3-17
Because 100 percent of the solder cannot be removed, the extraction method is not usually successful
with the plated-through solder joint. The component lead in a plated-through hole joint usually rests
against the side wall of the hole. Even though most of the molten solder is removed by a vacuum, the
small amount of solder left between the lead and side walls causes a SWEAT JOINT to form. A sweat
joint is a paper-thin solder joint formed by a minute amount of solder remaining on the conductor lead
surfaces.
MOTORIZED VACUUM/PRESSURE METHOD.—The most effective method for solder joint
removal is motorized vacuum extraction. The solder extractor unit, described in topic 2, is used for this
type of extraction. This method provides controlled combinations of heat and pressure or vacuum for
solder removal. The motorized vacuum is controlled by a foot switch and differs from the manual vacuum
in that it provides a continuous vacuum. The solder extraction device is a coaxial, in-line instrument
similar to a small soldering iron. The device consists of a hollow-tipped heating element, transfer tube,
and collecting chamber (in the handle) that collects and solidifies the waste solder. This unit is easily
maneuvered, fully controllable, and provides three modes of operation (figure 3-12): (1) heat and vacuum
(2) heat and pressure, and (3) hot-air jet. Some power source models provide variable control for pressure
and vacuum levels as well as temperature control for the heated tubular tip. The extraction tip and heat
source are combined in one tool. Continuous vacuum allows solder removal with a single heat
application. Since the slim heating element allows access to confined areas, the technician is protected
from contact with the hot, glass, solder-trap chamber. Continuous vacuum extraction is the only
consistent method for overcoming the resweat problem for either dual or multilead devices terminating in
through-hole solder joints.
Figure 3-12A.—Motorized vacuum/pressure solder removal. VACUUM MODE.
