Digital Signal Processing Reference
In-Depth Information
Antipads (holes precisely created in the copper planes) allow the PTH to pass
through (L2 and L5 in the figure) without connecting.
The nonfunctional pad on L3 does not provide an electrical connection to trac-
es on L3, but it does help anchor the PTH in the stackup. The parasitic electrical
effects of these pads can be detrimental to very high-speed signals. Depending on
the board thickness, high-end fab shops can selectively remove nonfunctional pads
on selected vias.
Blind vias (shown connecting L1 to L2) are PTHs that start on either of the
outer layers but do not pass entirely through the board. Blind vias can connect to
more than just the two layers suggested in the figure.
The buried via (sometimes mistakenly called a blind via) is shown connecting
together signals on L3 and L4. It is a PTH that does not pass completely through
the board and is not visible from the outer layers.
5.2.2 What Is the Via Aspect Ratio?
A via that is much longer than its outer diameter can be unreliable and is difficult
to manufacture [2]. The via aspect ratio calculated in (5.1) is a way to gauge this
difficulty:
PWB
_
thickness
OD
(5.1)
Via aspect ratio
=
..
Some shops call the via aspect ratio the drilling ratio, or simply the aspect ratio.
A via aspect ratio of 5 means that the fab shop can reliably produce boards that
can be five times thicker than the outer diameter (O.D.) of the smallest via (the pad
size is not a factor). For a 10-mil diameter via, this means that the board can be no
more than 50 mils thick.
Presently, boards with aspect ratios of 5 or less are considered as having main-
stream volume production worldwide. Aspect ratios in the 6-8 range are usually
considered
standard production
, ratios in the 9-14 range are considered
advanced
or
low volume
, and ratios greater than about 14 are considered
prototype
or
experimental
.
The signal integrity engineer uses the via aspect ratio provided by the manufac-
turer and the estimated board thickness to determine the minimum sized via that
can be used in a design (for example, when escaping from a small pitch BGA).
5.3
What Is the Signifi cance of Calling a Circuit Board “FR4”?
Although often used to describe the electrical characteristics of a typical circuit
board, the term FR4 has little electrical significance. It actually refers to the Na-
tional Manufacturers Association (NEMA) level of fire retardation.
Ratings FR1 through FR3 are paper-reinforced epoxy resin systems, while rat-
ings FR4 and 5 are glass fabric-reinforced epoxy resin systems with “reduced burn-
ing rate” [3].
