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SEMI F78-0304 © SEMI 2003, 2004 7 12.5.2 In the case of contam inated tubing, or if a cut- out or saw cut is necessary, the following cleaning procedure shall be used: 12.5.2.1 Primary rinse in cold running DI water. 12.…
SEMI F78-0304 © SEMI 2003, 2004 6
Sample test welds shall be made when any of the
following conditions exist:
11.5.1 Start of shift (in) or end of shift (out).
11.5.2 Change of weld parameters.
11.5.3 Change of material (heat number).
11.5.4 Change of tube size or wall thickness.
11.5.5 Change of ambient temperature ± 20°F
(± 11°C).
11.5.6 Change of source of power to power supply to
include addition or subtraction of extension cords.
11.5.7 Change or removal of the weld electrode.
11.5.8 Any change of equipment such as weld head,
weld head extensions, or power supply.
11.5.9 Any time that a weld discrepancy is noted by
the welding operator.
11.5.10 Any significant change of ID or OD purge gas
(source or flow rate).
11.6 All couponing shall use the same ID purge gas
and OD shielding gas as the production weld (Figure 2).
12 Procedure
12.1 Documented procedures shall exist for each weld
configuration including all parameters (including purge
times, orifice sizes, purge rates, and internal pressure).
12.2 Check parameters and verify that they are in
accordance with the qualified welding procedure.
12.3 Perform only one weld joint at a time.
12.4 Joint Preparation Procedure
12.4.1 All cutting of component or tubing weld ends
shall be done with a sharp-edged tool. No lubricants of
any kind shall be allowed.
12.4.2 All component and tubing weld ends shall be
de-burred after cutting.
12.4.3 Surfaces for welding shall be clean and shall be
free from oxidation, discoloration, oil, scale, chips, or
other material that is detrimental to welding.
12.4.4 Unless tubing is to be cleaned afterward, tubing
shall be opened, cut, faced, and deburred in a required
cleanroom environment, leaving no visible particulates
inside the cut end.
12.4.5 The tube shall be faced to remove all
necking/wedging caused by the tube cutters (Figure 3).
For tube cutting, use a wheel cutter with lathe-type
facing tool or a special designed power saw with
alignment guide. Do not use lubricant. If any “nicks”
are found, reface or discard the tube.
12.4.6 Unless tubing is to be cleaned afterward, all
weld end preparation shall be done in such a manner as
to minimize the introduction of contaminants into the
system. When bending, cutting, or facing tubing a
positive purge must be used to remove any particles.
12.4.7 The prepared end shall conform to ASTM A
632 (or ASTM A 269 ≥ ½ in. OD) tubing specification
with regard to ovality and wall thickness.
12.4.8 The weld fit-up gap shall not exceed 0.003 in.
(0.08 mm) when the entire circumference is affected
(Figure 4). The maximum gap in any one area shall not
exceed 0.006 in. (0.15 mm) (Figure 5). The prepared
end shall be square to tube run within ¼° (angle).
12.4.9 After preparing, debur the inside diameter
carefully and lightly. Do not scratch the inside
diameter. Any scratched tubes shall be reprepped or
scrapped.
12.4.10 Chamfering is undesirable. The maximum OD
or ID chamfering shall be less than 10% of the wall
thickness or 0.005 in. (0.13 mm) whichever is less
(Figure 6).
12.4.11 All components shall be maintained in a clean
condition until welded into the system.
12.4.12 All benders, cutters, facing tool collets, or
brushes that are to be used on stainless tubing or alloy
tubing shall not be used on carbon steel tubing and care
shall be used on mixing alloys. All tools shall be
maintained in clean condition and shall be free of
grease, oil, dirt, and other foreign matter. Avoid cross-
contamination from dissimilar materials.
12.4.13 Bends on the tubing shall not be made in the
weld area.
12.4.14 Use only tools and handling techniques that
will not mar, disturb the shape of, or in any way reduce
the conformance to specifications of the materials used
in this system.
