Its not the connection of pins that makes PCBs so hard.
Its the random creation of antenna that suddenly throws your electrons off the board and suddenly causing nearby traces (to also turn into antenna) to get crosstalk leading to failure. Or throwing your electrons off the board and failing EMI regulations.
With 1ns rise/fall times on typical microcontrollers (even a 4MHz ATMega328 / Arduino), an antenna can be inadvertently created with just a 75mm trace. With a faster chip, you could have antenna at 25mm or even shorter traces.
-----------
PCB board design today is about understanding the physical attributes of the PCB: how the stackup causes your board to act like a capacitor (and therefore a source of voltage when the bulk / decoupling capacitors run out of energy). Seriously, this is called "PDN" or power-distribution network. You turn Layer2 into a ground-plane and add a plane to Layer3, calculate the dielectric thickness and estimate the amount of latent capacitance you get between a 30mm^2 ground vs power plane seperated by 0.1mm of dielectric.
And not only do you calculate this, you *take advantage of it* to reduce high-frequency (ie: 1GHz+) power-delivery issues.
Which could happen as low as ~100MHz in practice, because its more about rise/fall times than it is about the actual clock speeds.
--------
There are two kinds of PCB designers. Those who know that they're designing an antenna... and those who do not know they're designing an antenna. The PCB itself is an antenna and you need to design it in a very peculiar manner to minimize radiation. You can't just guess-and-check this (I mean, I guess with enough time/money you can guess/check... but ideally you lay things out with a mental simulation of inductances, eddy currents, an idea of electric fields and return paths of your signals across low-through-high frequencies)
Hmm that's fair. I think for most hobbyist applications though it really is gluing a bunch of things together, up until the point where you have two get FCC certified or stuff behaves erratically and you have to go and learn this stuff.
If you're dealing with low speed (ie: 50MHz bandwidth aka 20ns rise/fall times or slower) circuits, then you don't need an autorouter. Just connect every pin of your circuit together and you're done. You might need a via to cross into and route inside of your ground plane, but at low-frequencies that's not a big deal because it'd be literally too small to be an antenna and radiate off. Its just an effect that you can almost entirely ignore.
If ~20MHz is the highest speed signal on your entire design (ie: ~20ns rise time, which STM32G0 or PIC or AVR beginner-friendly chips are around), then your wavelength is 15-meters, meaning the quarter-wave antenna would be (at a minimum) 3.75 meters (12 feet long), which is just far larger than any PCB-circuit a beginner will ever come across. (And will only in practice, be an issue on cables running off-board).
Its the random creation of antenna that suddenly throws your electrons off the board and suddenly causing nearby traces (to also turn into antenna) to get crosstalk leading to failure. Or throwing your electrons off the board and failing EMI regulations.
With 1ns rise/fall times on typical microcontrollers (even a 4MHz ATMega328 / Arduino), an antenna can be inadvertently created with just a 75mm trace. With a faster chip, you could have antenna at 25mm or even shorter traces.
-----------
PCB board design today is about understanding the physical attributes of the PCB: how the stackup causes your board to act like a capacitor (and therefore a source of voltage when the bulk / decoupling capacitors run out of energy). Seriously, this is called "PDN" or power-distribution network. You turn Layer2 into a ground-plane and add a plane to Layer3, calculate the dielectric thickness and estimate the amount of latent capacitance you get between a 30mm^2 ground vs power plane seperated by 0.1mm of dielectric.
And not only do you calculate this, you *take advantage of it* to reduce high-frequency (ie: 1GHz+) power-delivery issues.
Which could happen as low as ~100MHz in practice, because its more about rise/fall times than it is about the actual clock speeds.
--------
There are two kinds of PCB designers. Those who know that they're designing an antenna... and those who do not know they're designing an antenna. The PCB itself is an antenna and you need to design it in a very peculiar manner to minimize radiation. You can't just guess-and-check this (I mean, I guess with enough time/money you can guess/check... but ideally you lay things out with a mental simulation of inductances, eddy currents, an idea of electric fields and return paths of your signals across low-through-high frequencies)