Update README_fab.txt

Moved board size and material description to stack up section and added via description

Author: victoriamoorelbl

docs/README_fab.txt

@@ -10,10 +10,28 @@ Gerber, Drill, IPC-D-356, Board stackup, BoM, and X-Y placement.
 The design is created in, and these manufacturing files are mostly
 exported from, KiCad version 6.0.11.
 
-This is a 12-layer, 1.72mm (nominal) board.  Materials and thicknesses
+The overall board size is 150 mm x 180 mm, similar to the
+AMC double-width standard.
+
+This is a 12-layer, 1.72mm (nominal) board. 
+- PCB Material: FR4
+- Soldermask color: Black
+- Silkscreen color: White
+- Copper weight for inner layers is 0.5 oz and for outer layers is 1oz + 0.5oz = 1.5oz
+
+The SI conversion of the traditional PCB industry unit "1 oz" (really ounce/square-foot)
+is 305.15 g/m^2.  That _might_ convert to 34.15 micrometers layer thickness,
+but only if the copper plating has the density of pure bulk material at 20 C,
+Materials and thicknesses
 according to the board stackup description:
   marble-stack.txt
 
+Two Class 2 via types are utilized: 
+- 0.152mm (6mil) finished hole size, +0.0762mm/-0.152mm(+3mil/-6mil), with a 0.35mm (13.7mil) pad
+- 0.35mm (13.7mil) finished hole size, +0.0762mm/-0.35mm (+3mil/-13.7mil), with a 0.6mm (23.6mil) pad
+The above standard hole size toleracing indicates that the via thickness after plating is not critical, in fact, it is acceptable if the via is completely filled. 
+Vias are to be filled with non-conducive epoxy and capped according to IPC-4761.
+
 Three heat sinks are called out in the BOM and shown on the schematic:
   HS1  for U1 FPGA
   HS2  for U35 Power Supply
@@ -30,18 +48,6 @@ Design rules are 0.10 mm space, 0.10 mm trace, 0.152 mm drill
 allowing 0.152 mm soldermask dams between pads that are separated
 by 0.200 mm.
 
-The overall board size is 150 mm x 180 mm, similar to the
-AMC double-width standard.
-
-- PCB Material: FR4
-- Soldermask color: Black
-- Silkscreen color: White
-- Copper weight for inner layers is 0.5 oz and for outer layers is 1oz + 0.5oz = 1.5oz
-
-The SI conversion of the traditional PCB industry unit "1 oz" (really ounce/square-foot)
-is 305.15 g/m^2.  That _might_ convert to 34.15 micrometers layer thickness,
-but only if the copper plating has the density of pure bulk material at 20 C,
-
 Copper Gerber files, in order:
   marble-F_Cu.gbr
   marble-In1_Cu.gbr