A two-step plasma process has been developed as a means of removing surface oxide layers from indium bumps used in flip-chip hybridization (bump bonding) of integrated circuits. The two-step plasma process makes it possible to remove surface indium oxide, without incurring the adverse effects of the acid etching process

Dickie, Matthew R.

Greer, Harold F.

Hoenk, Michael E.

Jones, Todd J.

Nikzad, Shouleh

Vasquez, Richard P.

NASA Technical Reports Server

An improved assembly for inert-gas
shielding of a metallic joint is designed
to be useable during any of a variety of
both laser-based and traditional welding
and brazing processes. The basic pur-
pose of this assembly or of a typical prior
related assembly is to channel the flow
of a chemically inert gas to a joint to pre-
vent environmental contamination of
the joint during the welding or brazing
process and, if required, to accelerate
cooling upon completion of the process.
In a typical prior welding gas-shielding
assembly, depicted in the left part of the
figure, the inert gas is fed to the joint
through a central nozzle in the welding
torch. This arrangement does not always
provide adequate protection against con-
taminants because the inert gas flows tur-
bulently into the joint region and is not
evenly distributed in the heated region
surrounding the joint. The lack of inert
gas in some places in the region surround-
ing the joint can result in oxidation,
which, in turn, can lead to porosity and ul-
timately to cracking. Improper distribu-
tion of shielding gas can also lead to the
formation of plasma and to insufficient
cooling of portions of the region sur-
rounding the joint that are meant to be
protected against excessive heating.
The present improved welding gas-
shielding assembly, depicted in the right
part of the figure, provides a column of
evenly distributed gas flow directly sur-
rounding the pool of molten metal in the
weld joint to prevent oxidation and forma-
tion of plasma.
The assembly includes a conical inner
body and an outer shell comprising a hous-
ing body and an end cap. The outer shell
contains chambers sized and shaped to act,
along with the conical inner body, to dis-
tribute the inert gas evenly around the con-
ical inner body and to encapsulate the flow
of the gas and direct the flow onto the de-
sired region surrounding the weld joint.
The gas enters the assembly through a
supply tube from an external source. The
passage through which the gas enters is
tangential, so the gas flows in tangentially,
creating a vortex flow in the chambers be-
tween the outer shell and the conical
inner body. The gas circulates as it fills
the first chamber, then flows through an
annular orifice to fill a second chamber
and then flows through a series of diffus-
ing screens. After flowing through the
screens, the gas is directed to the weld
joint area by means of a nozzle. The com-
bination of the vortex flow, chambers,
and screens provides even columnar flow
around the entire heated area.
This work was done by Paul Gradl and
Kevin Baker of Marshall Space Flight Center
and Jack Weeks of Pratt & Whitney. For fur-
ther information, contact Sammy Nabors,
MSFC Commercialization Assistance Lead, at
sammy.a.nabors@nasa.gov. Refer to MFS-
32644-1.
NASA Tech Briefs, June 2009 21
Manufacturing & Prototyping
A two-step plasma process has been
developed as a means of removing sur-
face oxide layers from indium bumps
used in flip-chip hybridization (bump
bonding) of integrated circuits. This
process has considerable commercial
potential in that flip-chip hybridization
is used in the manufacture of cellular
telephones and other compact, portable
electronic products.
The need for this or another, similar
cleaning process arises as follows: In-
dium bonding bumps tend to oxidize
during exposure to air. As the duration
of exposure and the level of oxidation
increase, the electrical resistances of the
bonds subsequently formed via the
bumps also increase. In some cases, the
resistances can become so large that the
bump bonds may act as open circuits,
preventing proper functioning of the
bump-bonded devices.
There is a patented process for re-
moval of surface indium oxide layers by
Two-Step Plasma Process for Cleaning Indium Bonding Bumps 
This process could increase yields in the manufacture of consumer electronic products. 
NASA’s Jet Propulsion Laboratory, Pasadena, California
Improved Assembly for Gas Shielding During Welding or Brazing
