Cleaning and liquid contact with solids – Processes – Using sequentially applied treating agents
Reexamination Certificate
2001-03-22
2003-07-15
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Using sequentially applied treating agents
C134S025400, C134S033000, C134S140000, C134S157000, C134S164000, C134S902000
Reexamination Certificate
active
06592680
ABSTRACT:
BACKGROUND OF THE INVENTION
The continuing increases in the functional capacity of integrated circuits (ICs) over the last few years have been both astounding and beneficial. However, accompanying these increases are attendant technical problems that demand creative solutions. One such problem has been the increase in input/output (I/O) pads that typically result from increases in the amount of circuitry that can be incorporated onto an IC. The number of I/O pads on a traditional wire-bonded IC, which involves bonding wires from the I/O pads of the IC die to the substrate, is generally limited by the length of the IC perimeter because such I/O pads typically reside at the edges of the IC. Thus, reductions in the size of transistors and other electronic devices incorporated on a single die generally create a need for more I/O pads than what traditional wire-bonding technology can offer.
To satisfy this need, alternatives to wire-bonding techniques have been devised to increase the overall interconnection density of ICs. One such alternative is the “flip chip,” which utilizes I/O connections across the top surface of the die. Thus, the connections are not restricted to the perimeter of the IC. Typically, solder “bumps” are formed on these connections. The solder bumps are then covered with solder flux, the die is flipped over so that the bumps make contact with the connection points of the IC substrate, and the die-substrate assembly is heated to reflow the solder. Hence, the necessary electrical contacts between the die and substrate are made by way of the solder bumps with the aid of the solder flux.
FIG. 1
is a simplified perspective view of a typical flip chip assembly
100
, with a die
110
connected to a substrate
120
by way of solder bumps
130
, with die
110
and substrate
120
defining a narrow, substantially planar space
140
therebetween.
Tests on flip chip devices have shown that repeated heating and cooling of the IC during normal use tends to place sufficient thermal stress on the integrated circuit (die-substrate) assembly to cause some of the connections made via solder bumps
130
to break, creating electrical discontinuities between die
110
and substrate
120
. To prevent such breaks, an underfill material (generally an adhesive) is normally employed to fill planar space
140
to maintain the structural integrity of the assembly and prevent the electrical connections from breaking. However, after the solder reflow, some flux residue remains in planar space
140
that must be removed by way of an IC cleaning solution before the underfill can be applied. The cleaning process is vital since leftover residue within planar space
140
prevents the underfill from reaching the entirety of planar space
140
, thus adversely affecting the structural integrity and overall reliability of flip chip assembly
100
.
Complete cleaning of the flux residue from planar space
140
of flip chip assembly
100
has proven to be rather difficult. The distance between die
110
and substrate
120
is normally quite narrow, on the order of 70 um or less. Further complicating the process is the fact that several rows of solder bumps
130
may exist in planar space
140
, thus making access to all of planar space
140
even more problematic.
Currently, IC assemblies are normally cleaned using commercially available centrifugal cleaners and cleaning solutions. As shown in a simplified manner in
FIG. 2
, a centrifugal cleaner
200
employs a tank
210
that is filled with an IC cleaning solution
220
during the cleaning process. Centrifugal cleaner
200
usually holds several IC assemblies, such as flip chip assembly
100
, using a cleaning fixture
230
immersed in cleaning solution
220
inside tank
210
. A tank-filling mechanism (not shown) of centrifugal cleaner
200
is used to fill tank
210
with IC cleaning solution
220
. Cleaning fixture
230
is then spun or agitated on a central vertical axis in cleaning solution
220
by way of a motor
240
. Cleaning solution
220
is then drained from tank
210
, and water rinse and spin-drying cycles in centrifugal cleaner
200
then normally follow. Cleaning fixture
230
holds several flip chip assemblies
100
, or other similar IC assemblies, horizontally within IC cleaning solution
220
.
Cleaning fixture
230
may be implemented in a variety of ways. For example, cleaning fixture
230
may consist of a central carousel to which one or more cassettes are attached. Each carousel would then be loaded manually with flip chip assemblies
100
prior to the cleaning process. Also, flip chip assemblies
100
may be held in boats
300
(FIG.
3
), each of which holds several flip chip assemblies
100
throughout a majority of the IC manufacturing process. In that case, a cleaning fixture holds several such boats
300
containing flip chip assemblies
100
to be cleaned. Other methods of implementing cleaning fixture
230
not disclosed herein are also employed in the industry.
Unfortunately, as displayed in
FIG. 4
, which shows a top view of flip chip assembly
100
after being agitated or spun in a bath of cleaning solution
220
in centrifugal cleaner
200
, tests have shown that cleaning solution
220
almost always fails to penetrate the entirety of planar space
140
(not shown explicitly in
FIG. 4
) between die
110
and substrate
120
, leaving some flux residue behind because an air pocket
400
becomes trapped in planar space
140
. When flip chip assembly
100
is positioned horizontally, cleaning solution
220
encroaches from all sides of planar space
140
simultaneously, trapping air pocket
400
approximately in the center of planar space
140
. Air pocket
400
then acts as a countervailing force against the entry of cleaning solution
220
into planar space
140
. Cleaning solution
220
is thus prevented from reaching all of planar space
140
, allowing some of the flux residue from the solder reflow phase to remain. The remaining flux residue thus prohibits the underfill material subsequently applied from occupying all of planar space
140
. Tests also confirm that no amount of spinning or agitation in cleaning solution
220
will force air pocket
400
from planar space
140
so that cleaning solution
220
may occupy all of planar space
140
.
To remedy this problem, the use of a apparatus and method of cleaning the tight spaces in integrated circuit assemblies, such as, for example, between the die and substrate of a flip-chip IC, that would result in the complete removal of the flux residue in the space would be advantageous. Without any flux residue present in the planar space, the underfill material to be applied for purposes of structural integrity may fill all of the space, thus preventing the breakage of the various connections between the substrate and die. The cleaning of other types of integrated circuit assemblies involving similar tight spaces, such as, for example, ball grid arrays (BGAs) and direct chip attach (DCA) assemblies, whereby a die is attached directly to a printed circuit board (PCB), would also benefit from such an apparatus and method.
SUMMARY OF THE INVENTION
Specific embodiments according to the present invention, to be described herein, provide an effective way of cleaning a space within an integrated circuit assembly without trapping air inside the space. For example, one embodiment of the invention provides a method of cleaning an IC assembly, such as a flip chip IC. To allow the cleaning solution to enter the space without trapping an air pocket inside, the IC assembly is held at an incline from horizontal. The IC assembly is then immersed slowly in the cleaning solution so that the space is completely filled with the cleaning solution prior to the integrated circuit assembly becoming completely submerged within the solution. Since the cleaning solution fills all of the space, all flux residue will be dissolved, allowing the underfill material used later in the IC manufacturing process to fill the entire space, helping to create a structurally reliable IC assembly.
Another e
Bjorlie Russell
Christison Pamela L
Hanna William H
Houdek Lawrence E.
Pierce Perry H.
Agilent Technologie,s Inc.
El-Arini Zeinab
Way Kyle J.
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