Page 130 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

Useful Electronic Circuits and Construction Techniques to Get You Going 123

Turn the board and the tracing layout over to view the underside (Fig. 6.1c,d).
The components will still be obvious by comparison with the traced view, if it
hasn’t been weeks since you made the layout. Following the traced wiring, you
now need to “draw” the wiring with solder. This is a rapid process. The board
should be held firmly in a small vice or clamp, as you will need to apply some
pressure from the soldering iron to get fast, thermally conductive connections
to the pads. A small rotational oscillation of the iron in your fingers scrubs the
tip against the pad and greatly improves heat transfer. First put a solder blob
onto each square pad along the wiring route, using a touch of the hot iron and
then a couple of millimeters of flux-cored solder. Once all the solder bumps are
in place, they can be joined together. This involves melting the second bump in
a row, dragging the molten solder over the insulating gap back to the first solid
bump, adding a small additional amount of fluxed solder, and then quickly
removing the iron. If this is judged correctly, the molten blob will melt into the
previous solid blob, form a good connection, and promptly solidify. Solid solder
will then have bridged the gap, and the second and third blobs can be joined in
the same way. If too much heat is applied, usually by performing the backward
sweep too slowly, both blobs will melt and the high surface tension will sepa-
rate the solder into two unconnected islands. Immediately trying again will
often frustratingly just add to the size of the two molten blobs, because the tem-
perature of the whole area is now too high. If this happens blow on the board
or wait ten seconds for everything to cool down and try again. This is all easier
to do than to describe, although it does take a little practice. Figure 6.1d shows
the final result.
Once you have the touch, this drawing in solder process can be very efficient
and extremely fast and rarely results in misconnected circuits. You have, after
all, sorted out all the topology beforehand on paper. Nevertheless, a final check
of the physical board against the underside paper view is recommended, again
using the magnifier or stereo microscope.
When all wiring is complete, you will have used a lot of solder, so much of the
circuit will be covered in excess flux; this must be removed. The commercial
aqueous flux cleaners are ideal for this. Immerse the whole board in the cleaner
for five minutes, and then scrub the board underside with an old toothbrush.
Finally, rinse the board under running tap water until all trace of a soapy feel
has gone. If you have any, rinse also in deionized water, dry the board on labo-
ratory paper or a paper towel, and then place in a warm airing cupboard or
under a tepid hair dryer to dry. As we discussed in Chap. 3 failure to remove
the flux can lead to odd performance in detector circuits, due to its significant
conductivity. A few components, notably some piezo sounders, batteries,
switches, and unsealed relays, should not be immersed. They will be added after
washing and drying.
The act of laying out on paper helps to make the layout perfect, so that I
rarely need to make any corrections. Gross errors can be corrected using copper
braid or a “solder-sucker” to aspirate solder along a line of connection bumps.
These work very well on the single-sided, nonplated hole boards. Similarly,

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