I thought I'd be able find this easily via google, but apparently not, so: What pressure can 'normal' consumer-packaged chip withstand?
(To give a specific example, say the OMAP4430)
I'm presuming it's complex parts like this that would break first under high pressure, instead of simpler passive parts, or the PCB itself.
I'm also assuming the pressure is applied hydrostatically - so air gaps under components, provided they are not completely sealed, will be the same pressure as the external pressure.