requires little or no adjustment to produce
an acceptable weld.
The latest technology, represented by the
AMI Model 205, allows for ease of use. When
the starting amperage is entered, the power
supply gradually reduces the current to produce a uniform weld bead width around
the entire weld joint. The amount of current
slope can be adjusted to achieve the desired
amount of penetration, and the unit can be
programmed in degrees, which assures that
the weld head will perform a complete 360
degree weld plus tie-in before beginning
the downslope.
Autogenous orbital tube welding is completely automatic in that the welding operator
installs the tubing and/or fittings in the weld
head, starts the ID purge, initiates the weld
sequence by pushing a button and does not
make any adjustments during the weld.
Remote operating pendants are typically available for power supplies for use
in the field. Operators can initiate the weld
from the pendant where the power supply
may be located at some distance from the
weld joint.
Types of Orbital Weld Heads
The type of weld head used for high-purity and ultra-high purity semiconductor applications is fully enclosed so that
the entire weld joint and electrode are
protected by the shielding gas. The head is
filled with inert gas, usually argon or a mix
of argon with no more than 5% hydrogen,
prior to striking an arc. A tungsten electrode installed in a rotor moves the electrode around the weld joint while the head
and the tube remain stationary.
SEMI F-78 requires that weld heads and
fixtures be clean and free of any particulates
and excessive discoloration. They must rotate
freely and smoothly at all speeds and hold
the tubes or fittings firmly in place. There
must be a provision in the fixture for viewing
the weld joint to insure proper fit up.
The electrodes must be precision ground
to the factory specification for the head and
weld type. The electrodes may be thoriated,
ceriated, or lanthanated3 rather than pure
tungsten to facilitate arc strike. However, in
our experience, ceriated provide the most
reliable arc strike. The tungsten length must
be fixed for each size tubing to assure a
uniform arc gap set to within 0.002 inches
(0.051 mm) that remains the same for every
joint of similar outside diameter (OD) and
wall thickness.
Each weld head can accommodate a
range of tube diameters, which for most
heads is accomplished by changing the
tube or fitting clamp inserts on either side
of the head.
The most commonly used head for semi-conductor process gas lines is the Model
9AF-750. This head can weld sizes from
0.250 inch ( 6.35 mm) OD to 0.750 inch
( 19.05 mm) OD. It is a very rugged head
suitable for use in the field, or in the shop
but can be used in the cleanroom as well.
The Model 9-500 welds diameters from
0.250 inch OD ( 6.35 mm) to 0.500 inch ( 12. 7
mm) OD. It uses a set of tools to configure
the head for tube-to-tube, tube-to-fitting
or fitting-to-fitting welds. Initially this
head did not have water cooling and that
limited weld productivity; but eventually,
water cooled heads became available and
Electrodes must be
precision ground for
the head and weld type.
with this feature up to 300 welds per day
became possible.
Water cooling is particularly recommended for tube diameters of 3 to 4 inches
OD and larger. While most semiconductor
gas lines are 0.250, 0.375, or 0.500 inches
in diameter, sizes up to a 6 inch pipe and
larger have been done with orbital welding,
however, a different type of equipment is
required for tube sizes greater than 6 inches
(152.4 mm) OD.
End Preparation
A precision fit between components being
welded is essential for achieving repeatable
high quality welds. In order for orbital weld-
ing to be successful in high purity or ultra-
high purity applications, the weld ends must
be machined with tolerances of 0.003 inches
(0.762 mm) from a plane perpendicular to
the centerline of the tube with an OD/ID
burr of less than 0.005 inches (0.127 mm).
No oils or lubricants are permitted and the
cut curl must be controlled so that metal
particles do not enter the tubing or scratch
the ID surface.
Inert Gas Purging
Purging is an essential part of the GTAW
process in which inert gas protects the
tungsten electrode and weld pool from discoloration caused by oxidation during the
weld, and while the weld cools to ambient
temperature. A purge on the ID surface is
particularly critical as discoloration is associated with loss of corrosion resistance, and
oxidized metal may particulate from the surface and place the semiconductor product
in jeopardy.
SEMI F78 12. 6.1 states that all welds must
use a positive and repeatable form of ID
purge pressure control. The technique of
using a Magnehelic pressure gauge to control the flow rate during welding is shown
in Figure 3. Table 1 in SEMI F78 details the
flow rate, purge pressure and size of flow
restrictor for each tube diameter and wall
thickness from 1/16 inch (1.588 mm) up
to 6 inch (152.4 mm). The purge gas must
be measured and controlled with separate
flowmeters used for both the ID and the OD
purge. This technique is useful for controlling
the weld bead profile and preventing weld
blow-out that may occur in field welds with
excessive ID pressure. The weld assembly
must be kept under a continuous purge until
all welding is complete. The purge gas apparatus must be stainless steel tubing with face
seal fittings for high purity and UHP applications. Less critical applications permit PFA
plastic tubing for the final run.
The ID purge gas must be certified to less
than 3 parts per million (ppm) total moisture, oxygen and other contaminants. While
color is the final weld criterion, oxygen analyzers are frequently used to show that gas
exiting the ID purge is of the same level of
purity as that directly from the source. Purifiers are used in the purge lines to remove
any trace amounts of water or oxygen from
