Hello Bluebird,
More than 2-layers....... well, it depends on what the end-target is.
Over the past 15 years or so, most of my designs have been 4-layer, with a handfull of 6-layer boards and a couple of noteworthy 8-layer boards, and yes, they are all used in high-end analog audio.... no digital clocks
I do have the advantage of having in-house pick & place, so most designs are SMD, however, putting that aside for the moment, the advantage of multilayer are many if you seeking high-performance in regard to crosstalk control and power distribution.
Don't be scared of multilayer; the advantages are many.
This particular project required 4 and 6 layer PCBs:
http://www.cmlaudio.com/nm1249-v-mini-master-sidecar-control-panel/
The reasons were:
a) to achieve a particular set of facilities in a pre-defined physical space.
b) to achieve high levels of performance (mimimal crosstalk) between a number of signal paths that are in close proximity.
Both requirements were met and would not have been possible with double sided PCBs.
There are always traps that can be fallen into; others contributing to the thread have noted some of them such as parasitic capacitance. This is more of a problem where circuitry is at medium to high impedance (DI input stages, equalizers, signal processing circuitry) and less of a problem in amplifier stage outputs and switching matrix designs, however, an awareness of component and trace proximities, signal levels, impedances and their effects on each other will go a long way to designing-out the "banana skins" that can cause you to slip-up and create a less than optimal design. I have been doing PCB design for over 40 years and still slip-up at times..... usually when trying to get a design done in a hurry, or squeezing in that accidentally omitted connection.
A decent quality CAD package is also a high priority. You don't need to go as high-end as Altium (you are very fortunate if you can) but it is not a necessity. What is a necessity is that the CAD can really do multilayer well and that you can control the copper-flood / copper-pour / power-plane / ground-plane parameters. A rock-solid forward-annotation and back-annotation ability is another essential facility. Also, totally trustable dimensional error checking that can be set for custom trace-trace, trace-pad and pad-pad gap checks is an essential. CAD is a really personal choice, my own preference being Seetrax Designer XL. Trust and reliance on your CAD to tie the schematic to the PCB and
always keep the two in snyc are essential. JR noted that inner-layer traces are impossible to debug and you have to know that the CAD has done what you have expected it to do.
One really obvious advantage of hiding traces on the inner layers is that it is really hard to reverse engineer a design.
Multilayer is a good way to add a level of protection to your design IP.
Always, and I really do mean
ALWAYS use thermal relief on any hole that connects to a solid copper area. If you dont, you
will break plating on through-holes when trying to de-solder a component. Also, by making holes bigger than necessary, you can often suck out solder around all of a component leg, therefore reducing hole plating damage when removing components.
Where multilayer really scores in analog audio is the ground-plane. The number of designs I have seen over the years where performance has been trashed or compromised by poor ground connections (usually scrawny-thin traces) is depressingly high. A multilayer with a groundplane would have solved many problems.
I have never needed or wanted to put blind or buried vias into a design. Too expensive, special manufacturing processes required, specific layer pairs required, definitely need to discuss the manufacturing parameters and requirements with the PCB manufacturers...... in short: blind & buried vias - just say no!
Through-vias are not a problem at all. Matador describes them perfectly.
Choosing what to do with your layer stack is something that some designers fret over and over-analyse.
This, in overview, is what works for me as a 4-layer stack and has been proven to be highly successful:
Layer 1 (component side): inter-component signal traces & any other short point-to-point traces (decoupling caps to ICs).
Layer 2: Groundplane.... all of it.... no breaks.
Layer 3: Signals & power, mainly signals. Flood open spaces with power or ground.
Layer 4: Signals & power. Ground flood of open spaces.
Using L3 & L4 for power allows wide low-impedance power traces. Use of multiple parallel vias to link power traces between layers is a neat trick to adopt. The same is true when linking the L2 groundplane to other ground-flooded areas of other layers.
In a 6-layer board, my layer assignment is typically:
L1: Top components & interlinking signals & short point-to-point traces.
L2: Groundplane
L3: Signals & power & screening ground
L4: Groundplane
L5: Signals & power & screening ground
L6: Bottom components, Signals & power & screening ground
An 8-layer board is initially as-per the 6-layer, then,
L6: Groundplane
L7: Signals & power & screening ground
L8: Bottom components, Signals & power & screening ground
Some design-rules for adjacent layers with signals on them include:
Avoid trace-crossings, except at 90 degrees,
Never run traces parallel above each other,
Use copper floods for screening of adjacent layer signals.
Most of the PCBs in this console design are 4-layer, with 6 & 8 layer where needed.
https://www.soundtechniques.com/our-gear-test/
The heart of the console is the centre-section card cage. This has 26 plug-in cards (all multilayer) that deal with balanced inputs, balanced outputs, mix amps, console logic, monitor selection, group signal paths, etc, etc. The backplane system for the card cage comprises of a pair of 8-layer boards, one deals with inter-card connection in the card-cage and the other handles the cabling to the centre section modules, the console channel buckets and the console external connections.
Cheers,
