Can Aluminum and Copper Grounding Conductors Land on the Same Bar?
Yes, provided the bar is listed for both copper and aluminum and each conductor gets its own terminal. The rule people are quoting when they say dissimilar metals cannot touch is NEC 110.14, and it applies to a single terminal or splicing connector, not to a shared bus bar.
What 110.14 says about dissimilar metals
Section 110.14 states that conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors, unless the device is identified for the purpose and conditions of use. The concern is galvanic corrosion at the point of contact, plus the very different thermal expansion rates of copper and aluminum working the connection loose over years of load cycling.
Notice the scope. A terminal or splicing connector. A ground bar in a panel is a bus with many individual terminals. Copper under screw four and aluminum under screw nine are not intermixed in a terminal, they are two separate connections that happen to share a bus. Almost every ground bar kit sold for residential and commercial panels is listed AL/CU, and that listing is what permits both metals on the bar.
What actually gets an inspector's attention
The violations that show up on real inspections are usually different from what the homeowner or the apprentice was worried about. Section 408.41 requires each grounded conductor to terminate in an individual terminal, and while equipment grounding conductors have more latitude, the terminal must be listed for the number of conductors landed. Two grounds under one screw on a bar listed for one conductor per terminal is a 110.3(B) violation regardless of metal.
Section 250.8 lists the permitted means of connecting grounding and bonding conductors: listed pressure connectors, terminal bars, pressure connectors listed as grounding and bonding equipment, exothermic welding, machine screw type fasteners, thread forming machine screws, connections that are part of a listed assembly, and other listed means. Sheet metal screws are specifically not permitted. That one catches people more often than dissimilar metals ever does.
When the inspector is right
There is a version of the objection that holds up. If aluminum and copper are landed under the same screw, or twisted together under a single wire nut that is not identified for combining the two metals, that is a straight 110.14 violation. The same is true of an old aluminum lug that is not marked AL or AL/CU with a copper conductor jammed in it. Check the marking on the bar itself, since a lug marked CU only is a copper conductor lug and nothing else.
Aluminum also needs its own prep. Most manufacturers call for a wire brushing and an antioxidant compound on aluminum terminations, and 110.14 requires that any compound used be suitable for the application and not adversely affect the conductors, equipment, or both. Follow the instructions in the box per 110.3(B), and torque the connection to the marked value per 110.14(D), which matters more for aluminum than for copper because aluminum cold flows under pressure.
If you want to end the argument on site, photograph the AL/CU marking stamped on the ground bar and the one conductor per terminal arrangement. That is the documentation that resolves it.