Package variant with unconnected pins

What part of this question is difficult to understand? I'm assuming it must be, because other questions get answered, but not mine.

On 04/09/2013 09:46 PM, Clifford Heath wrote:

I'm making a symbol for the QFN20 and SSOP24 variants of the SIlicon

Laboratories Si473x famiy devices. Apart from the SSOP having more NC

and earth pins (which I can connect as duplicates), there's an extra pin

(DYBP) on the SSOP which isn't brought out on the QFN. If I add this pin

to the device, I get errors using the QFN package because it isn't

connected.

 

Is it possible to have a pin in a device which is not available in all

package variants? How?

...

What part of this question is difficult to understand? I'm assuming it

must be, because other questions get answered, but not mine.

It's not it's difficult to understand, but more likely that no one has a

good answer to offer.

Just off hand, I'd say that it's not possible to have a pin defined in

the device's symbol, and not have it connected to any pad in the package.

When you say "I get errors" isn't real clear.  Exactly what is the error

message, and when do you get it?  I assume you're getting an error when

you try to add the QFN package to the device, but the QFN package

doesn't have enough pins.

One suggestion I would make is to create a separate device with a new

symbol that doesn't have the DYBP pin defined.  Then adding the QFN to

the new device shouldn't be a problem.  Down side: you now have multiple

Si473x devices.

Alternatively, if the DYBP pin is not needed or should be left

unconnected for proper operation of the IC, you can remove this pin from

the symbol, then add the QFN.  Down side: The DYBP pin is inaccessible

for all packages.

I don't consider these as "good" answers, but merely work-around

suggestions.

HTH

    - Chuck

Thanks, that's what I figured. I had just hoped that there was a different answer .

The errors I was getting were a result of not connecting the pad to anything in the

SSOP, so you get an error when you try to place the SSOP. I hadn't yet added DPYB

to the symbol, but that would cause the same problem with the QFN.

Eagle's connection tool needs the ability to say "unavailable" for any pin in the symbol.

Would it be possible to place an "infinitely" tiny pin with no keep-out etc, and connect

to that, or would that create other issues?

On 04/10/2013 06:08 PM, Clifford Heath wrote:

Would it be possible to place an "infinitely" tiny pin with no keep-out

etc, and connect

to that, or would that create other issues?

Sure, you could add an extra pad to the package for that purpose.

Position it somewhere where you're sure there will be no connection.

When you place the pad, be sure to turn off the stop mask and cream

layer properties for the pad itself.  If I recall correctly, these are

check boxes in the properties dialog.  Just copy the QFN.pac to

something like QFNx and modify the latter.

Enjoy,

    - Chuck