12.4.15 Tube ends shall be covered while the purge is
removed using a technique that will minimize the
amount of infiltration or contamination. Covers shall be
of non-particulating material.
12.4.16 Remove protective cover immediately prior to
performing the weld.
12.5 Tube Cleaning
12.5.1 It is recommended that all cut tubing be cleaned.
At a minimum, tubing contaminated during preparation
shall be cleaned using a high purity cleaning procedure.
SEMI F78-0304 © SEMI 2003, 2004 7
12.5.2 In the case of contaminated tubing, or if a cut-
out or saw cut is necessary, the following cleaning
procedure shall be used:
12.5.2.1 Primary rinse in cold running DI water.
12.5.2.2 Secondary rinse in hot [80°C (176°F)] DI
water.
12.5.2.3 Final rinse in DI water with pressure flush.
12.5.2.4 Blow dry with pressurized hot [150°C
(318°F)] N
2
. Ensure that drying occurs immediately
after final rinse.
12.5.2.5 Use immediately or cap and seal in plastic
sleeves.
12.6 Purging
12.6.1 All welds must use a positive and repeatable
form of ID purge pressure control. See Table 1 for
suggested settings and refer to Figures 7 and 8.
12.6.2 Production welds must use the same flow rates
and ID purge pressures as the qualified coupon weld.
12.6.3 During welding, all tubes, fittings, valves, sub-
assemblies, and all other components shall be
continuously purged.
12.6.4 Automatic orbital welding equipment shall
supply a constant gas shield to the weld head during
welding.
12.6.5 During all welding, a sufficient amount of
purge/shield gas shall be maintained until the weld has
cooled to a temperature where it can be handled, and
until the weld head is removed from the newly welded
parts.
12.6.6 Both purge/shield gas supply lines shall contain
flow indicators to ensure proper purging.
12.6.7 For welding of installed systems that will not be
subsequently cleaned, once construction begins, an ID
purge shall be maintained, either a flowing purge of 3
to 5 scfh (1 to 2 L/min.) or a block purge of 30 psi (206
kPa), until the system is complete. A flowing purge is
recommended on UHP systems.
12.6.8 The purge supply shall have a means to
manifold it so that there is a single point of connection
for each line under construction.
12.6.9 Extreme care shall be taken to ensure that all
contiguous flowpaths are fully purged.
12.6.10 All dead legs must be purged out completely
prior to welding.
12.6.11 All welds shall be performed with the purge
flow established by the weld procedure specification
sweeping the weld area during and after welding.
12.6.12 Vacuum devices may be required to overcome
back pressure in components such as regulators, filters,
purifiers, check valves, or others. Dead-end
components such as gauges may be purged using a
small-diameter tube placed inside the tubing to be
welded and back-flowing purge through the weld zone.
12.6.13 Pre-purging and post-purging shall occur for as
long as necessary to avoid unacceptable weld
discoloration.
12.6.14 Light external oxidation may be removed with
a stainless steel wire brush immediately after welding.
Purge shall be maintained during the brushing process,
and care shall be taken to perform the brushing process
in an appropriate area so as not to contaminate the work
area.
12.7 Welding Electrode
12.7.1 Welding electrodes shall be changed as
frequently as necessary to prevent weld deterioration.
Typical number of welds per electrode is as follows:
Tube diameter Welds per electrode
¼-in. and under 25 to 50
3/8 to 1.0-in. 20 to 25
1.0 to 2.0-in. 10 to 20
2.0-in. and above 10 to 15
12.7.2 Electrode shall be cut, not broken to length.
12.8 Additional Requirements
12.8.1 Maintain sufficient distance between weld joints
and valve seats to avoid damage to valve seats or valve
stem tips when purging through the valve. Purge
through the valve to the weld when possible.
12.8.2 Valves shall be located so as to allow space to
operate the valve after installation at the job site.
12.8.3 Valves must be cool to the touch after welding
and prior to cycling to avoid damage to the seat.
12.8.4 Clean Room Welding
12.8.4.1 Welders shall follow all clean room protocol
and use non-powdered latex gloves any time that the
tubing or component to be welded is removed from
protective covering.
