Steampunk

“I tell you all the time, you will never be able to replace me with a brass and steam contraption.” ― Maeve Alpin

This field of technology focuses on creating devices using steam as the main source of energy. It is sometimes confused with Clockwork tech as it also includes quite a few gears among the pistons, hydraulics and pneumatics.
This tech is second most common after Magitech. It is most commonly used for vehicles and guns.
But it doesn't mean that there aren't uses for it in other fields as well.
You can make massive exoskeletons with steaming engines on the back and cannons to provide offense.
Or mysterious steaming boxes that only you know the purpose of.
Robots are probably the third most common area where this tech is used.

Steampunk for Battle Engineer

There are no particular requirements to choose Steampunk.

Materials

Raw materials for steampunk machines aren't rare. But if you collect them raw and then craft them into usable parts by yourself it will take some time. Though most steampunk engineers simply buy or order the stuff they need from other craftsmen. 
Steampunk machines mainly use materials like:

• Metals like copper, brass, steel and iron
• Rivets
• Gears and cogs (need to be custom made in most cases)
• Wood
• Glass
• Light bulbs (need to be custom made in most cases)
• Leather 

There aren't any stores that sell steampunk parts, so most artists have to do a lot of legwork to find materials for their projects. Materials like metal need to be then turned into adequate parts either by you if you possess the right tools and skills or by local artisans.
The exact materials needed for each creation is either written in the description of the creation or defined by the DM at the moment of need. 

Sources of energy

The source of energy for steampunk machines is in the name, Steam. Most creations use coal-burning stoves or other sources of heat powering steam-driven engines.
The stoves are bulky and the bigger the energy requirement the bigger they will get and the more coal or other source of heat will be used.

Size	Coal per 3 uses or hour of activity
Tiny	0.5 lb
Small	1 lb
Medium	5 lb
Large	25 lb
Huge	60 lb
Gargantuan	250 lb

Particularities

You start with some materials and basic parts (worth total of 25gp) in your Mobile Tinkering Platform. 
Steampunk creations are easy to repair even without additional materials. 
If repaired without the adequate materials/tools, the creation has a higher chance of failure when used. 
You can replace one of your skill proficiencies in granted by this class by Persuasion proficiency. This indicates that you're used negotiating with merchants and artisans.

Exoskeleton example 
Artificial life/robot example
Gadget example