Microvia_-_pouziti

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PRINTED CIRCUIT DESIGN

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Cost-Effective Use of Microvias

The tiny holes can be a big benefit, or a big pain in the FR-4

High prices have been associated with the microvia

since the inception of this technology. This may have
been true in the early days, but as processes have
evolved and equipment prices have declined, the
costs have dropped significantly. But this doesn’t

mean that microvias are right for every design.

A microvia is defined as a blind via with a diameter 0.005˝ or less that

is usually drilled from the top and/or bottom layer(s) to the first or sec-
ond adjacent internal layer. Normally the adjacent internal layers can be
used to redistribute the signals to other areas of the board where con-
ventional through-hole vias can be used, or they can be used as
power/ground planes or a combination of both (Figure 1).

Microvias are not appropriate for every PCB. For boards designed

for the latest products from Kenmore or Whirlpool, I wouldn’t rec-
ommend them. But very dense designs (200-plus I/Os per sq. in.),
or package pin spacings at or below 0.8 mm, are good candidates for
microvia technology. While there are different types and configura-
tions of microvias, the most cost-effective to produce in lower vol-
umes are blind, laser-drilled microvias using standard woven glass
materials such as FR-4. Most PCB manufacturers are familiar with the
processes used to produce boards using these materials, and the ini-
tial capital investment for equipment to produce microvias is on the
decline.

Compared to conventional plated through-holes, microvias offer a

distinct advantage in terms of overall board size reduction, layer count
reduction and increased route or interconnect density. Getting all of
the parts to fit onto a specific board size is only half of the equation;
completely routing all of the connections is the other. Microvias per-
mit reduced component-to-component spacing, which may lead to an
overall reduction in board size or reduced layer count, by increasing
the number of routing channels available on each layer (Figure 2).

Depending on the conductor width and spacing requirements,

using microvias can increase the routing density of a board by as
much as 33%.Therefore the price of the bare board could decrease as
much as 15-20% by eliminating additional signal layers.

There are three main methods for fabricating microvias: laser abla-

tion, photovia and plasma etching. The cost and merits of each were
described in a 1998 study by Kevin Arledge and Tom Swirbel1:

Multiple studies have presented the relative merits of each of these
methods, with cost one of the common comparisons. Detailed
cost models typically show that the mass via generation methods,
such as photolithography (photovia) and plasma etching, have a
significant cost advantage at very high via counts. Laser-formed
vias are typically presented as being more expensive at higher via
counts because laser-drilled vias are formed in a sequential man-
ner. However, at lower via counts the laser is portrayed as being
more economical. The cost crossover point between lasers and
mass via production methods moves up to higher via densities as
laser speeds increase, but there is always the assumption that at
some point the mass via production processes gain the advantage.