PCB Design and Prototyping

[PL Liew]

i have solved the 2 questions below but i'm not sure whether my answers are correct or not.

a. Explain the significance of the top and bottom layers of a PCB. What are they usually used for?

b. Can you think of any rules or constraints to follow when routing it?


Thanks if you all could help me.

 

[ronsimpson]

There is no exact answer.
....It depends on through hole or surface mount.
....Digital or analog or power.
Surface mount the signal traces are on the same side as the parts. Through hole usually the signal traces are on the other side of the board.
I often have one side mostly ground and the other side mostly signal.

Connect the power supply by pass capacitors very close to the supply pins. No long traces.

 

[misterT]

a) If the board is multilayer, then keep power planes in the inner layers and traces on the top/bottom layers. This is especially important if you have fast signals. Top and bottom layers have less parasitic capacitance than the inner layers (air has low dielectric constant).

b) It is good practice to run signal traces vertically on one side and horizontally on the other side. This makes routing easier. Electrically there is no difference between top and bottom layer.

 

[ronsimpson]

Mister T,
Item (B) is very good.
Item (A): for every rule there is an exception. Most of the time I keep the power layers on the inside. Partly because if I need to cut a trace (make a change) the inside layers are very hard to cut and jump. I want signal layers out where I can see them. Just like you said. BUT When I need a very quite board. (no radio frequency interference) I use the power layers on the top and bottom layers as a shield. The noise signal are in a copper box.

 

[NorthGuy]

There are rules imposed by manufacturing process, such as you cannot have traces close to each other or traces must be of certain width etc. These you must obey.

There are also good practices, such as using ground planes, distributing power in a star pattern rather than daisy chain, keeping certain traces short, make sure that sensitive traces are somehow isolated from high frequency traces and so on ... However, when you get many components and traces, it is very hard not to bend the rules.

 

[jpanhalt]

This tutorial is a bit old but may still be useful: https://alternatezone.com/electronics/files/PCBDesignTutorialRevA.pdf

As to PL Liew's question, I suggest that she post her answers so we can comment. That might lead to a better learning experience. If there are errors, they can be better explained and understood. As Ron says, for every rule, there are exceptions.

John

 

[eTech]

Hi

i have solved the 2 questions below but i'm not sure whether my answers are correct or not.

a. Explain the significance of the top and bottom layers of a PCB. What are they usually used for?

I think the significance is that it provides design flexibility. Signals can be separated and placed on the appropriate layer that helps improve the quality of the design. For example, if power is on top and ground is on bottom, the parasitic effect of the two adjacent layers can be used to help reduce noise.

b. Can you think of any rules or constraints to follow when routing it?

There are many correct answers to this question, but generally, it depends on the type of circuitry being routed, fabrication requirements, and manufacturing requirements.
Different circuitry types (analog, digital, RF, etc.) have different rules and constraints. An example constraint would be "signal A must not be closer than X to signal B", or "analog components must be placed on left side of board and digital on right", or "connector A must be located at left edge of board", etc.
Example design rules would be min or max trace widths, spacing between traces, pad sizes, hole sizes, and are closely related to board fabrication or manufacturing requirements.

Hope that helps...

eT