james tornes wrote:
> A guy giving a presentation to us was looking at various motherboards
> and making assesments like "this is a 4 layer board", "that one is 6
> layers". Can someone actually differentiate with an average eye,
> between boards with 4 layers and 6 layers? Thickness of boards can
> vary a bit even in-between runs so what's the technique here?
> 
> Jim

One easy way is to look at the layer window if one is present.

A layer window is an area on each layer with no copper on either side 
except for a layer number in it.  The window might be roughly 4mm wide 
with the length equal to the number of layers times 4mm.

Each layer's window will be near the previous one so they end up in a 
row.  When held up to the light, you can read each layers numbers.  This 
allows you to verify that all the correct internal layers were assembled 
in the correct sequence.  Thus a four layer board will have a 
rectangular window with 1234 in it.  Un less the pcb house screwed up 
and it reads 1324!

I have done 4 layer windows but they might be difficult to read with 
something like a 16 layer board.

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