you mean this?  you have to go to your device MFG to get the accurate info...
https://www.cirrus.com/en/pubs/appNote/AN315REV1.pdf

> Fellow TechNetters:
>
>    Perhaps the third time is the charm:
>
>    Is there any data out there regarding the integrity of a Thermal Header
> Solder Attachment/Bottom Termination Component solder joint integrity
> over time.   Most BTC that I have x-rayed exhibit some degree of
> voiding.   Can the solder joint voiding degrade the solder connection
> to a point that it would result in a shorted device internally due to
> heat build.   Is a solder joint a considered a Thermal Insulator?
>
> Victor,
>
> From: Hernandez, Victor G
> Sent: Monday, January 16, 2017 3:15 PM
> To: TechNet E-Mail Forum <[log in to unmask]>
> Cc: Hernandez, Victor G <[log in to unmask]>
> Subject: FW: IPC Standard, Thermal Header Solder Attachment Devices, or
> Bottom Termination Components
>
> Fellow TechNetters:
>
>    My original post last week did not generate any feedback.   Therefore,
> I post again in hope of reaching more audience the beginning of the
> week.   The holidays are over with and the serious work begins.
>
> Victor,
>
> From: Hernandez, Victor G
> Sent: Thursday, January 12, 2017 7:46 AM
> To: 'TechNet E-Mail Forum' <[log in to unmask]<mailto:[log in to unmask]>>
> Cc: Hernandez, Victor G
> <[log in to unmask]<mailto:[log in to unmask]>>
> Subject: IPC Standard, Thermal Header Attachment Devices, THAD
>
> Fellow TechNetters:
>
>     Is there any data out there regarding the integrity of a Thermal
> Header Solder Attachment/Bottom Termination Component solder joint
> integrity over time.   Most BTC that I have x-rayed exhibit some
> degree of voiding.   Can the solder joint voiding degrade the solder
> connector to a point that it would result in a shorted device
> internally.   Is a solder joint a Thermal Insulator?
>
> Victor,
>