Method for surface mounting electrical components to a substrate

A method for surface mounting electrical components to a substrate, such as a printed circuitboard, involves use of an anisotropically conductive adhesive or Z-Axis adhesive between facing conductive surface areas on the component and substrate. Pressure is applied to the conductive adhesive by a nonconducting adhesive that is first cured between oppositely facing nonconductive surface areas of the component and substrate. This fixes the thickness of each layer of the conductive adhesive at a dimension no greater than its design conductive thickness. In a first submethod, the nonconducting adhesive is a fast setting adhesive subjected to mechanical pressure only as it is assembled on the substrate prior to the subsequent curing of the conductive adhesive. In a second submethod, it is a high shrinkage adhesive that applies compressive force between the component and substrate as it cures and shrinks dimensionally while at a temperature below the subsequent curing temperature of the conductive adhesive.

TECHNICAL FIELD 
This disclosure pertains to methods for surface mounting electrical 
components to a substrate by use of directionally conductive adhesives. 
BACKGROUND OF THE INVENTION 
Adhesive surface mounting of devices to a supporting substrate is 
exemplified by the disclosure of U.S. Pat. No. 3,818,279 to Seeger, Jr. et 
al. It discloses use of a conductive elastomeric material containing 
electrically conductive particles for establishing random conductive paths 
between a substrate area and an overlying device. Such adhesives have no 
directionally conductive properties. 
U.S. Pat. No. 4,339,785 discloses the mounting of a component to a printed 
circuit board by use of a structural adhesive. 
U.S. Pat. No. 4,774,634 to Tate et al., discloses use of a structural 
adhesive connecting to the body of a device and used in combination with a 
flexible conductive adhesive at electrical contact areas. This combination 
produces a conductive connection between the leads of a circuit component 
and a supporting substrate, but the connection is again multidirectional. 
The need to confine the area of conductivity when surface mounting a 
component by use of a conductive adhesive has led to development of 
anisotropically conductive adhesives, also known as Z-axis adhesives. 
Anisotropically conductive adhesives are a mixture of a nonconductive 
adhesive binder and conductive particles capable of forming electrically 
conductive paths between facing conductive surface areas when subjected to 
heat and pressure. By delineating the opposed areas at which pressure is 
exerted on such an adhesive, one can eliminate stray and unwanted 
conductive paths outside that area, where the conductive particles will be 
sufficiently separated from each other so that current will not flow 
through the composite mass. This eliminates the flow of current between 
adjacent conductive areas on the same substrate and between facing 
conductive surfaces separated by a distance greater than the minimal 
distance required to complete an electrical path through the adhesive 
mixture. 
Surface mounting of components by use of an anisotropically conductive 
adhesive currently requires application of pressure during the course of 
setting or curing the adhesive. This typically requires usage of 
mechanical fixtures that must remain in place as the component and 
substrate are heated within an oven. Present use of such adhesives also 
requires redesign of the mechanical fixtures as any component features 
within a circuit are changed by subsequent developments. 
The present invention has been developed to take advantage of the ability 
to apply conductive adhesive by screen printing or other types of printing 
technology. The use of printing technology to create electrical 
connections between components and a substrate is very rapid and 
economical. The inventive method also provides a high degree of 
flexibility for meeting changing circuit requirements, eliminating the 
necessity of redesigning mechanical fixtures to hold the circuit 
components in place as the conductive adhesive is being set.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This disclosure of the invention is submitted in furtherance of the 
constitutional purposes of the U.S. Patent Laws "to promote the progress 
of science and useful arts" (Article 1, Section 8). 
The present method is schematically illustrated by the accompanying 
drawing. It can be utilized for surface mounting an overlying electrical 
component 10 to a supporting substrate 11. The component 10 and substrate 
11 have oppositely facing surfaces that include corresponding conductive 
surface areas 12, 13 and 14, 15, respectively. The paired corresponding 
conductive surface areas establish electrically conductive paths between 
the component 10 and substrate 11. 
