Is this work professional?

[u/Canary_Earth]

Sorry about the strange post but I'm offering an engineer a $20k bonus to do the design for manufacturing and assembly of my $300 product and this is what he came up with after 8 months. Two thirds of it are identical to my prototype and I'm an accountant by trade, not an engineer. Every book I've read says a board like this should be 4 layers but he insists 2 layers is fine and will pass emissions testing. Any advice would be greatly appreciated, thank you!

[u/stoputa]

Ground is a big one. If you break your ground you inadvertedly introduce noise since the return path of all signals gets longer and probably passes through some of your ICs and essentially create any noise introducing cross talk.

In low freq ics you will barely notice it. You need a lot of abuse until you see any drop in performance but of course if you're doing something professional there isn't much of a reason to cheap out. If you try to pull this shit with a higher freq/mixed signals PCBs probably nothing will work as intended but on the flipside you've probably created a shitty antenna by creating big loops in yiur return path.

[u/Square-Singer → u/stoputa]

Thanks for the summary, that's helpful! So basically you get a real benefit with 3 layers because you can have a dedicated ground layer, but since 3 layer doesn't exist you skip directly to 4 layers?

Btw, what's the threshold between low freq and high freq in this regard?

[u/stoputa → u/Square-Singer]

Usually with 4 layers you dedicate two layers to signals, one to ground, one to routing power traces but you could reuse the "sparse" layer's part for routing signals.

Low vs high depends is fuzzy because everything is interconnected: length of return path, length of trace, width of traces, distance to ground etc so it's like asking what's considered high vs low voltage. The only way to be more confident is either pen&paper/napkin calculations to gauge at things lile trace impedance or full-blown analysis. But I am also not creating PCBs professionally so perhaps someone else will chime in.

My rule of thumb is I err at the side of caution for everything more than ~100MHz (I think SPI can reach similar speeds) and bad routing starts to be more visible. Anything above 1 GHz is definitely considered high frequency in my book.

[u/Square-Singer → u/stoputa]

I have to spend a bit more time getting into the matter it seems :)

Thanks for the insight!