Inert gas is distributed evenly over the region surrounding the weld joint.
Marshall Space Flight Center, Alabama
Workpiece
Laser Beam or
Welding ElectrodeWeld Joint
Workpiece
Laser Beam or
Welding Torch
First
Chamber
Second
Chamber
End Cap
Conical
Inner Body
PRIOR DESIGN IMPROVED DESIGN
Housing Body
Annular
Orifice
Shielding
Gas Flow
Shielding
Gas Flow
Weld Joint
These Simplified Cross Sections show the main differences between the present improved welding
gas-shielding assembly and a typical prior such assembly.
https://ntrs.nasa.gov/search.jsp?R=20090022331 2019-08-30T07:05:15+00:00Z
etching with hydrochloric acid. Unfor-
tunately, once the oxide is removed, the
acid can continue to attack the indium,
reducing the size of the bumps and even
undercutting them. The acid can also
attack metal layers on and under the
bond pads, potentially creating open
circuits and thus negating the benefit of
removing the oxide. In contrast, the
two-step plasma process makes it possi-
ble to remove surface indium oxide,
without incurring the adverse effects of
the acid etching process.
In the first step of the plasma process, a
device on which indium bonding bumps
has been formed is exposed for a suitable
amount of time (typically, 20 minutes) to
a plasma generated in a gaseous mixture
of 1/3 argon + 1/3 methane + 1/3 hydro-
gen. During this step, the oxygen in the
indium oxide is removed through incor-
poration into CO and CO2 gas molecules,
while the indium in the indium oxide is
removed through incorporation into
In(CH3), which is volatile. Following this
step, a carbonaceous surface film is also
formed on device surfaces that are not
covered by indium.  
A second step for removing the car-
bonaceous film is as follows. The device
is exposed to a plasma generated in a
gaseous mixture comprising 72 percent
of argon and 28 percent of hydrogen.
This step greatly reduces the carbon
content without exerting any significant
adverse effect on the indium. The power
used to generate the plasma in this step
must be chosen carefully: the power
should be high enough to ensure effec-
tive removal of the carbonaceous film,
but not so high as to melt or otherwise
damage the indium bumps (see figure). 
This work was done by Harold F. Greer,
Richard P. Vasquez, Todd J. Jones, Michael E.
Hoenk, Matthew R. Dickie, and Shouleh
Nikzad of Caltech for NASA’s Jet Propulsion
Laboratory. 
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-45911, volume and number
of this NASA Tech Briefs issue, and the
page number.
PROCESSED AT POWER GREATER THAN OPTIMUM PROCESSED AT OPTIMUM POWER
Indium Bumps 
Bond Pads Indium Bumps 
These Indium Bonding Bumps were treated by two different versions of the two-step plasma process. The pockmarks on the left bump were caused by
using greater-than-optimum plasma-generating power in the second step of the process. The right bump was processed at optimum power.
A hand tool has been developed for
crimping leads in flexible tails that are
parts of some electronic circuits — es-
pecially some sensor circuits. The tool
is used to cut the tails to desired
lengths and attach solder tabs to the
leads. For tailoring small numbers of
circuits for special applications, this
hand tool is a less expensive alternative
to a commercially available automated
crimping tool. The crimping tool con-
sists of an off-the-shelf hand crimping
tool plus a specialized crimping insert
designed specifically for the intended
application. 
The components of the insert and
their roles include the following:
• A pin guide aligns pins on the solder
tabs with the leads in a tail that is part
of the flexible circuit.
• A punch pushes the pins through the
pin guide and crimps them onto the tail.
• A forming plate aligns the tail over
grooves that form the pins from the
solder tabs.
• A spaceplate includes a surface that serves
as a stop for positioning the tail when the
tail is inserted in the forming plate.
• A dowel pin enables semi-permanent
assembly and alignment of the punch,
pin guide, and springs.
• A pin holder holds and helps to align
the solder tabs before crimping.
This work was done by Aaron Hulse and
Myron A. Diftler of Lockheed Martin Corp.
for Johnson Space Center. Further informa-
tion is contained in a TSP (see page 1). MSC-
23461-1
Tool for Crimping Flexible Circuit Leads
Lyndon B. Johnson Space Center, Houston, Texas
22 NASA Tech Briefs, June 2009


Two-Step Plasma Process for Cleaning Indium Bonding Bumps

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