Welcome on board and welcome aboard have the same meaning.
Pilots and cabin crew usually prefer to say Welcome on board.
Welcome aboard is more common to use when someone joins a team. It is a friendly way to let someone know that they are welcome to become part of the team.
So, Do you have any questions about the company?
No, everything is clear!
I want to welcome all passengers on board this Boeing 747 today.
The phrases welcome on board/aboard mean that you are welcome to board this vessel(boat, train, plane, etc).
In a modern context, this also includes teams(especially in business) which comes from the idea that the business is moving forward like a train.
The phrase probably comes from when boards/planks were used to enter a boat.
Is it welcome on board or welcome aboard for a new employee?
You can say either really, It doesn’t matter.
For me, I would use welcome on board if I needed to be formal (in an email for example).
If I wanted to be friendly and welcoming, I would use welcome aboard.
On Boarding a new employee meaning
On boarding a new employee is the process that a new employee goes through to become familiar with the new company.
That could include setting up a company email address, learning where everything is, and becoming familiar with the software that a company uses.