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That is a tough question. Unlike wooden studs, steel studs are not the best material to hang anything from, let alone cabinets! The first time most people see a steel stud they are amazed at how flimsy and seemingly weak they are. And they are! That is, until they are part of a completed wall. The weaknesses of the stud, mostly related to twisting and bending, are eliminated when attached together and then to drywall to make a complete wall. Here are some tips… use them to help you decide the BEST way for your wall!
In a kitchen, wood or plywood strips are often installed between the metal studs to give a strong nailing/screwing surface to attach the cabinets to. Some contractors install additional metal cross pieces between the studs instead of wood to allow for this additional screwing surface. If you are dealing with old work, though, you have to first try to determine if there are any "nailers". You can use a magnetic or electronic stud finder to look for horizontal nailers. There are special self-tapping screws that are used with steel studs that are available at most hardware stores and lumberyards. An air or electric powered screwgun or an electric drill with a screwdriver bit must be used.
If there are no nailers to be found, you have a few options. Screwing the cabinets up to the metal studs with special self-tapping screws used for this purpose will offer some measure of support, but I wouldn't trust them as the sole support for the cabinets, especially if they are large and heavy. You can try these two techniques to add additional support… 1) retrofitting nailers or 2) using toggles.
1) Retrofitting nailers can be dusty and messy but allows you to install the cabinets in the traditional way… all screws! First, cut out neat, horizontal strips in the dry wall behind the cabinets at the height you are planning on screwing them to the wall. Then install wood crosspieces between the steel studs, screwing them to the studs. The size and type of wood will depend on the size and type of studs... the largest piece you can get into the wall! You might have to custom-rip the 2x3s or 2x4s to get a good fit. Replace the drywall scraps to make the wall level and put up the cabinets. You don't even have to "finish" the entire repair, since it will be mostly behind the cabinets.
If for some logistical reason you cannot attach the wood strips to the studs or if it appears that the wood strips will be too flimsy, you can still make use of this technique by reinforcing the wall instead. This can be useful if the wall is a very thin, nonstandard (less than 4 ©" thick) interior partition wall. Cut a piece of 3/4" plywood and insert it into the wall so that it overlaps the uncut drywall above and beneath the cut at least a few inches and extends between the studs. Screw and glue the nailer in place. Positioning it can be tricky since there isn't anything to grip once the entire nailer is in the wall. To give myself something to hold onto so I can pull the plywood to the wall, I partially screw in a couple of 2 to 2 1/2" screws as "handles" to help me position the wood. I remove them once additional supporting screws have been installed through the drywall. Again, replace the scraps to level the wall and put up the cabinets.
2) You can also add additional support by using 1/4" toggle bolts or togglers. I have done this with good results, but I have to warn you that it is a little tricky to get the cabinets up using toggles!
As always, it helps to have a helper! Figure the location of all toggles and self-tapping screws and mark them on the inside of the cabinet. Pre-bore holes for the toggles that are just large enough for the "toggle" to go through the cabinet. When you install the toggle, use a large washer to keep the screwhead from dropping through the hole. Figure on one toggle between each stud. The easiest way to install the toggles is after the cabinet is hung with the self-tapping screws... minimum two per stud. Once the cabinet is up, use the toggle holes you already drilled into the cabinets as guides and drill through the drywall. Install the toggles or togglers (with washers preinstalled) and tighten them down securely.
With either of these methods… depending on the room aesthetics… you can also attach a 1x2 wood strip on the wall to give the cabinets some support along the bottom edge during installation. This makes alignment a little easier, especially if you are working solo. This strip can be left on the wall after cabinet installation to provide additional permanent support or removed and the wall repaired.