Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-07-17
2003-06-24
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S743000, C029S852000, C174S262000, C228S256000, C361S780000, C427S097100
Reexamination Certificate
active
06581280
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for filling holes in a substrate and such resulting holes and, more particularly, relates to a method for filling holes in electronic substrates that have high aspect ratios of at least 5:1 in a process for forming vias and interconnects and the resulting holes and vias.
BACKGROUND OF THE INVENTION
In the electronics packaging industry, there is often a need to fill holes in various substrates for forming vias and interconnects. One such use is for a high performance chip carriers having high small diameter aspect ratio holes. The hole filling process is affected by a number of processing and material parameters, for instance, the diameter of the via hole, the depth of the hole and the type of the substrate material, etc.. Typically, via holes are through holes and are used to electrically connect a top surface and a bottom surface of a substrate and, thus, the filler material utilized in forming the vias must be electrically conductive. A number of techniques have been used to perform the via hole filling process. Depending on the type of the substrate, the techniques may include electroplating, electroless plating, solder paste screen printing and conductive paste screen printing.
A fundamental processing problem arises when the depth-to-width ratio, or the aspect ratio grows. The problem gets more complex when the hole diameter is made aggressively small, i.e., 125 &mgr;m or less. For instance, for holes having depth-to-width aspect ratios of 5:1 or larger, and the hole diameter is less than about 125 &mgr;m, it is no longer possible to use a conventional technique such as paste screening to fill the holes. Even the more advanced plating techniques cannot be used alone when diameters further decrease and aspect ratios further increase.
An attempt to fill via holes that have 17:1 aspect ratio with conductive epoxies by a screen printing method was found ineffective, even when the screening process is conducted from both the top and the bottom side of the substrate. One other attempt to fill a glass substrate that has via holes of 17:1 aspect ratio was carried out by utilizing a conductive material that has extremely low viscosity, i.e., an eutectic solder that has a viscosity of only 2 centipoise in its molten state, approaching that of water. Even at such low viscosity, known techniques for filling the high aspect ratio via holes were found ineffective. For instance, the conventional techniques tried include an injection molded solder technique with only pressure utilized. The injection molded solder technique was not able to fill deep vias holes that have very small diameters, i.e., smaller than 25 &mgr;m.
A second technique of vacuum injection molding was also tried. A graphic illustration of the vacuum injection molding process is shown in FIGS.
1
A~
1
D. The vacuum injection molding process was disclosed in a co-pending application that was assigned to the common assignee of the present invention under Ser. No. 08/518,874. The vacuum injection molding method utilizes a pressure differential formed between either ambient and vacuum or positive pressure and vacuum. The pressure and the vacuum are both supplied on the same surface of a substrate
10
, as shown in FIG.
1
A. The process is carried out by utilizing a shallow vacuum link
12
that allows a continual evacuation of air from via holes
20
that have a large aspect ratio such as 5:1. The vacuum link
12
must be sufficiently shallow such that the surface tension of molten solder prevents cross-leaking during the operation. Such a shallow link
12
effectively choke a significant part of the full pressure differential and thus producing only partial filling of via holes
20
that have high aspect ratios.
As shown in
FIG. 1A
, an injection head
14
which includes spaced apart vacuum slot
16
and injection slot
22
is positioned on top of a mold plate, or substrate
10
in fluid communication with the mold cavities, or via holes
20
contained therein. Relative axial sliding is effected between the injection head
14
and the mold plate
10
for sequentially evacuating gas from the mold cavities
20
using a continuous vacuum and injecting into the evacuated mold cavities
20
a liquid fed from a continuous source (not shown). The sliding of the injection head
14
over the mold plate
10
, as shown in
FIGS. 1B and 1C
, automatically provides self valving for sequentially evacuating and filling the mold cavities
20
from the same side, i.e., the top side
18
of the mold plate
10
. In a preferred embodiment, the vacuum slot
16
and the injection slot
22
are linked together at the mold plate
10
so that surface tension of the liquid restrains flow of the liquid (not shown) from the injection slot
22
to the vacuum slot
16
while allowing gas flow thereinbetween for effecting vacuum in the mold cavities
20
.
As shown in
FIG. 1A
, at the start of the process, i.e., before scanning begins, the mold cavities
20
are empty. As the scanning process begins, as shown in
FIG. 1B
, some cavities
24
are evacuated. As the scanning of the injection head
14
continues, some cavities
26
are filled with the liquid. In the final step of the process, as shown in
FIG. 1D
, the vacuum supply to the vacuum slot
16
is turned off while the scanning of the injection head
14
is completed, i.e., all the cavities
20
,
24
and
26
are filled with liquid.
The vacuum injection molding method illustrated in FIGS.
1
A~
1
D applies a vacuum and a liquid injection on the same surface of the substrate. To allow the continual evacuation of air from mold cavities that have a larger aspect ratio such as 5:1. The drawback of the process is that the vacuum link must be sufficiently shallow such that the surface tension of molten solder prevents cross-leaking during the operation. The shallow link chokes off a significant part of the full pressure differential and thus only produces partial filling of the mold cavities, or the via holes that have high aspect ratios.
It is an object of the present invention to provide high aspect ratio filled via holes in an electronic substrate and a method of forming certain of these filled holes.
SUMMARY OF THE INVENTION
In accordance with the present invention, high aspect ratio filled via holes in electronic substrates are provided, as well as a method of forming certain of these holes.
In a preferred embodiment, an apparatus for filling a liquid in high aspect ratio holes in a substrate is provided which includes a filler plate adapted for receiving an injection head therein, the filler plate has a bottom surface adapted for receiving an injection slot provided on the injection head, a vacuum plate adapted for receiving a vacuum slot in a top surface, the vacuum slot is in fluid communication with a vacuum source, and a connection means for connecting the filler plate and the vacuum plate together in a face-to-face, spaced-apart relationship forming a gap thereinbetween with the bottom surface of the filler plate positioned parallel and opposite to the top surface of the vacuum plate, the connection means further includes an adjustment means for adjusting a relative axial position of the filler plate to the vacuum plate, the gap formed between the filler plate and the vacuum plate is sufficiently large to slidingly engaging a substrate therein and to form abutting contacts between the substrate and the bottom surface of the filler plate and between the substrate and the top surface of the vacuum plate so that holes in the substrate are first evacuated by the vacuum slot and then filled with a liquid by the injection slot in the filler plate.
In the apparatus for filling a liquid in high aspect ratio holes in a substrate, the adjustment means may include a veneer adjustment for fixing a relative axial position of the filler plate to the vacuum plate. The adjustment means may further include a veneer adjustment for transforming a circumferential displacement into a linear axial displacement. The filler plate may be
Curcio Brian Eugene
Gruber Peter Alfred
Maurer Frederic
Papathomas Konstantinos I.
Poliks Mark David
Hogg William N.
International Business Machines - Corporation
Nguyen Donghai D
Vo Peter
LandOfFree
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