Larger traces is the same as thicker wire.
Think about fuses, or check out the Kicad calculator on the front page. 
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Have a play with the Calculator Tools . . .
Example 100mm long track, 1oz copper thickness, 0.25mm wide . . .
now 0.5mm wide . . .
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Unless he edited it in the mean time, thinner is less resistance.
Wider tracks also mean more capacitance, which most often a bad thing. 0.25mm wide tracks is wide enough for easy manufacturing for most PCB manufacturers, and it is good for up to about 200mA, so it’s plenty for all logic signals. Only for power tracks wider tracks have an advantage because it lowers the resistance and voltage drop. I think a bit of resistance is even good for logic level signals, as it dampens oscillations, but it’s probably negligible on a PCB scale.
Your GND plane is atrocious though. And from the way I understand it (contrary to what Jonathan_Haas wrote) a poorly designed GND plane can be detrimental to the design. Area’s of copper that are only connected on one side can act as antenna’s to both pick up and radiate noise, and you can have standing waves in such area’s.
A proper GND plane is a quite important part of a PCB, especially in these modern times with ever higher speed logic (I’ts the flank steepness, not the signalling frequency that counts). A properly designed GND plane is continuous over the whole PCB (except for designed in exceptions such as real antenna’s and isolation barriers and such) In a proper GND plane there should not be a single interruption that is bigger then about 3 to 5mm. That means modifications such as only using short hops to get under other tracks, making the standard clearance smaller so the GND plane sneaks in between the pads of your THT headers and IC’s, and also avoiding groups of via’s that can create bigger holes.
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I don’t understand who’s right and who’s wrong now haha
It’s true that with the calculators, thicker traces leads to lower resistance…
I know my GND place is atrocious and that’s why I shared it but I feel like I don’t have the experience to make a better one. It’s the first circuit I make with so many wires everywhere…
The speed logic isn’t going to be very important here as I’m not trying to retrieve any data from the SD card. I am only measuring the resistance between every pin (with a voltage divider) and plotting the voltage-current characteristic of each pin relatively to the ground. It’s only like less than 15 measures per second so no high speed logic or anything (everything works analogically except the command of my MUXs).
So your main recommandation would be to try and make the holes in my GND plane smaller, even if it leads to more of them ?
physics 101, resistance of a wire (near DC): R = ρ (L / A) with A being the cross section. So bigger cross section = less resistance, with cross section being a (linear) function of the width of the wire → wider wire = less resistance.
first thing you could do is adding “bridges” for GND everywhere you have long divided sections. also try to minimize the routing on the bottom layer by using it only for jumping over some conflicting wires, not for long tracks like you have it right now. I assume you don’t want to manufacture the PCB yourself so adding more vias is nothing you should be afraid of.
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Was just about to add something similar, instead I’ll just add this for further reference: How to Calculate Resistance Using Resistivity | Physics | Study.com
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I can confirm. It is valid recommendation always.
But in amateur designs may not be taken very seriously.
I used to design my 2 layer PCBs with all bottom being GND with no breaks at all.
Here is an example of such PCB:
Blue is everywhere except one 3 pin THT connector.
All vias are GND.
That is definitely a big resounding YES with dominating GOD voice and double echo’s. Big holes hurt the integrity of your GND plane, and the smaller each hole is the better. The total number of holes hardly matter at all.
To underline the importance of a good GND plane you can watch the video below from Rick Hartley. He made an over two hour long presentation of the importance of the GND plane combined with background explanations and design suggestions. All two hours are worth watching, and this again emphasis the importance of a GND plane.
As long as you are working at a hobby level and with relatively slow microcontrollers (it’s the signal flank steepness, not the switching frequency) then you can still hump a long with impaired grounding, but as you go into more faster digital IC’s, sensitive analog stuff and also have to comply with EMC regulations, then a good GND plane becomes essential to your PCB.
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I knew that formula but messed it up in my head between the section S and the length.
Thanks for the other tips!
Thanks for all the tips everybody, I will improve the PCB with your recommendations when I have the time. Then I’ll post the GND place again to see if there are any improvements.
Thanks again:))
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I’m sure I echo everyone’s thoughts when I say . . . You are Welcome 
Try and watch some of the video above, I stole some time at work today and watched 90% of it, it will help you loads with GND and Power planes and good practice in relation to signal tracks.
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This is extremely surprising.
Remembering this relationship in the form of a formula is a mistake as you just can missed it up in your head, as happened to you.
The longer the riverbed and the greater the distance the water has to cover, more stones in the way, creating more resistance in total.
The wider the channel, the easier it is for the water to flow through, i.e. it encounters less resistance.
And also V=RI.
The higher resistance (the longer the riverbed or narrower channel) a greater level difference (voltage) is needed to ensure the same flow of water (current). To increase flow level difference have to be increased.
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I also watched a great part of the vid and found it very interesting not only for PCB design but in general regarding my electronic culture around grounding and interference. I learnt a lot of things even though I don’t see exactly how I could apply the different advices given in the video without redesigning the whole PCB… And I am unfortunately running out of time on this project. But now at least I don’t have any very “big holes” anymore as you can see :
But I still have those long wire holes though… At least they’re thin so the surface isn’t that big meaning they might not act like antennas that much.
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I find this very amusing because my whole interpretation of wires was the other way around. I considered imagined when you make a wire wider there is more volume to go through for electrons so more obstacles… just like for the length.
And of course the longer it is the more resistance that’s something I always understood and made sense for me.
Thanks for sharing your analogy!
But if electrons ‘decide’ to use only part of wire (as it was before making it wider) then R is not changed. The rest of wider wire gives them other ways to go in paralel so in the same time with the same voltage more of them will cover the distance so resultant resistance is lower.
So for your home’s wires to handle a huge amount of current, they should be made as thin as possible. Something wrong with that logic, I think. 
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The analogy I’ve always used is a water tank with and input pipe (with tap) at the top and an output pipe (with tap) at the bottom. It covers many electrical fundamentals.
I’m not sure if you follow his thinking well.
Huge amount of current obviously needs a wide wire (to have a place for that current) but unfortunately the wider wire has higher resistance but who cares…
Isn’t folk physics fun? Like when Wile E Coyote runs off a cliff horizontally for a distance then drops vertically. What’s more he doesn’t start dropping until he realises he’s in mid-air. 
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