Due to increasing clock rates and use of faster logic, the need for controlling
PCB trace impedance has arrived. To this end Art Jackson and I are suggesting
the following spec for the arrangement and thickness of the layers in our PCBs.
Note that the center insulator thickness is not specified, only the overall
sandwich thickness.
4-LAYER

ALL INTERNAL LAYERS ARE 1 OZ. COPPER (0.0014")

/			----------------------------	<-- TRACE PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
|			----------------------------	<-- POWER/GROUND PLANE
|			////////////////////////////
 > 62 MIL OVERALL	////////////////////////////
|			////////////////////////////
|			----------------------------	<-- POWER/GROUND PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
\			----------------------------	<-- TRACE PLANE
6-LAYER

ALL INTERNAL LAYERS ARE 1 OZ. COPPER (0.0014")

/			----------------------------	<-- TRACE PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
|			----------------------------	<-- POWER/GROUND PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
|			----------------------------	<-- TRACE PLANE
|			////////////////////////////
 > 62 MIL OVERALL	////////////////////////////
|			////////////////////////////
|			----------------------------	<-- TRACE PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
|			----------------------------	<-- POWER/GROUND PLANE
| 5-7 MIL INSULATION ->	////////////////////////////
\			----------------------------	<-- TRACE PLANE
The nominal impedance for an 8 mil trace on these boards should be around 50
ohms. 

The reason for putting the second 2 trace layers between the power and ground
planes on the 6-layer PCB is that this keeps all signal layers a constant
distance from a plane and thus keeps the impedance more or less constant
regardless of which trace layer you are on. This also means that the impedance
you see on a 6-layer PCB should  be similar to that of a 4-layer PCB which will
(we hope) make your 4-layer PCB behave just like your prototype. 

If the loss of visibility of the second two trace layers is a problem for you,
you can always override the spec and put them on the outside.

I'd like to get comments (yea/nay/other) on this from everyone before getting
out the granite and the chisel so please respond before Friday 9/18 or forever
hold your tongue.

	Tim