Soldering Techniques - Crown PSA-2 Service Manual

Phase Meter
AC Voltmeter
100my low range, flat fre-
quency response to 100K Hz
Filter 20-20K Hz bandpass, low noise
20Hz-20K Hz
Intermodulation Residual (.002% or lower)
Distortion Analyzer
7.3 Soldering Techniques
The difference between success and failure in service
repair is often determined by the thermal characteristics
of an iron and how well it matches the job at hand. One
would not use a large flat-head screwdriver to work ona
delicate Swiss watch. Likewise, the proper size iron and
tip should be used when soldering delicate electronic
parts in position.
Iron wattage classification is actually not a very good
method of choosing an iron. The reason for this is
because of the possible inefficiency of heat transfer to the
tip internally. A large wattage iron (125W) may, in effect
produce lower tip temperatures than another iron smaller
in wattage. Likewise, tip size and shape does not
necessarily work in proportion to temperature.
Therefore, it is impractical to compare soldering irons by
their wattage but more feasible to refer to them by their
maximum tip temperature.
Usually, the skilled service technician can pick the right
iron and tip for the job from experience or
recommendation. In most cases, the miniature or small
electrical soldering iron will work well with delicate
semiconductor devices (Fig. 7.1). When the proper size
iron is used (usually around 700° F. tip temperature), a
joint is almost instantly heated (approx. 500-550°) and
application of iron and solder melting is simultaneous.
7-2
Set output level for test-
ing; check noise level
Between preamplifier and
voltmeter in noise test
Check IM distortion
Hewlett-Packard 400F
or equivalent
Information available
from CROWN
Information available
from Crown
Fig. 7.1 Miniature Soldering Iron
When clean metal is exposed to air a chemical reaction
takes place known as oxidation. When heat is applied to
metal, oxidation is speeded up and creates a non-metallic
film that prevents solder from touching the base metal.
By applying a small amount of solder to a hot iron tip, a
desirable process known as tinning occurs. The main
reason for tinning an iron is to help prevent it from
oxidation as well as to aid in heat transfer. Tinning
should be performed prior to each use as well as after long
idling times.
To help prevent oxidation or remove existing oxidation
while soldering, a natural rosin flux core solder should be
used. Not only does flux aid in cleaning, but acts as a
catalyst in that it helps speed up the joint formation
without actually entering itself, into the bond. Never use
an acid flux except to clean a highly oxidized tip that will
not tin correctly. Crown recommends 63% tin/ 37% lead
composition with a rosin flux core of 2.5% (melting
temperature is approx. 361°F.).
Fig. 7.2 shows the correct and incorrect method of
applying rosin flux core solder to a joint. Never apply
solder to the iron tip directly and allow solder to run onto
the joint (flux is burned away and does not clean the
joints). Always apply heat to the connection and allow
the joint to melt the solder, not the iron. This insures
proper wetting and flow of the solder.