reason to have entire Cu layer as GND connected through via (KiCad)

Question

This is the first time I am designing a PCB and I was looking at example PCBs drawn in KiCad (the image below is my first PCB, to remove any sources of confusion).

I noticed that oftentimes, when all components are just SMDs and all of them are placed on the front Cu layer, there are vias connecting the GND of the front Cu layer to the back Cu layer. The entire back Cu layer is just GND.

Is there a good reason to do this? As you can see in the image below, my circuit is simple enough that I feel that the top layer ground pour is good enough for good grounding.

Answer 1

Component mount:

There is no specific reason to mount the components at the top side. there are some board that double sided mount. However, it is conventional to use the top side for the Components mount.

GND layers: Ground layer are useful especially in building Anlog mixed Board. any voltage can be written in V= (V+)-(V-),so the Ground here is the V-=0V. However if we limit the path of the returning current ( GND path ), we can have an induced voltage there like V-= 0.001V . so, For some components, this can be an issue.

so to overcome this issues we use the ground plane as all the components will always have the same V-.

Answer 2

The main reason one creates a copper zone on any PCB is to get low impedance current return paths without needing to think about it too much. (If you don't have a ground plane you would need to ensure to give the current a return path near to the signal to get the same result.)

For this reason one would aim to have an uninterrupted ground plane on some copper layer (In most cases one of the inside layers of a multi layer board is used for this. As the outer layers are already used for signals in most pcbs. A pcb like yours where all signals can fit on one layer is really rare.) Every GND connection is then connected using vias to this plane. (Example the ground contact of decoupling caps is connected in the shortest way possible using a via. This via will be very near the pad. Depending on manufacturing technique it can even be inside the pad.)

We can not answer the question "does my pcb need a ground plane" from a screenshot of the pcb. We do not know enough about the application. (What is the function of the PCB? What type of signals are there? Does it need to adhere to some EMC standard? ...)

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SMD parts are mostly placed on one side for manufacturing reasons. (Typically the top side of the design program is used for this.)

Reflow soldering is a lot easier if all SMD components are on the same side. It is possible to have components on the bottom during reflow but this will be more expensive. Some larger components might even need adhesive if they are on the bottom during reflow.