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shadders
United Kingdom
224 Posts |
 Posted - 14 Dec 2009 : 22:20:17
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Hi,
I have scanned the forum texts, but still unsure how to create a specific component PCB footprint.
The IC is a 32pin QFP device with the underneath of the package a ground connection. The manufacturer has stated that the pad underneath the IC should be 3.45mm x 3.45mm but with 9 vias/holes distributed through the surface to allow air to conduct the heat.
What is the optimal was of creating this footprint ?
A copper filled polygon with 9 separate plate through pads added ?.
Although this looks as required – will the manufacturing process accept this as a valid ground connection once the design has been processed into a PCB Output format ?.
Is there a preferred method ?
Thanks and Regards,
Richard.
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DavidM
United Kingdom
458 Posts |
Posted - 16 Dec 2009 : 08:57:52
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Richard,
Adding pads to the footprint is probably the best way to do it. I would suggest you add the central ground plate/tab as a surface-mount pad as well, making a 42-pin footprint (32+1+9). You would only need 33 pins on the schematic symbol, 32 for the main QFP pins and one for the ground connections.
If you then go into the Component editor and set the component pin number for the 33rd scm pin to the following:
33+34+35+36+37+38+39+40+41+42
this will ensure that when you connect pin 33 to ground in the schematic, all 9 of the through-hole thermal sinks are connected to ground as well.
The only issues you may then have is with solder resist. Being pads, the ground tab and the thermal sinks will all want to plot on the solder resist plot, which may or may not be what you want. If you want instead to place a particular shape 'window' in the resist, you would need to add this to the footprint as a shape on the resist layer, and set the padstyles to zero-sized on the resist layer using the 'Add Exception' button on the Design Technology dialog.
I hope this is enough to get you moving along.
David.
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shadders
United Kingdom
224 Posts |
Posted - 16 Dec 2009 : 22:22:54
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Hi David,
Thanks. I did create a copper fill area- but this would only be connected to the ground plane through the vias, or if i manually connected.
You approach seems better since the method will definitely connect the large pad to ground - and easy change to the PCB symbol i have created.
I have not completed the latter stages you have mentioned - this is my first project and i keep on changing the chip sets as i find better or a different way of implementing the circuit.
Thanks for the guidance on this, much appreciated.
Regards,
Richard. |
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PCB Symbol Ground Area