PCB Layout Perspective - Is it how you look at it?

[paulvdh]

The schematic and the PCB are made with completely different rules in mind.

For a schematic, the main purpose is to be clear and easy to read by humans. Even whether the schematic itself “works” or not is secondary to that. The reason for this is simple to understand: If the schematic is made “correctly” but it’s difficult to read, then it’s also difficult to verify if the connections themselves are correct. When a schematic is drawn in a logical and easy to comprehend way, then errors in the wiring between parts are generally easy to spot.

For a PCB, the rules are completely different.

• EMC considerations.
• Layout that minimizes track length, especially for high current tracks.
• Sometimes a set of tracks must have specific lengths for high speed digital interfaces. (For example DDR memory interfaces).
• Solve the puzzle of “crossing track on the same layer”.
• Separation of digital and analog stuff.
• Placement of heatsinks in convenient and effective places.
• Placement of connectors in logical places.
• Placement of electrolytic capacitors where they stay cool.
• Physical constraints, such as 4 opamps in a single IC. Wires that are bundled in a connector, The size and form of the enclosure, etc.
• Many, many more…

So the main purpose of the schematic is to make it easy to understand for the people reading it, while the PCB layout has lots of rules that affect the quality of how good the end product works. A bad PCB layout can even cause a circuit to not function at all.

A big of digging in my memory and this forum brings up your older thread:

Placing tracks on the back side of PCB in Eschema

Hi, I’m working on a board that has numerous vias where tracks suddenly end on the front side and continue on the back of the board. I cannot find a way to designate tracks in Eschema as being on any side other than the front side. Am I missing something? Placing all tracks regardless of which side they will actually be on in the PCB seems to be an inefficient way of laying out the schematic? Should there be a choice of FRONT SIDE wire AND BACK SIDE WIRE in Eschema? Seems to me the abilit…

There too you are making some mental connection between the schematic and the PCB that simply does not exist.
Because the goals of the schematic and the PCB are so vastly different, the only link wetween the schematic and the PCB is the mapping from schematic symbols to footprints, and the netlist.

So stop doing that. There is no correlation between placement of parts on the schematic and the PCB. There is no “backside of the schematic” on which to place wires, etc. So stop pretending there is. The whole concept of thinking along these lines is flawed.