best PCB routing practices

Hi everyone,

I recently did a PCB board and have few questions which are lingering in my mind.
Let me put down my routing strategy(placement done),
First fanout all the SMD components to make good pwr and gnd connection through vias.I think this was my first mistake ,as near the chip I had lot of power and gnd pins,which after fanout blocked the exit of other signals.I think auto fanout board creates lot of problems for SM chips(specially chips with very small pitch).How should we make sure that for the IC the tool uses only smaller vias?After fanning out board,I routed critical nets(differential signals,clock etc.) and locked them.Then I just used auto router hoping it will finish the routing of all other nets.But to my surprise it created lot more problems.Then I had to route other signals manually.I think auto router should never be used.

People who have experience in PCB layout please comment.Is there a good strategy which we should follow to route a PCB?specially packages where the pins are closely placed.What should be the first step in routing a board?what type of vias to be used for densely placed chips?What track lengths should be used near these chips?I am new comer to this field so if my questions are stupid please forgive me.

Regards
DB

Normally I do the component layout on the board and then start the auto routing. Move the component layout a bit to compensate for obvious mistakes. Then auto route again. Most of the times you will see something will work better if on or other track was routed differently. I then route that track manually and start auto route again. Up to a point where you will see that the software can not do anything better anymore. Then I go over to manual route and complete the last portion. Normally I am happy if the software can do at least 80% of the routing for me.

Hi Archerman,

So idea is first do the component layout manually and then let auto router do other stuff.Do fanout everything on board helps?I found that it makes life more difficult to route.

One more question?Are there design constraints for differential routing in ORCAD?

Regards
Deba

Change your fanout settings, increase the via to pad distance etc.

fanout settings like sharing vias and use free vias wherever it fits.How to determine the via to pad distance?

hi
I think every software is good if your know how to working that other wise all is bundle ..hehehehee
sorry
I am using sprint layout it is fantastic software it is very easy and also very cheep and i think all requirements about the pcb designing include of this software it is very fast

Hi DB,

Placement of components w.r.t Isolation and track distance is an important criteria. The ground and supply tracks should be wider than signal tracks. The minimum track for single and double layer is 12 mils and 5 mils for multi layers. After component placing, start autorouting and make necessary track changes later.

Hope this helps you!

Route power and ground first (supply not pull ups):
Clocks and critical signals next:
Standard signals last but in order of signal speed:
Before you do this you of course need to determine the complexity of the PCB and from this the number of layers required, track and gap sizes etc.
Autoroute is not the way to go, interactive routing is, the critical stuff still tends to be better manualy routed, so you use a mixture of manual and interactive routing.
The main thing is placement, if it aint placed right you will never route the board.

Hi Marce thanks for your suggestion.
Hi Nick007,what do you mean by changing the fanout settings?

BGA Breakouts and Routing - Mentor Graphics

Download the book "bga-breakouts-and-routing" by Charle Pfeil, though covering BGA's the techniques etc are applicable for all devices. As a basic rule we only break out BGA's at the onset of a design cycle. Other components we break out interactively as the design develops.