Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2002-02-01
2004-10-12
Edmondson, L. (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S175000, C438S612000, C252S512000
Reexamination Certificate
active
06802446
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention generally relates to conductive adhesives suitable for use in making electrical connections in electronic assemblies. More particularly, this invention relates to a conductive adhesive material whose composition enables electrically-conductive particles to be metallurgically bonded to each other and to metal surfaces contacted by the adhesive material during bonding, thereby promoting the electrical continuity and structural integrity of the resulting electrical connection.
(2) Description of the Related Art
Conductive adhesives (CA) have been used in electronic assemblies to make electrical connections, such as attaching components to circuit boards and within flexible and rigid circuit boards. Conductive adhesives can be categorized as isotropic (ICA) or anisotropic (ACA), the latter of which is also known as a z-axis conductive adhesive. ICA's generally comprise a polymer matrix of a thermosetting or thermoplastic material in which is dispersed small conductive particles that may be metal coated or formed entirely of metal. In ACA's, the conducting particles are typically solid metal or polymer spheres ultimately coated with a noble metal, usually gold. In a typical ICA application, a conductive adhesive is dispensed so as to be between a pair of terminals, such as a lead of a component and a trace on a substrate, and then heated to cure the polymer matrix, forming an interconnect that bonds and electrically connects the lead with the trace. In a typical ACA application, a film or paste adhesive with randomly dispersed conducting particles is tacked, printed or dispensed onto a substrate throughout the entire contact area.
Conductive adhesives suffer from several shortcomings, one of which is that the adhesive strength of existing conductive adhesives is generally inadequate to withstand mechanical shocks that can occur during the assembly process or in service. Another and more limiting disadvantage of conductive adhesives is that the conductive path through the interconnect is defined by the conductive particles that physically contact each other, but are bonded to each other and to the terminals by the polymer matrix of the adhesive.
FIGS. 1 and 2
represent two types of interconnections that illustrate this shortcoming. In
FIG. 1
, an isotropic conductive adhesive
112
is shown as adhering the lead
114
of a surface-mount component (not shown) to a metal trace
116
on a laminate substrate
118
, forming an electrical interconnect
110
. The conductive adhesive
112
conventionally contains metal particles
120
dispersed in an adhesive matrix
122
. The particles
120
are maintained in physical contact with each other, the lead
114
and the trace
116
by only the adhesive matrix
122
. Consequently, the robustness of the electrical interconnect
110
is not determined by the conductivity of the individual particles
120
, but instead by the interfacial conductivity between the particles
120
and between the particles
120
, lead
114
and trace
116
. Because the particles
120
inevitably oxidize due to oxygen and/or moisture intrusion into the adhesive matrix
122
, an oxide layer is typically present at the interfacial surfaces of the particles
120
. For this reason, the particles
120
are typically formed from a material whose oxide is conductive, such as silver. However, silver is expensive and silver migration is detrimental to the integrity of the electrical connection. Regardless of what material the particles
120
are formed of, the resulting conductive path through the interconnect
110
is not mechanically robust.
FIG. 2
illustrates the use of an anisotropic conductive adhesive
212
, such as in the manufacture of a flat panel display. The adhesive
212
is shown as forming an interconnect
210
between an oxide-free metal post
214
on an I/O pad of a silicon chip
215
to an input/output trace
216
on a glass substrate
218
. The conductive adhesive
212
contains conductive particles
220
(typically nickel or gold-coated polymer spheres or solid metals) dispersed in an adhesive matrix
222
. As represented in
FIG. 2
, the standoff is on the order of the diameter of the particles
220
(made possible because the surfaces of the chip
215
and glass substrate
218
are extremely flat). As a result, interparticle resistance is not an issue with the interconnect
210
. However, as with the isotropic interconnect
110
of
FIG. 1
, the particles
220
of the anisotropic interconnect
210
are maintained in physical contact with the metal post
214
and trace
216
by only the adhesive matrix
222
. Consequently, the robustness of the electrical interconnect
210
is again primarily determined by the interfacial conductivity between each particle
220
and its corresponding die and metal substrate contacts. Any swelling of the adhesive matrix
222
or oxygen or moisture intrusion can reduce or interrupt the physical contact between the individual particles
220
and the die and substrate metals, increasing the electrical resistance of the interconnect
210
above that allowed for the application.
In view of the above, one can appreciate that in a relatively hostile environment, the interconnects
110
and
210
may electrically open or increase in resistance as a result of degradation or failure of the mechanical bond provided by their polymer adhesive matrices
122
and
222
, such that device functionality can be compromised. A more robust interconnect system could be obtained by substituting the interparticle mechanical bonds with metallurgical bonds. However, previous attempts to use fusible alloys as the conductive particles of a conductive adhesive have been unsuccessful because of the aforementioned oxidation of the particles
122
and
222
which, in addition to significantly increasing the electrical resistance of the interconnects
110
and
210
, prevents the particles
122
and
222
from wetting each other and the terminals when heated above the melting or solidus temperature of the particles
122
and
222
. Without wetting, true metallurgical bonds cannot be obtained.
In view of the above, it can be appreciated that existing conductive adhesives are not suited for harsh environments as a result of their limited adhesive strength being inadequate to withstand mechanical shock and provide a robust electrical interconnect. Accordingly, it would be desirable of an electrical interconnect material were available to overcome the shortcomings of the prior art.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a conductive adhesive material and a process by which the conductive adhesive material is used to produce an electrical interconnection characterized by metallurgical bonds between electrically-conductive particles, instead of the mechanical bonds of prior art conductive adhesives. The metallurgical bonds formed between conductive particles yields a more robust electrical interconnection that can more readily withstand mechanical shocks typical of assembly processes and operating environments.
According to the invention. the conductive adhesive material comprises the conductive particles and a polymer material in which the particles are dispersed. At least the outer surfaces of the conductive particles are formed of a fusible material, and the polymer material contains a fluxing component capable of reducing metal oxides on the surfaces of the particles. As a result, heating of the oxide-free particles to a temperature at which the fusible material is at least partially molten, e.g., near the melting temperature or above the solidus temperature of the fusible material, causes the particles to metallurgically bond to each other and to metal surfaces contacted by the adhesive material. The metallurgical bonds between particles provide superior electrical continuity and structural integrity as compared to the mechanical
Chaudhuri Arun K.
Stepniak Frank
Walsh Matthew R.
Chmielewski Stefan V.
Edmondson L.
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