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So I've got a design in which I would like to use the AT25SF321B-MHB-T chip by Dialog Semiconductors, but its datasheet does not mention whether the exposed thermal pad should be connected to ground or not.

Their documentation is lacking in general, for example, no information provided as to recommended land pattern/footprints. Funny thing is that their website, doesn't have any mention of the AT25SF321B-MHB-T whatsoever, thus no step files to be found either.

Does anyone have experience with the UDFN package and their exposed pad?

As far as I know, these are generally connected to ground on the PCB for most effective thermal relief, but I've also had cases where the datasheet says to specifically leave the exposed pad unconnected.

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    \$\begingroup\$ with standardized packages, I'd argue you don't really need your own landing pattern, it's standardized, hence the "(EIAJ SOIC)" after the package name. The UDFN package drawing says "JEDEC MO-229", which is also a standard with official landing pattern. \$\endgroup\$
    – Marcus Müller
    Mar 7, 2022 at 14:42
  • \$\begingroup\$ Thank you for that, I hadn't noticed that! \$\endgroup\$
    – Cyborgium
    Mar 7, 2022 at 14:47
  • \$\begingroup\$ can't jhelp you with "connect it or not", though. \$\endgroup\$
    – Marcus Müller
    Mar 7, 2022 at 14:47

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Usually, when the datasheet does not mention the exposed pad for UDFN devices, you can assume that it is not connected inside the chip, meaning that you can connect it to whatever you want (typically ground or leave it floating).

I found a datasheet for a similar device from the same manufacturer, stating the following:

The metal pad on the bottom of the UDFN package is not internally connected to a voltage potential. This pad can be a “no connect” or connected to GND.

Although, assumptions are as always dangerous to make.
For a low power like this where heat sinking won't be an issue anyway, I'd take the safe route and leave it unconnected if the datasheet doesn't tell you otherwise. If it was needed to ground the pad, that surely would have been mentioned.

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  • \$\begingroup\$ That similar datasheet is a nice find! I think I might add an open solder bridge to ground on the first proto just in case. Of course once I actually get the chip, I can measure whether the pad is in fact connected to ground. \$\endgroup\$
    – Cyborgium
    Mar 7, 2022 at 15:01

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