Page 297 - Analog and Digital Filter Design

294 Analog and Digital Filter Design

completed boards also causes problems. If you have to rework a surface-mount
board, use a low melting point (LMP) solder and add extra flu?c; LMP solder
has a higher tin content than standard solder and is actually stronger. LMP
solder also has a small amount of silver that prevents leaching of the compo-
nent terminals.

On the surface, there is a temperature coefficient mismatch between components
and the board. Leadless Chip Carrier (LCC) devices have an expansion coeffi-
cient of GppdT, but for the board it is 14ppd"C (below the glass transition
temperature) in the X-Y plane. Above the glass transition temperature the
PCB has a coefficient of expansion of 50ppd"C. Again, temperature cycling
strains the solder joints and can lead to failure. Small gull-wing ICs (integrated
circuits) do not have a problem in this respect.

Copper-clad invar is used within some PCBs to restrain expansion and to
distribute heat. This should be used with polyamide boards, rather than glass
and epoxy types.

Solder resist can be used to restrain solder, but this can create large blobs on
the lead or pad area. Surface-mount ICs use smaller packages than conventional
leaded devices, and thin tracks of solder resist between the pads are not
practical.

PCBs that have a fine track pitch tend to have 0.05,um gold plating. If the gold
is thicker it causes embrittlement. Gold or nickel plating gives a flat surface and
makes surface-mount component placing easier.

Assembly and Test

When a filter is assembled, the inductors and capacitors usually have to be
selected for value. Lowpass and highpass filters are not too critical for exact
values, but bandpass and bandstop types are sensitive to value variations. Band-
pass and bandstop filters comprise a number of LC circuit branches that are
series or parallel tuned. With these types of filters it is best to select or adjust
the components to within 1% of their design values before connecting them into
circuit. Final adjustment can be made in-circuit, in one of two ways.

One method of in-circuit adjustment is to tune each LC pair separately. This is
the best method for narrowband iilters (bandwidth less than 10% of the center
frequency), but it can be difficult to carry out. This is because each LC pair must
be electrically separated from the others to prevent circuit interactions. This
could be considered during the PCB layout design phase. Links could be pro-
vided to connect each stage together after fine-tuning and testing has been
carried out.