Microvia_-_pouziti

The following are a few simple ways to use microvia technology

to help save time, money and effort:

Microvia-in-pad. One technique gaining popularity and producing

results is microvia-in-pad. Placing microvias directly in the compo-
nent pad can reduce overall circuit length, improve EMI and conserve
real estate that would normally be used for conventional fanout or
dog bone patterns. Since the finished microvias have a very small hole
diameter, with an aspect ratio of about 1:1, there is virtually no com-
promise of component solderability or solder joint integrity, even on
BGA patterns (Figure 3).

Microvias for power or ground.You may have noticed that when the number

of I/Os on a device increases, the number of power and ground con-
nections also increases. Typically, for every 10 signal pins on a device,
three or four are ground connections. If the design calls for a dedicated
via for every power or ground connection, a number of valuable routing
channels will be lost by using conventional through-holes for fanout pat-
terns.This microvia technique can be used only when power and ground
planes are located adjacent to the top and/or bottom layers of the board.

Pin escapes in dense areas. Components placed on the bottom side directly

By Charles Capers

COVER STORY

Figure 1 –

A typical microvia construction

under components mounted
on the top may limit the
clear area for through-hole
vias to escape from the com-
ponent pins. Blind microvias
may provide a good alterna-
tive; they do not penetrate
the board, thus enabling the
stacking of top- and bottom-
side microvias on top of each
other without electrical
interference.

On some

boards, high-speed design
rules may specify that certain
components need to be
placed in close proximity in
order to minimize trace or
bus lengths, while other
components on the same
bus can be located further
away. Microvias permit the
connecting of components
in close proximity without the need for additional real estate to complete
fanouts. Microvias also permit redistribution of the signals to an area
where through-holes can be used to complete the connections on other
signal layers.

Avoid buried vias. The myth thatmicrovias are costly to build typically

stems from the use of buried vias in conjunction with microvias. The
reality is that buried vias are cost-prohibitive on any type of multi-
layer PCB structure. Most PCB designers associate the use of buried
vias with the use of microvias, but as mentioned, there are alternate
ways to use microvia technology without the use and cost burden of
buried vias. In general, buried vias can add a cost premium of about
20-60% depending on the number of different buried drill layers
specified. On a six-layer board, a single buried drilled via from layers
2 to 5 can increase the cost by as much as 20%. Subsequent drill
processes can add cost premiums of 20% each.