Difference between IC layout and PCB layout

Hi,

In multilayer PCB layout, it is very common to have dedicated power and ground plane layers during layout. This results in good signal integrity.

But in IC layout, I have rarely seen anyone using a particular metal layer solely for power or ground plane purpose. Has anyone done a IC layout where few of the metal layers is only for power or ground? Why is the strategy for IC and PCB layout different? Would appreciate if someone can throw some light on this.

Thanks

Depends entirely on the design really. Top metals are usually allocated for supply and ground but if there's more than one supply it is not unusual to use the same metal for two supplies and ground. Mainly because top metals are usually thicker and better to use them for high current lines, rather than giving it all to a single ground plane.

Other reason is that they are not as beneficial to have on ICs. If you isolate things noise will still couple through substrate (triple well processes used to prevent this) and substrate is probably hotter as well (you use exposed pad to connect it to places)

And one more reason for not having large planes is that you'll need to open slots in them for them to not break during processing, so your ground plane on an IC probably won't look like a ground plane you're used to on PCBs.

Still if you have done all your routing in a few layers there is no reason not to allocate upper layers solely for ground and supply because they'll be wasted anyway. And in most cases metal layers are allocated for supply and ground. They just don't usually form a plane that covers everything (especially in digital design). Also if you're designing an RF circuit you may want to keep lines shielded from other signals or change transmission line characteristics. So there are still applications where you'd use a ground plane, but it's more like a pseudo ground plane in most cases compared to ideal ground planes in PCBs.