Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Die bond
Reexamination Certificate
2001-09-04
2004-11-30
Clark, S. V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Die bond
C257S782000
Reexamination Certificate
active
06825570
ABSTRACT:
TECHNICAL FIELD
The present invention relates to resistance-reducing conductive adhesives, and to apparatus and methods of attaching electronic components using resistance-reducing conductive adhesives.
BACKGROUND OF THE INVENTION
Semiconductor chips (or die) may be mounted to circuit boards or other electronic components in several ways.
FIG. 1
shows a die
10
mounted to a circuit board
20
in a “flip chip” or “chip on board” (COB) assembly. In this assembly, the die
10
has a pair of bond pads
12
that are attached directly to a corresponding pair of contact pads
22
on the circuit board
20
. The bond pads
12
and contact pads
22
are typically formed from aluminum, although other electrically conductive materials may be used. The bond pads
12
may be attached to the contact pads
22
by soldering or by some other suitable method. Electrical signals from the circuit board
20
may then be transmitted to the internal circuitry (not shown) of the die
10
through the contact pads
22
to the bond pads
12
, and vice versa. It is customary to provide an encapsulating layer (or “glob top”)
14
over the die
10
to hermetically seal the die
10
, thus insulating and protecting the die
10
from humidity, oxidation, and other harmful elements.
It is known to use a layer of conductive material to attach the bond pads of a die to the contact pads of a circuit board, as disclosed in U.S. Pat. No. 5,789,278, and in commonly-owned co-pending patent application Ser. No. 09/389,862, both incorporated herein by reference. For example,
FIG. 2
is a partial cross-sectional view of a bumped die
40
attached to a circuit board
20
using an anisotropically conductive layer
50
. In this assembly, solder bumps
42
are formed on the bond pads
12
of the die
40
. The anisotropically conductive layer
50
is formed between the bond pads
12
and the contact pads
22
on the circuit board
20
.
The anisotropically conductive layer
50
includes a plurality of conductive particles
52
distributed in a suspension material
54
, providing electrically conductive pathways
56
through the suspension material
54
in one direction (e.g. the “z” direction as shown in FIG.
3
). The conductive pathways
56
may be formed, for example, by compressing the solder bumps
42
against the layer
50
, causing the conductive particles
52
to contact each other to form columns of conductive particles. Electrical signals are then transmitted from the circuit board
20
to the die
40
through the conductive pathways
56
, and vice versa. The layer
50
is electrically insulative all other directions, hence it is “anisotropically” conductive.
Anisotropically conductive layers
50
may be formed in a number of ways, including as a film or as a viscous paste that is applied (e.g. stenciled, sprayed, flowed, etc.) to the circuit board
20
and the contact pads
22
. The anisotropically conductive layers
50
may then be cured by, for example, subjecting the suspension material
54
to certain environmental conditions (e.g. temperature, pressure, etc.), exposing to suitable curing compounds, irradiating with ultraviolet or ultrasonic energy, or other means depending on the composition of the suspension material
54
. The suspension material
54
may be composed of a variety of materials, including thermoset polymers, B-stage (or “pre-preg”) polymers, pre-B stage polymers, thermoplastic polymers, or any monomer, polymer, or other suitable material that is non-conductive and can support the conductive particles
52
. Various suspension materials are taught, for example, in U.S. Pat. No. 5,221,417 to Basavanhally and in U.S. Pat. No. 4,737,112 to Jin et al. The conductive particles
52
are commonly formed from silver, nickel, or gold, however, a variety of electrically conductive particles may be used.
Isotropically conductive layers may also be used for attachment of electronic components. For example,
FIG. 3
is a partial cross-sectional view of a die
10
having a pair of bond pads
12
, each bond pad
12
being attached to corresponding contact pads
22
of a circuit board
20
by an isotropically conductive layer
60
. Like the anisotropically conductive layer
50
described above, each isotropically conductive layer
60
includes a plurality of conductive particles
62
suspended in a suspension material
64
. The isotropically conductive layer
60
, however, is electrically conductive in all directions and therefore does not extend between adjacent bond pads
12
(or contact pads
22
) to prevent shorting or erroneous signals. Electrical signals from the circuit board
20
are transmitted through the isotropically conductive layers
60
to the die
10
, and vice versa. Both isotropic and anisotropic conductive materials are commercially-available from, for example, Ablestik of Rancho Dominguez, Calif., or A.I. Technology, Inc. of Trenton, N.J., or Sheldahl, Inc. of Northfield, Minn., or 3M of St. Paul, Minn.
Although successful results have been achieved using the above-referenced die packages, there is room for improvement. For example, each of the electrical connections between the bond pads
12
and the contact pads
22
are electrically resistive which may reduce signal strength, increase power consumption, and increase waste heat generation. These characteristics may undesirably degrade the performance of an electronic assembly.
SUMMARY OF THE INVENTION
The present invention is directed to resistance-reducing conductive adhesives, and to apparatus and methods of attaching electronic components using resistance-reducing conductive adhesives. In one aspect, a resistance-reducing conductive adhesive comprises a first quantity of conductive adhesive, and a second quantity of a chelating agent combined with the conductive adhesive. The chelating agent reacts with a metal, typically an oxidized form of the metal such as an oxide or metal ion of a metal-containing conductive lead (or other electronic component) to form a soluble metal-ligand complex. The chelating agent may also react with an oxide-free form of the metal on the conductive lead to passivate the metal by forming hydrogen bonds. The resistance of the resulting electrical connection is reduced in comparison with prior art methods of conductive adhesive coupling, providing improved signal strength, reduced power consumption, and decreased waste heat.
In various alternate aspects, the conductive adhesive may comprise an anisotropically conductive adhesive, an isotropically conductive adhesive, a conductive epoxy, or a hydrophilic adhesive. In other aspects, the chelating agent reacts with a lead comprising another conductive material, particularly a metal. Typically, the metal is a divalent or trivalent metal, including but not limited to, aluminum, copper, gold, nickel, platinum or silver. In a preferred aspect, the metal is aluminum. Alternately, the chelating agents may be any suitable agent that provides the desired reactive mechanisms, including, for example, oxalic acid, malonic acid, citric acid, and succinate succinic acid. In a further aspect, the second quantity of the chelating agent comprises a value within the range from approximately 0.1 percent by weight to approximately 20 percent by weight, inclusive.
In another aspect, an electronic assembly comprises a first component having a first conductive lead formed thereon, a second conductive lead, and a resistance-reducing conductive layer extending between the first conductive lead and the second conductive lead. The resistance-reducing conductive layer comprises a conductive adhesive having a plurality of conductive particles disposed within a suspension material, and a chelating agent approximately uniformly blended with a portion of the conductive adhesive, the chelating agent being chemically reactive with an at least partially oxidized metal ion or metal-oxide to form a soluble conductive metal-ligand complex. The portion of the conductive adhesive may include substantially the whole volume of the conductive adhesive or be a local volume locally disposed between the c
Jiang Tongbi
Lee Whonchee
Clark S. V.
Dorsey & Whitney LLP
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