Correct Ground Trace

[mike_mike]

Hello, I just found that my layouts does not have correct ground traces, so I want to learn something about designing the ground trace.
I took the attached schematic and I want to make the correct ground design.
Please have a look at the PCB layout that I draw and tell me what should I improve.

[tggzzz]

Hello, I just found that my layouts does not have correct ground traces,

What makes you think that? In what way do you want to change performance?

In general having a solid ground plane is A Good Thing. But without a definition of your problem, we can't say what a solution might be.

[RoGeorge]

The dedicated therm you are looking for is "ground loops", not ground traces.  The name 'ground loops' suggests it should be something about GND, but this is not about GND only.  It applies in any circuit where two independent current loops have a common path.

The common path/trace have a small resistance that is not zero, so the current from one loop will create a voltage drop on that portion of the common circuit, because of that small resistance of the common path (the common path in practice is a portion of a wire or a segment of a PCB trace).  The voltage drop created by one current loop will appear as a perturbing voltage source for the other current loop, and this is usually an unwanted effect that must be avoided.  For example, a ground loop can make a perfect amplifier to auto-oscilate, and so on.

Search about ground loop, why are they bad, and how to avoid them in practice. 

For a good PCB layout, ground loops should be avoided.  This is about ground loops in general, I didn't check the layout of your PCB.

[Rerouter]

A starting fix would be the large input capacitor, general rule of thumb is to have your loads connected to the capacitor, and then from the capacitor to the supply rails (decoupling)
same for your switch, have it connect to its decoupling capacitor then the power input plug.

This kind of circuit really should not need a star ground, and would also agree that a solid plane would meet most roles (Ironically I am working on a similar circuit where start grounding matters, but that is with a 6.5 digit ADC)

[tggzzz]

The dedicated therm you are looking for is "ground loops", not ground traces.  The name 'ground loops' suggests it should be something about GND, but this is not about GND only.  It applies in any circuit where two independent current loops have a common path.

There is insufficient information in the OP's post to draw that conclusion - which may or may not be the case.

[RoGeorge]

Indeed. It just happened that I already knew the project where from this question is derived, and just wanted to clarify the name for the OP:  https://www.eevblog.com/forum/beginners/arduino-pid-soldering-station/