A layer of anisotropically conductive adhesive 16 is applied to at least 
one surface in each of the paired conductive surface areas 12, 13 and 14, 
15. Such an adhesive includes a mixture of a nonconductive adhesive binder 
and electrically conductive particles. The conductive particles within the 
adhesive are capable of forming electrically conductive paths between the 
surface areas 12, 13 and 14, 15 when subjected to heat and pressure to 
reduce the layer thickness to an operable conductive dimension. This 
thickness dimension is a function of the conductive particle sizes within 
the composite adhesive mass. 
Anisotropically conductive adhesives, also known as Z-axis adhesives, are 
well known in surface mounting technology for electronic components. A 
more comprehensive description of such compositions can be found within 
the disclosure of U.S. Pat. No. 4,588,456, which was issued May 13, 1986 
and which is hereby incorporated into this disclosure by reference. 
One specific example of an anisotropically conductive adhesive amenable to 
the practice of this new method is Zymet ZXUV 101, a Z-Axis epoxy adhesive 
manufactured by Zymet, Inc., of East Hanover, N.J. Other examples are ZME 
series Z-Axis epoxy adhesive paste and ZSP series Z-Axis epoxy adhesive 
paste, both manufactured by Al Technology, Inc. of Lawrenceville, N.J. 
In place of the conventional step of mechanically applying pressure to the 
anisotropically conductive adhesive layers 16 by using mechanical fixtures 
or devices that must remain in place to exert pressure on the assembly as 
the adhesive is set by curing within an oven, the present invention 
utilizes a second adhesive to assure application of adequate curing 
pressure for the conductive adhesive. This second adhesive is a 
nonconductive adhesive 17 applied to at least one of the oppositely facing 
surfaces of the component 10 and substrate 11 at a location spaced from 
the conductive surface areas 12, 13 and 14, 15. 
In the case of a symmetrical circuit component 10, adhesive 17 might be 
applied at its center, assuming that no conductive surfaces are at this 
location. Adhesive 17 might also be applied about multiple locations on a 
particular electrical component 10, depending upon its size and surface 
configuration. 
The choice of adhesive 17 will be dependent upon the specific manner by 
which pressure is to be applied to the adhesive layers 16 during the time 
of curing. 
The respective thicknesses of the adhesive layers should be such that the 
thickness of the applied anisotropically conductive adhesive layers 16 is 
only slightly greater than the operable conductive dimensions at which 
conductive paths are formed across their composite masses. After 
application of adhesive layers 16 and 17, the component 10 and substrate 
11 are next assembled in their desired overlying positions relative to one 
another. The corresponding conductive surface areas 12, 13 and 14, 15 will 
then be in spatial registration with one another. At this point the 
anisotropically conductive adhesive will be in surface-to-surface contact 
with the corresponding conductive surface areas 12, 13 and 14, 15 and the 
nonconductive adhesive will be in surface-to-surface contact with 
oppositely facing nonconductive surfaces of component 10 and 11. 
Illustrative nonconductive surfaces are designated in the drawing by the 
reference numerals 18 and 19, respectively. 
Curing of the adhesive layers 16 and 17 is a sequential two-step process. 
First, the nonconductive adhesive layer 17 is cured to bond the component 
10 to the substrate 11 in a manner that subjects the anisotropically 
conductive adhesive to pressure as a result of the bonding forces applied 
to it by the set nonconductive adhesive. The assembling equipment for 
placing and/or holding component 10 on the substrate 11 can then be 
released. The anisotropically conductive adhesive is subsequently cured 
without further external curing pressure being applied to component 10. 
This results in bonding of the corresponding conductive surface areas 12, 
13 and 14, 15 to one another, causing the conductive particles within the 
anisotropically conductive adhesive to form electrically conductive paths 
between the corresponding conductive surface areas as a result of the 
holding pressure of adhesive 17 while adhesive layers 16 are being cured. 
