Figure 2. A Model 9-500 weld head mounted on an ICF Micro-Align Plate; the plate
provides for precision alignment of components for welding. ( Photo courtesy of Arc
Machines, Inc.)
same way by welding personnel. However,
at that time there was no clearly established
practice and no common set of weld criteria
that could be applied to the entire industry.
The existing Semiconductor Equipment and
Materials International (SEMI) standard at
that time, SEMI F3-94, mentioned orbital
welding but provided no guidelines.
In 1993 and 1994, SEMI introduced two
new standards that apply to orbital GTA
welding in semiconductor fluid distribution
systems. These standards, which replaced
SEMI F3-94 are: SEMI F78-0304 Practice for
Gas Tungsten Arc (GTA) Welding of Fluid
Distribution Systems in Semiconductor
Manufacturing Applications, which is a
guideline for fabrication, and SEMI F81-
1103 Specification for Visual Inspection
and Acceptance2, which details weld acceptance criteria.
The two documents were intended to
be complementary: if the procedures out-
lined in the Practice are followed, a weld
capable of passing the criteria outlined in
the Specification should result. The new
SEMI standards provided guidelines for
welders and welding operators while also
providing a set of criteria by which quality
assurance/quality control inspectors could
evaluate welds. A common terminology
was defined so that welding personnel and
QA/QC could effectively communicate with
each other.
Types of Orbital Welding
Equipment
Orbital welding power supplies for SEMI
F-78 specified GTAW include, constant current, DCEN (direct current electrode negative) and electronically controlled with rapid
dynamic response capable of 5 Hz (CPS) or
greater pulsed welding. Initially the power
supplies were large and cumbersome, but
when the first compact autogenous welding power supply that could run on 110 VAC
was developed, it was immediately seized
upon by contractors who could easily move
it around a construction site. The early
portable power supplies, such as the Arc
Machines, Inc. Model 107, featured digit
switch controls. Each time the tube size was
changed, the operator would have to dial
in all the weld parameters for the new weld
schedule.
Weld parameters controlled by the
power supply include: prepurge time when
the enclosed weld head fills with inert gas;
rotation delay time, during which the arc is
struck and a liquid weld pool is established;
primary and background amps sufficient
to achieve full penetration; pulse times for
high and low current pulses; travel speed
(RPM), typically 5 inches (127 mm) per
minute; and downslope time during which
the arc is gradually extinguished, followed
by postpurge; which continues the gas
flow until the weld and heat- affected zone
(HAZ) have sufficiently cooled to prevent
discoloration of the weld.
