During combat, when can I trigger upgrade cards?

Shield wall for example can be exhumed to grant an extra armor. I can't find where it says it has to be used before the dice are rolled.

thanks

Shield wall is always in effect when you are the defender and have more trays than the attacker.

3 minutes ago, Drakoniss said:

Shield wall is always in effect when you are the defender and have more trays than the attacker.

Yes, but you have to exhaust it. Carnage4u is asking when do you need to actually do that. Shield Wall is a "while" effect so it takes place over the specified event, that attack. Under Abilities in the Rules Reference, you would use the upgrade when the event would occur, so during Phase 2 of an attack after an enemy targets the unit and it's a legal target (i.e. los).

Yes the second you become the defender you chose to use it or not.

4 hours ago, Drakoniss said:

Yes the second you become the defender you chose to use it or not.

Not exactly. Shield wall is a while effect and according to 83.6 in conjunction with 10.12 you can use it at any point during the attack. This means that you can wait until your opponent has finished modifying the dice, then use shield wall.