Make the ground plane as large as possible, i.e. flood the entire bottom layer with copper.
 

Thread Starter

captoro

Joined Jun 21, 2009
207
Hi,

I thought of that in the beginning, then I imagine the center being raised by the solder, causing the side to be harder to solder...could that be an issue ?, GND is on the second layer of this 4 layer board
 

nsaspook

Joined Aug 27, 2009
13,086
Hello,

I am making a PCB with a QFN-16 pads, I am having issue with heat so I modified the PCB so I can add a heat sink and fan.
I created a small hole on the PCB in the center of the QFN chip. Can this help with the heat issue ? could it be bad?

ken
Don't route inside the small QFN package. Put in a keep-out if it's auto-routed. Make sure any silkscreen is clear also.
 

Thread Starter

captoro

Joined Jun 21, 2009
207
Not sure what you mean.
I was working on this.
I get an error message saying the courtyard over lap. They dont, one is inside the other..
feedback please
 

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nsaspook

Joined Aug 27, 2009
13,086
Don't know what software you're using but that's a mechanical clearance issue. I wouldn't put a hole/routing via in that space. If it has an exposed pad use the correct method with properly sized thermals.

1677294824391.png1677295204674.png

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Thread Starter

captoro

Joined Jun 21, 2009
207
I am using Kicad 7 and the mosfet driver ISL155110. There is not thermal pad on this IC. Just want to do the most to prevent heating. It is heating quit a bit now. But I dont have a heat sink or the space for one. I am designing a PCB for heat dissipation now.
 

ronsimpson

Joined Oct 7, 2019
2,989
There is not thermal pad on this IC.
From the data sheet:
Exposed thermal pad. Connect to GND and follow good thermal pad layout guidelines.
I would connect all the "NC" pins and the "EP" and the "GND" pin. Have copper on all layers under the part with vias. See post #6. Keep the traces as thick as you can to help spread the heat.

If I has a hot gate driver I would extend the heatsink pad to include the GND and NC pins and maybe add copper out into the corners. (on all layers)
1677339831160.png
Next quesion is why is it hot?
 

Thread Starter

captoro

Joined Jun 21, 2009
207
I googles an image of the IC and it does have EP.
So all the Ep and NC are connected to ground ?
Why is it hot?. because its going at 100kHz 50% PWM, 12v, and I guess thats why.
 

MrChips

Joined Oct 2, 2009
30,720
It is getting hot because it is spending too much time in the linear region.
I would suggest that you review your drive circuitry and application.
Examine the switching times and voltages on an oscilloscope.
 

ronsimpson

Joined Oct 7, 2019
2,989
So all the Ep and NC are connected to ground ?
You do not have to connect the NC pins to ground but grounding will be better so you don't break up ground for NC pins. Also the heat will get out and away better with solid grounds.

I do not use "thermals" in my grounds. I do not use thermals at all. I want the best connection for heat and electrons.
hy is it hot?. because its going at 100kHz 50% PWM, 12v, and I guess thats why.
100khz is not that fast. That depends on what transistors you are driving. If the Gate charge is large then it might take time to charge the Gate and that causes heat. Please post the schematic. (at least the driver, resistors, and transistors) Included part numbers.

How hot?
 

nsaspook

Joined Aug 27, 2009
13,086
https://www.analog.com/en/design-notes/exposed-pads-a-brief-introduction.html
  • If the exposed pad is intended to provide significant power dissipation, the PCB designer should add vias from the exposed pad's land area to a copper polygon on the other side of the PCB. This lowers the thermal impedance from the IC to the ambient air.
1677387366156.png
This is a prototype PCB so the bottom has thermal relief pads to ease (vs a giant heatsink ) possible rework soldering and the thermals also connect to a internal ground plane.
https://www.microchip.com/content/d...B-Design-Guide-QFN-DQFN-Packages-00001843.pdf
 

ronsimpson

Joined Oct 7, 2019
2,989
the bottom has thermal relief pads
I think that is not a good idea. I want the heat to spread out.
For rework; We have a hot plate to heat the bottom side of the board while hot air heats up the top side. I agree rework is harder with good heatsinking. The photo in #13 does not help much because there are internal layers at 100% fill. In my opinion.
 

nsaspook

Joined Aug 27, 2009
13,086
I think that is not a good idea. I want the heat to spread out.
For rework; We have a hot plate to heat the bottom side of the board while hot air heats up the top side. I agree rework is harder with good heatsinking. The photo in #13 does not help much because there are internal layers at 100% fill. In my opinion.
I want the heat out too but it's not always a simple decision for one type of design.. The photo in #13 just shows that thermal via's are one part of more complex 3D structure and must be evaluaded electrically and thermally in 3D, not as a single solid 2D layer.

I've used plain solid pads too.
1677438720130.png1677438981652.png
 
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Thread Starter

captoro

Joined Jun 21, 2009
207
Hello,
Actually this is the first time I am using QFN IC in my design. I didnt see one in Kicad so I built my own PCB layout. When I saw the big number 17 in the center it gave me errors about bing floating, so I deleted them. SO vias or pads I see are options. Are all of them connected to the GND plane ?
 

Thread Starter

captoro

Joined Jun 21, 2009
207
You do not have to connect the NC pins to ground but grounding will be better so you don't break up ground for NC pins. Also the heat will get out and away better with solid grounds.

I do not use "thermals" in my grounds. I do not use thermals at all. I want the best connection for heat and electrons.

100khz is not that fast. That depends on what transistors you are driving. If the Gate charge is large then it might take time to charge the Gate and that causes heat. Please post the schematic. (at least the driver, resistors, and transistors) Included part numbers.

How hot?
How hot ? I didnt have my laser thermometer, but If you touch it it burns..
I have a 10 ohm resistor on the output of the driver, Then Its a wire to the base of the MOSFET.
 

ronsimpson

Joined Oct 7, 2019
2,989
Actually this is the first time I am using QFN IC in my design.
I am using KiCAD6 and have used 5. Post #9 is from 6. There is QFN with a heat pad. (there are several) I don't know 7 but do a search (filter) for QFN-16.
If you made a QFN-16 you need to add a connection in the schematic for pin 17. Or you can make the heat pad have the same number as the ground pin. It is OK if not good to have more than one pin with the same number.
How hot ? I didnt have my laser thermometer, but If you touch it it burns..
More than 50C.
Again, please post a schematic of the IC to MOSFET. I don't care about the rest of the circuit. Do a screen capture or something.

In KiCAD you can edit a QFN-16 but you cannot save it back. You will have to save it to a new library. Or you can save it to your board. Again I am not using version 7.
 

Thread Starter

captoro

Joined Jun 21, 2009
207
I will get back you on the Schematic, I am away from home this week. I doubt my schematic will give you more information, its very basic circuitry
 

ronsimpson

Joined Oct 7, 2019
2,989
I ask because I am using GAN transistors with a turn on voltage of about 1V and a max voltage of 6V. (normal 5V) Some people use MOSFET drivers that output 20 or 30V and then add a "Micky Mouse" circuit to divide down the gate drive to a lower level. That makes the drivers hot. I have seen 100 ohm G-S resistors that heat. Also, long wires have inductive kick back that cause trouble. We drive 1000A power modules hard because of the high Gate capacitance.
 
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