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SEMI F78-0304 © SEMI 2003, 2004 5 may contaminate the tub ing being faced. The cutting or facing shall not be of a abrasi ve type. 8.7 Severing or parting equip ment shall be of the machining type that will sep arate the…

SEMI F78-0304 © SEMI 2003, 2004 4

5.1.54 tungsten — non-standard term for tungsten

electrode.

5.1.55 tungsten electrode (2) — a component of the

electrical circuit that terminates at the arc, molten

conductive slag, or base metal. A non-filler electrode

made principally of tungsten and used in arc welding.

5.1.56 undercut (2) — a groove adjacent to the base

metal at the edge of the weld left unfilled by weld

metal.

5.1.57 underfill (2) — a groove weld condition in

which the weld face or root surface is below the

adjacent surface of the base metal.

5.1.58 weld bead (2) — a weld resulting from a weld

pass.

5.1.59 weld level — a segment or portion of a weld

schedule in which one or more weld parameters can be

changed independently; part of a weld sequence.

5.1.60 weld sequence — a series of steps executed by

the welding power supply to make a particular orbital

weld.

5.1.61 welder — a person who does welding

(sometimes used to refer to a welding machine or

power supply).

5.1.62 welding equipment — power supply, weld

heads, torches, and associated cables and accessories

used for welding.

5.1.63 welding operator — a person who welds with

an orbital or machine welding system.

6 Summary of Practice

6.1 The welding procedure is shown in the flow chart

in Figure 1.

7 General Requirements

7.1 All welding performed under this practice shall

conform to the applicable requirements of the ASME

Boiler and Pressure Vessel Code, Section IX, ANSI

B16.25, B31.3 Chapter V, and AWS B2.1, to the extent

that they are included herein.

7.2 All welds shall be based upon Welding Procedure

Specifications (WPS) and be documented with

associated Procedure Qualification Records (PQR) in

accordance with ASME Boiler and Pressure Vessel

Code, Section IX, or with AWS B2.1.

7.3 Qualification of the welding procedures to be used,

and of the performance of welders and welding

operators, shall conform to the requirements of the

ASME Boiler and Pressure Vessel Code, Section IX,

Articles II and III, or AWS B2.1.

7.4 All welding shall be performed only by certified

welders and welding operators. Certification procedures

shall include, at the minimum, producing three

acceptable welds in a row of typical GTA weld joints of

the smallest and largest diameters of each alloy to be

welded. Welding parameters shall be set by the welder

or welding operator. Certification shall expire after six

months of inactivity.

7.5 The weld assembly shall be kept under a

continuous purge until all welding is complete.

7.6 Welders shall use clean room-compatible gloves

any time that the tubing or component to be welded is

removed from protective covering.

8 Apparatus

8.1 Welding equipment shall be of the GTAW,

constant current, DCEN (direct current electrode

negative) and electronically controlled type with rapid

dynamic response capable of 5 Hz (CPS) or greater

pulsed welding.

8.2 All welding fixtures and weld heads shall be clean

and free of any particulate and excessive discoloration.

Weld heads shall rotate freely and smoothly at all

speeds. All clamping and holding fixtures shall fit

tightly around applicable fittings/tubing, allowing no

movement after clamping in excess of 0.003 inch

(0.008 cm). The welding fixturing shall allow viewing

of the weld joint to insure proper fit-up.

8.3 Electrodes shall be precision ground to the factory

specification for head and weld type. Electrode gap

shall be set using tooling or procedures that provide

accurate and repeatable gaps to be set to within 0.002

in. (0.005 cm). The use of 2% Ce-doped or 2% La-

doped tungsten electrodes is recommended.

8.4 Purge gas apparatus shall be stainless steel tubing

and components with face seal fittings, when possible.

PFA plastic tubing is acceptable as the final run to

allow flexibility for hook-up. Lengths shall be restricted

to less than ten feet. All components that come into

contact with the weldment shall be stainless steel. Only

heavy wall PFA or stainless tubing shall be used on the

ID purge. Only stainless tubing shall be used on HP

systems.

8.5 Purge gas flow shall be measured and controlled.

8.6 Facing equipment shall be of the dry end

machining style. The equipment shall be capable of

tolerances of 0.003 inches from a plane perpendicular

to the centerline of the tube, the OD and /or ID burr of

less than 0.005 inches. The equipment shall be capable

of controlling the cut curl so as that it does not enter the

tubing or cause scratching of the ID surface. The

equipment shall not use oils or lubricants in a way that

SEMI F78-0304 © SEMI 2003, 2004 5

may contaminate the tubing being faced. The cutting or

facing shall not be of a abrasive type.

