rule of thumb for choosing gow many pcb layers needed?

[pute]

Hi,

I'm new in designing a complex digital pcb design. I'm wondering is there any rule of thumb for choosing how many pcb layers needed? For example, what is pcb layers needed when the SOC has 400 pins (although pins used around 300)?

thank you,

Pute

 

[dmd4raj]

First we have to confirm the list of digital high speed signals(Diff pairs),and rest of digital single ended(Controlled imp., traces) sginals.Differential pairs should be routed with specified trace width and air gap.Minimum of 100 mila pair-pair spacing to avoid cross talk

if it is any analog signals then this should be seperated from digital high speed signals (clock0to avoid the coupling ofswitching noise .

Also first confirm the board size....Also every signal layer(routing)need a adjacent ground reference place.Also if u are handling different power with higher current rating then we need a seperate power plane

No of layers should be EVEN

If any high speed interfaces(USB 3.0,HDMI,GIGABIT ETHERNET,SATA)then we nneed a separate Signal layers with Rogers or NELCO as Dielectric.(to minimize the dielectric losses)

preferably this can be on top(microstrip line)to have high speed propagation speed.

While designing stack up copper weight should be balanced to avoid the warpagee

ie -------------------top(routing)
-------------------(ground)inner 1
------------------------------------------------------------------------------------------
--------------------power-inner2
--------------------bottom(routing)


if split the stack up into 2 equal parts then the layers should be mirror image of another part

Based on this select the no of layers


regards
Rajan .K

 

[vijay s]

hai Rajan .. so u have improved urself lot....

what is that rogers and NELCO

 

[lhademmor]

gigabit and sata needs Rogers ??? We do without any problem make FR4 fiber board designs for both 4 and 100 GIGA and the 2 sata defines.

And Yes are working in the fields to day without any issues.

 

[Rossano]

Hi,

an additional rule of thumb for BGA 0.5mm pitch.
You need a layer to coming out of two BGA rings.