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how to use thermal via for TI ic bq24650? [message #168975] Wed, 25 January 2017 17:38 Go to next message
srinivasan m
Messages: 5
Registered: January 2017
Junior Member
In the data sheet it has a typical layout for the device. It shows thermal vias underneath the package through the central GND connection Pad. These are then connected to the GND plane on the lower side of the board to enhance the thermal properties of the device.

How to do the same. i downloaded cad soft package and bxl file. In the TI forum , they mentioned that its better to create 3 x 3 vias for 3 x 3 mm ics. But when i created board layout i found that via's has been already placed(green colour). Is my perception correct? or how to create thermal via's on the gnd pad. I also want to know about the normal size of the thermal via's to be used in my board. I'm going to use my board as an heat sink. Suggestions are welcome.


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Re: how to use thermal via for TI ic bq24650? [message #168977 is a reply to message #168975] Wed, 25 January 2017 18:13 Go to previous messageGo to next message
rachaelp
Messages: 596
Registered: March 2015
Location: UK
Senior Member
> srinivasan m wrote:
>
> How to do the same. i downloaded cad soft package and bxl file.
I've just looked at the TI page for this part. I assume you followed the instruction to download UltraLibrarian and this is where the library component came from?

Personally I would NEVER use tools like this (or the bundled libraries which come with EAGLE or any other CAD tool!). You have to thoroughly check the resulting parts anyway which takes as long as just creating the part yourself in the first place. In addition the schematic symbol won't follow any consistent design guidelines so with library parts downloaded from all over the place you end up with messy schematics which are horrible to read.

Creating your own library parts isn't hard. If you don't know how to do it then Google for some tutorials. Adafruit and Dangerous Prototypes do some good tutorials to get you started.

Now, back to the part in question. The datasheet has all the information you need to create the part once you know how to create library components. Then for the thermal vias, you need to set the drill size to be small enough so that the solder paste wont wick through the holes and away from the thermal pad when it reflows. I use 12mil drills for parts like this typically but you need to keep the capabilities of your PCB manufacturer in mind. I would place the vias in the board design itself and not in the library part.

Best Regards,

Rachael

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Re: how to use thermal via for TI ic bq24650? [message #168978 is a reply to message #168977] Wed, 25 January 2017 18:31 Go to previous messageGo to next message
srinivasan m
Messages: 5
Registered: January 2017
Junior Member
Yes i used ultra librarian. As u said , i will create my own library file.   Is Ultra libraian not reliable? I want to know whether is it possible to place the thermal via's after creating our own part in the ground pad for this ic? what does solder paste wick through holes mean? i'm  a beginner in PCB design in eagle. Please enlighten me

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Re: how to use thermal via for TI ic bq24650? [message #168985 is a reply to message #168978] Wed, 25 January 2017 22:31 Go to previous message
rachaelp
Messages: 596
Registered: March 2015
Location: UK
Senior Member
> srinivasan m wrote:
>
> Yes i used ultra librarian. As u said , i will create my own library file. Is Ultra libraian not reliable?
It's as reliable as whomever created the part for UltraLibrarian in the first place assuming there are no bugs in the tool which create inaccuracies when exporting for different CAD software library types. Are you willing to trust the integrity of your design to somebody you don't know and a tool that you don't have any proven validation data? If it's just for a one off hobby build maybe you can get away with it but if you are building a larger quantity of boards you could waste a lot of money if something you just trusted to be correct was actually incorrect. In the case of this part you found that the existing vias in the thermal pad didn't meet the data sheet recommendation, and who knows what else in wrong unless you check it? Which will take pretty much as long as just creating it yourself....

> srinivasan m wrote:
>
> I want to know whether is it possible to place the thermal via's after creating our own part in the ground pad for this ic?
You can either create the thermal vias in the library part or you can exclude them from the library part and place them manually in the board. I prefer this option as it gives me flexibility to do different options with the vias if I have a different board stack up.

> srinivasan m wrote:
>
> what does solder paste wick through holes mean? i'm a beginner in PCB design in eagle. Please enlighten me
A via has a certain hole size and in the case of these thermal vias they are through the PCB from top to bottom. When you put the paste on the pad and place the component on top of it the paste will slightly go into the via holes but it will mostly stay where it is meant to be under the part. When the board is reflowed however and the solder paste becomes much more liquid it can get sucked into the thermal vias to the other side leaving the thermal pad of the device not properly soldered. To avoid this you need to ensure you don't use a drill size for the thermal vias which is too large.

Best Regards,

Rachael

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