8.7 Severing or parting equipment shall be of the

machining type that will separate the tubing without

contaminating the ID of the tubing. Wheel type cutters

designed to cut stainless steel and CRAs are allowed

with purge. Dry saws of orbital type or cutoff are

allowed only when followed by cleaning to 12.5. All

cut ends shall meet the tolerances for facing equipment

or be followed by end preparation with a facing tool.

9 Materials

9.1 All materials to be welded shall be manufactured to

ASTM specifications and so certified by the

manufacturer. Certification shall conform to ASTM

A450, Section 25.

9.2 All seamless austenitic stainless steel tubing shall

be in conformance with SEMI F20 or customer

specification.

9.3 A backing (ID) gas is required during welding, and

while tacking (if tacking is used).

9.4 Weld parameters are affected by the choice of

shield gas. Argon, due to its effectiveness and material

compatibility, is the most commonly selected shielding

and purging gas. Argon and helium are inert and

therefore have no effect on the weld metal. These gases

do have very different ionization potentials, thermal

conductivity, and reactivity.

9.5 Argon/hydrogen mix is a reducing gas that avoids

the formation of oxides. It also reduces the amperage

required for a given ID weld bead width while reducing

the OD bead width. Argon/hydrogen mixes adversely

affect high ferrite materials (above 80% ferrite). Use of

argon/hydrogen mixes will shorten tungsten electrode

life. The weld parameters will be affected by hydrogen

to argon percentages. Hydrogen mixes above 5 vol%

are not recommended for safety reasons.

9.6 Nitrogen will cause instability of the arc in mixes

above 3 vol% in the shield (OD) purge gas. It is a

acceptable backing (ID) gas for austenitic stainless and

many CRAs. Nitrogen when exposed to welding

temperatures will cause nitride formation in some high

ferrite materials (above 80% ferrite). Use of nitrogen

mixes in the shielding (OD) gas will shorten the

tungsten electrode life.

9.7 The ID purge gas will be certified to 99.9997%, or

less than 3 ppm total contaminants (moisture, oxygen,

and other contaminants).

10 Safety Precautions

10.1 This practice does not purport to address all of the

safety issues associated with its use. It is the

responsibility of the users of this standard to establish

appropriate safety and health practices and determine

the applicability of regulatory limitations prior to use.

10.2 Welding equipment used to make welds shall be

operated in accordance with the manufacturer’s

operating and safety instructions.

10.3 All welding performed under this practice shall

conform to the applicable requirements of ANSI/ASC

Z49.1.

10.4 Welding gas mixtures containing more than

5 vol% H

are not recommended due to the potential for

fire hazard.

10.5 Do not reweld stainless steel that has been used

for corrosive gas delivery.

10.6 See Appendix 1 for information on stainless steel

and welding fume.

11 Test Specimens: Couponing

11.1 Prior to the welding of a particular size, wall

thickness, and alloy, a primary standard sample weld

shall be made, sectioned, and analyzed at the job site.

The primary standard sample weld shall become the on-

site work sample against which other welds of the same

size, wall thickness, and alloy are judged. This on-site

work sample may be used indefinitely or reproduced

each day at the discretion of the examiner.

11.2 The primary standard sample weld shall be

checked for compliance with SEMI F81, “Specification

for Visual Inspection and Acceptance of Gas Tungsten

Arc (GTA) Welds in Fluid Distribution Systems in

Semiconductor Manufacturing Applications.” Coupons

shall be cross-sectioned and inspected visually. Weld

coupon criteria are the same criteria for all system

welds.

11.3 Once a sample weld is found to be acceptable, all

essential and supplementary essential variables shall be

documented in the procedure qualification record.

11.4 Any significant deviation(s) from the on-site

work sample will cause the weld(s) to be rejected.

Rejected welds shall be removed and replaced.

11.5 Sample test welds shall be made periodically.

These sample test welds shall be compared to the on-

site work sample and checked for compliance with

SEMI F81. Deviation from the on-site work sample or

SEMI F81 shall be cause for rejection. If the weld

inside diameter is inspectable using a sight tube or other

device, sample test welds may be production welds.

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.