Weld Criteria SEMI F81
SEMI F81 provides visual inspection
and acceptance criteria for GTA welds on
stainless steel and other corrosion resis-
tant alloys in fluid distribution systems
for semiconductor manufacturing appli-
cations. A good weld is fully penetrated
with good alignment, and with a uniform
well-spaced weld bead; it has a flat inner
and outer weld bead with minimal con-
cavity or convexity; there are no visible
defects such as cracks, porosity or slag
islands, except a small slag inclusion at the
end of the downslope—less than 10% of
ID Purge Set-up using a Magnehelic pressure gauge
the wall thickness may be acceptable; and
there is no ID discoloration due to oxidation when viewed under a bright fluorescent light without magnification for HP
and UHP applications. Slight oxidation of
the weld OD is permitted. There must be
a downslope sufficient to prevent a crater
from forming at the end of the weld. Any
tack welds must be completely consumed,
that is, the arc has to re-melt the tack weld
so that it is not visible in the weld.
flowmeter egulator
Magnehelic gauge
Weld Coupons and Qualified
Welding Procedure
While borescopic or direct visual examination of welds may be practical for assem-
tee
flow
restrictor
• Set flow rate
• Install restrictor
• Install tee and Magnehelic gauge
• Adjust flow to achieve desired pressure
Figure 4. A good weld is fully penetrated
with a uniform weld bead, no visible
defects and virtually no discoloration from
oxidation. (Photo courtesy of Therma, Inc.)
ID Purge Set-up for Welding
flowmeter egulator
Weld head
flow
restrictor
• Remove tee and Magnehelic gauge
• install weld head
• make weld
0.01µm
filter
207 Power Supply
Figure 3. Top: Prior to orbital welding, a Magnehelic gauge is connected to the weld components via a tee, and the flow rate adjusted to give the appropriate pressure at the joint.
Bottom: Once the flow rate is set, the Magnehelic and tee are removed from the circuit,
the weld head installed and the weld is completed. (Courtesy of Arc Machines, Inc.)
blies of small diameter tubing in the cleanroom, it is not practical for examining welds
on closely spaced small diameter tubing
in the field. The semiconductor industry
has used couponing as a method of weld
examination, based on the assumption
that the orbital GTAW process is a repeatable process. So, if the sample weld is good,
the subsequent welds made on the same
tubing with the same weld parameters and
purge conditions will also be good.
Couponing is done in the context of a
qualified weld procedure which is detailed
in Fig. 1 of SEMI F81 Welding Procedure
Flow Chart. All materials must be verified;
the weld procedure and welding operators
qualified to ASME Section IX of the Boiler
and Pressure Vessel code or AWS (American
Welding Society) B2.1.
Prior to the start of production welding on a particular size, wall thickness and
alloy, a primary sample weld is made and
visually examined on the OD for weld
bead appearance, joint contamination,
joint soundness, surface oxidation, discoloration, pitting, cracking, and defects