12.8.4.2 All tools and fixtures used for the assembly
and welding shall be maintained clean and shall not be
removed for use outside of the clean room preparation
area.
12.8.4.3 As much welding as is feasible will be
performed in the clean room preparation area in the
form of sub-assemblies.
SEMI F78-0304 © SEMI 2003, 2004 8
12.8.5 Field Installation
12.8.5.1 If a system cut-out or cut-in is required, purge
shall be applied to both ends or the down stream end
must be discarded or cleaned, per Section 12.5. The
following cutout procedure shall be followed:
12.8.5.1.1 Set purge pressures to 5 psi max (35 kPa)
for safety.
12.8.5.1.2 Make initial cut. WARNING
: When the
tube is cut purge gas will escape the cut area at high
velocity. This gas may contain particles from the cut or
the tubing. Protect yourself, others, and nearby
equipment from injury or damage (Figure 8).
12.8.5.1.3 Make any additional cuts.
12.8.5.1.4 Face the ends.
12.8.5.1.5 Set single purge direction for welding.
12.8.5.1.6 All cuts shall be done in the horizontal when
possible.
13 Interpretation of Results: Weld Inspection
13.1 All welds shall be 100% inspected on the outside
surface and whenever possible on the ID surface to
insure conformance to the weld bead specifications
listed in SEMI F81.
13.2 Dimensional and Configuration Inspection —
Dimensional and configuration inspection shall be
performed as follows:
13.2.1 One hundred percent subassemblies shall be
inspected. If a subassembly is opened for inspection,
the inspection must be performed in a clean room of
same class as the assembly area.
13.2.2 Confirm fabrication drawing is attached to the
assembly; if not, reject.
13.2.3 Use check sheet to verify conformance of
assembly to drawing. Check dimensions, squareness,
offsets, straight edges, and levels to verify all
dimensions.
13.2.4 Inspect each weld externally in reference to the
coupon.
13.2.5 When subassemblies have passed inspection,
they shall be tagged as such, rebagged, and released for
final testing.
13.3 Any items found defective during inspection and
repairable may be repaired, if the repair will not
degrade the system conformance to installation
specifications.
13.4 Where a weld is found defective, the preceding
two welds shall be tested as indicated in Section 11. If
either of these welds is rejectable per SEMI F81, then
all welds made since the last welding procedure was
established shall be removed and replaced.
14 Report
14.1 Coupons shall be logged with the date and time
and operator identification. Coupons and coupon logs
shall be retained for one year. Sample test welds shall
be kept on file and may be reviewed at any time during
the construction.
14.2 A daily log shall be maintained on all system
welds and coupons (see Weld and Coupon Log, Table
2), and as-built drawings recording all data shall be
maintained in the welding area.
14.3 All welds shall be identified with a code number
and cross-referenced with the drawings for future
evaluation.
15 Related Documents
AWS C5.10 — Recommended Practices for Shielding
Gases for Welding and Plasma Arc Cutting
ANSI/AWS C5.5 — Recommended Practices for
GTAW
Table 1 Suggested ID Purge Setting for Argon and Argon Gas Mixes
Tube
Size
Wall
Thickness
Minimum
ID Purge Rate
ID Purge
Pressure
Restrictor
Size
1/16 in.
n/a
0.015 in.
n/a
3 scfh
1.5 l/m
13 to 16.8 torr
7 to 9 iwc
175 to 230 mmwc
n/a
1/8 in
3 mm.
0.028 in.
0.8 mm
5 scfh
2.3 l/m
9.3 to 16.8 torr
5 to 9 iwc
130 to 230 mmwc
1/16 in.
¼ in.
6 mm
0.035 in.
1 mm
7 scfh
2.5 l/m
5.2 to 6.3 torr
2.8 to 3.4 iwc
71 to 86 mmwc
1/8 in.
3/8 in.
10 mm
0.035 in.
1 mm
7 scfh
2.5 l/m
2.8 to 4.7 torr
1.5 to 2.5 iwc
1/8 in.