Page 296 - Analog and Digital Filter Design

Selecting Components for Analog Filters 293

purposes; it screens the components from tracks passing underneath, and it pro-
vides part of a low-loss transmission line. By using FR4 board in a standard
thickness of 1.6mm, 50-ohm transmission lines can be created by making the
printed circuit tracks 2.5mm wide. A transmission line is formed between the
earth plane and the track.

The technique of providing an earth plane on high-speed PCBs may cause prob-
lems when an inductor is placed on the board, because of the capacitive cou-
pling between the ends of the inductor and the earth plane. This capacitance
forms a parallel tuned circuit with the inductance and may cause the filter to be
de-tuned. One solution is to remove the earth plane from the area below the
inductor. An alternative solution is to mount the inductor on spacers above the
board, so reducing the capacitance.

Surface-Mount PCBs

Surface-mount components are used extensively in active filter circuits. Ceramic
capacitors are common but can be damaged by stress due to circuit board expan-
sion. One way of minimizing this problem is to use physically small devices;
devices larger than 1812 (0.18 x 0.12 inches) should be avoided.

Ceramic capacitors should be protected with a moisture-resistant coating. If
moisture is absorbed into the ceramic material, the capacitance value will
change. Moisture can also be absorbed into plastic packages, so a conformal
coating over the whole board is preferred. Some consideration should be given
to storage of components; metalized sealed bags should be used, perhaps with
desiccant material. This will prevent moisture being trapped into an assembled
board and avert the risk of damage during soldering (as the moisture boils off).

Conventional PCBs have plated through holes that are lmm or larger in dia-
meter. Surface-mount boards do not need holes large enough for component
leads; hence they tend to be smaller in diameter. Metalized "via" holes 0.3 mm
in diameter are common (used to connect two tracks rather than for component
leads). The problem arises when the board is heated. Glass and epoxy board,
such as FR4 type, has a high coefficient of expansion at temperatures above
125°C. Above 125°C the board goes through its glass transition temperature and
its coefiicient of expansion is greater than normal; Z axis expansion increases
the thickness of the board and can cause fractures between the tracks and the
via-hole pads.

Soldering causes a problem due to the heat applied to the board; in wave sol-
dering the board is heated to about 300"C, which is way above the glass transi-
tion temperature. To reduce the problem of via-hole damage, all plated through
holes should have a wall thickness of 35pm or more. Temperature cycling of