Hallo Mahmood,

may be it's possible to produce brilliant PCB with autorouter. But in my whole carreer I never saw any good one!
The major problem with autorouters is, that their intention is to fabricate optically nice looking layouts, but not physically nice looking! An autorouter places all parts in order to use PCB area very economically, so that no area is wasted and the best use of whole rectangle is made. This often results in an arrangement, where all parts are equally spaced and are sitting in rank and file like tin soldiers. Indeed very nice looking, but physically providing a desaster, sometimes!

Why?
Because now some parts are located at places where they are not needed, far away from that location where they should actually sit for providing optimum performance. And because of this distance often much too long and much too thin copper traces are used for according connections.

I will talk here about only one subject, because space is limited: Ground (0V)! If you have a look at schematic in most cases you will only see ONE ground. (Where analog and digital parts are mixed, sometimes two grounds are distinguished.)
The important fact is now, that pedantically spoken, not only ONE ground exists, but as many grounds as ground connections are shown in schematic!! And it can make a really big difference how these 'identical' grounds are connected to each other.

Some examples:

1. All kind of RFI filters show a very 'hot' ground point. Connected to PCB ground at the wrong place can result in a total eroding of dampening perfomance. Professionals distinguish between PCB ground and 'chassis ground', e.g.

2. All ground connections, where big and/or noisy currents can flow must be connected very carefully to other grounds. Otherwise ground noise will heavily rise. Typical example: Rectifier section of mains transformer power supply. If system's ground is connected to a ground track inside rectifier - strorage capacitor loop (instead of 0V pin of regulator!), an extreme amount of hum can be superimposed to Vcc. The dead of every low level analog signal application...

3. If cables with screens are used, shield may only be connected to very certain ground points. Also here a special 'chassis ground' should be distinguished. An autorouter would connect it somewhere to PCB ground, probably even using stupid pigtail technique.

4. Some parts need connection of decoupling components as near as possible. This very often collides with placing logic of autorouter. At the end, there can be sometimes a distance of even centimeters to decoupling component. Added inductivity is so high then, that use of low inductivity capacitors like SMD tantals, etc, makes absolutely no sense. This can be very tragical especially with negative voltage regulators and low drop voltage regulators (!!!). But with any kind of switcher this will surely end up in a desaster, sometimes with lots of smoke...
All these parts need very low and stable impedance of decoupling measure over a wide range of frequencies. If this is violated, instability results.

5. Ground connections often carry very high frequency currents. In order to avoid relevant loops (radiation!!) connection to PCB ground is critical. In microcontroller circuits especially their associated quartz oscillator with it's burden capacitances highly suffers from these currents.
I know from some microcontroller boards, which failed CE radiation test, only because parts of quartz oscillator were located in a wrong way!! Burden capacitance must be located directly to GND pin of microcontroller, no other ground current must flow across these ground connections!

6. Digital signal traces with high toggling rate (oscillator signals) contaminate adjacent parts of groundplane due to unavoidable stray capacitance. These ground areas should be located far away from sensitive parts of circuit. So, oscillator signals must be routed very carefully and adjacent groundplane areas are 'very special' ground points...

I could increase this list very easily. All these points tell me, that there IS a difference between certain ground points. I can manage it, by introduce of certain 'chassis ground', by specially arranging decoupling capacitors, by using a groundplane where possible, but using ground point star routing where necessary, by avoiding pigtails, by avoiding high frequency radiation loops, by keeping oscillator signals as short as possible and as far away as possble from sensitive parts of circuit or edges of PCB, by etc. etc. etc.

BUT MOST AUTOROUTERS WOULD NOT MAKE ANY DIFFERENCE BETWEEN THESE CERTAIN GROUND POINTS!!

Regards,
Kai