AVR-Chat

Russell Shaw wrote:
> Kathy Quinlan wrote:

>>I need someone reliable and able to do fine etches as I do not like the 
>>price for 4 layer (well I do not care, but customers do ;), so I tend to 
>>squeeze it all on 2 layer (things like M128's, 2x Max232's,74vhc08 and 
>>SD card all on a PCB 3" x 2").

Just as an aside.  This works almost all of the time but the thing to be wary of 
  that is making this more difficult is that as geometries shrink the edge rates 
are climbing. The part number of the chip may remain the same.

While the CPU might be running at a low clock speed, the rise time of the edges 
has fallen dramatically. This means that switching transitions may cause tracks 
to radiate where they may not have before. This in turn means grief when trying 
to meet EMC specs. While you may not be interested in actually doing the trip to 
the lab the bottom line is that if the gear can't meet the spec then it is also 
liable to intermittently failure in the field due to susceptability issues i.e. 
if tracks are radiating then there are also likely to be tracks that are making 
good receiver antennae.

Series resistance, filtering & ground planes are the obvious things that can 
help. In the case of the groundplane, on a two sided PCB the active layers are 
widely separated by the core & so the effect of the groundplane is greatly 
reduced, this is not the case where on a 4 layer board where the signal layer is 
far closer to one of the power planes.

This relates to the other current discussion re insurance, a 4 layer PCB is 
better insurance and could also be construed to better in terms of "Best 
Practice" if defending a product failure.

A few board houses in Aus are doing 4 layer proto's at a reasonable price, BEC 
is one that we have used. Peninsular and Entech are others but I haven't used 
them personally.

Having said that, I sympathise, sometimes the budget simply won't carry it. 
Lately I am looking more carefully at whether I can comfortably carry the 
resultant risk if required to cut corners as when it turns to doodoo the finger 
won't be pointing at the Clients purchasing officer.

Cheers

Don