From IPC-2221:
9.2.7 Blind and Buried Vias - Plated-through holes connecting two or more conductive
layers of multilayer printed board, but not extending fully through all layers of the
base material comprising the board, are called Blind and Buried Vias.

9.2.7.1 Blind Vias - Blind via plated-through holes extend from the surface and
connect the surface layer with one or more internal layers.  The blind via can be
produced by two methods:  (1) After multilayer lamination by drilling a hole from the
surface to the internal layers desired and electrically interconnecting them by
plating the blind via holes during the plating process; (2) Before multilayer
lamination by drilling the blind via holes from the surface layers to the first or
last buried layers and plating them through, imaging and etching the internal sides,
and then laminating them in the multilayer bonding process.  For the second process
if an interconnection is desired between the surface layer and more than one internal
layer, sequential etching, laminating, drilling and plating-through of these layers
together before final multilayer lamination is required.  Blind via holes should be
filled or plugged with a polymer or solder resist to prevent solder form entering
them as solder in the small holes decreases reliability.

9.2.7.2 Buried Vias - Buried via plated-through holes do not extend to the surface
but interconnect only internal layers.  Most commonly the interconnection is between
two adjacent internal layers.  These are produced by drilling the thin laminate
material, plating the holes through and then etching the internal layer pattern on
the layers prior to multilayer lamination.  Buried vias between non-adjacent layers
requires sequential etching of inside layers, laminating them together, drilling the
laminated panel, plating the holes through, etching external sides and laminating
this panel into the final multilayer panel.

9.2.7.3 Hole Size of Blind and Buried Vias - Small holes are usually used for either
blind or buried vias and may be produced mechanically, by laser or by plasma
techniques.  The minimum drilled hole size for buried vias is shown in Table 9-4 and
the minimum drilled hole size for blind vias is shown in Table 9-5.  In either case
plating aspect ratios must be considered; small deep blind vias are very difficult to
plate because of decreased throwing-power and limited plating solution exchange in
the holes.  Blind and buried via holes may be plated shut; thus, the master drawing
call out should be similar to that used for through-holes vias.  See sectional
standards for more information.

Table 9-4  Minimum Drilled Hole Size for Buried Vias

Layer Thickness Class 1 Class 2 Class 3
<0.25 mm  0.10 mm   0.10 mm   0.15 mm
0.25 - 0.5 mm  0.15 mm   0.15 mm   0.20 mm
>0.5 mm 0.15 mm   0.20 mm   0.25 mm

Table 9-5  Minimum Drilled Hole Size for Blind Vias

Layer Thickness Class 1 Class 2 Class 3
<0.10 mm 0.10 mm   0.10 mm   0.2 mm
0.10 - 0.25 mm  0.15 mm   0.20 mm   0.3 mm
>0.25 mm  0.20 mm   0.30 mm   0.4 mm


Lisa Williams
Technical Project Manager
IPC
2215 Sanders Road
Nortbrook, IL  60062
www.ipc.org

>>> Bock Ming Gai Joanne <[log in to unmask]> 10/16/98 01:29AM >>>
Hi all,

I'm currently looking into all the aspects of blind and buried via
(Design constraints, manufacturing and the do & don't). Can anyone out
there help me?

Your help is very much appreciated.
Joanne

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