it depends:  your board layout (potato chip), board material (FR4 or  
polyimide for example, CTE mismatch between chip/BT material used in  
the device), size of the device, number of the ball and its layout  
(some of them has corner ball larger than center for example),  
etc.etc.  You need to do your homework and better- do a FEM or get  
FEM data from your device vendor and recommendation on top your  
homework.  IMHO.
If you doubt, underfill it with PROPER material (some of them  make  
it worst not better).  best of luck.
jk
On Jun 9, 2018, at 11:11 AM, haviv gendelis wrote:

> Are there any rules for types of component to be under filled, for  
> high reliability application?
>
> Sent from my iPad