The pressure applied to the layers of adhesive 16 by action of the cured 
adhesive 17 reduces the thickness of the conductive adhesive to the 
operable conductive dimension at which conductive paths are completed 
through the particles within the adhesive composite during its curing 
process. 
Two submethods for effectively setting the anisotropically conductive 
adhesive 16 under the curing pressure imparted by action of adhesive 17 
have been identified to this point. 
The first submethod involves use of a high shrinkage nonconductive adhesive 
that cures at an energy level or temperature substantially lower than that 
required to set or cure the anisotropically conductive adhesive. 
Conversely, the anisotropically conductive adhesive is curable at an 
energy level or temperature substantially greater than that required to 
cure the high shrinkage nonconductive adhesive. This submethod requires 
the component 10 to be placed on substrate 11 at a position wherein the 
thickness of adhesive layers 16 is not greater than the sum of its design 
conductive thickness plus expected shrinkage during the setting of 
adhesive 17. The shrinkage that occurs in the high shrinkage nonconductive 
adhesive will result in the exertion of compressive forces between the 
component 10 and substrate 11. These forces will pull the two elements 
toward one another and apply the required compression to the uncured 
anisotropically conductive adhesive layers 16 to reduce their thicknesses 
to the dimensions at which they form conductive paths. After the 
nonconductive adhesive 17 has cured and shrunk, these compressive forces 
will remain intact as the assembly is further heated or subjected to 
radiation or other energy to set or cure the anisotropically conductive 
adhesive layers 16. Curing of adhesive layers 16 can be accomplished in 
the absence of external forces since the adhesive layers 16 will continue 
to be subjected to the pressure exerted by the previously-cured adhesive 
layer 17. 
The second submethod involves use of a very fast setting nonconductive 
adhesive 17 to quickly bond the component 10 to the substrate 11 during 
the component placement step. The required curing pressure to achieve 
electrical contact through the particles within the anisotropically 
conductive layers 16 will be provided externally to the component 10 
through conventional component placement equipment not shown that 
spatially locates the component 10 relative to substrate 11 at the design 
conductive thickness of adhesive layers 16. After the fast setting 
adhesive 17 has cured, this pressure will be maintained during subsequent 
curing of the anisotropically conductive adhesive layers 16 without any 
requirement of continuing external forces being applied to component 10. 
A typical Z-axis epoxy adhesive, as identified previously, will cure at 
elevated temperatures of approximately 150.degree. C. The identified Zymet 
adhesive requires a pressure of 0.5-1.0 Kgm/cm.sup.2 during curing to 
obtain good electrical contact. The AI Technology adhesives specify 
application of 0.30 Kgm/cm.sup.2 during curing at 150.degree.-160.degree. 
C. 
Such pressures can be achieved by use of the first-described submethod when 
utilizing a nonconductive adhesive 17 that shrinks approximately 4-12 
percent as it is curing and that will set or cure at a temperature below 
that necessary to set the anisotropically conductive adhesive layers 16. 
Adhesives that shrink approximately 4-12 percent as they are cured will 
exert adequate pressure to set and activate available anisotropically 
conductive adhesives in adjacent areas about rigid electronic components 
and substrates, such as a printed circuitboard. Specific examples of such 
high shrinkage nonconductive adhesives that can be used in this process 
are Loctite 420 adhesive, manufactured by the Loctite Corporation, of 
Newington, Conn. and Pacer M5 adhesive, manufactured by Pacer Technology, 
of Rancho Cucamonga, Calif. Loctite 420 is a cyanoacrylate adhesive having 
10 percent shrinkage during curing, and cures at elevated temperatures 
below 150.degree. C. Pacer M5 has similar properties. 
Examples of fast curing adhesives amenable to the second submethod are 
Loctite UV curable acrylics, produced by Loctite Corporation of Newington, 
Conn. and Emcast 1720 epoxy adhesives, manufactured by Electronic 
Materials, Inc. of New Milford, Conn. Both are cured in less than 10 
seconds by application of ultraviolet radiation. They can be flashed with 
ultraviolet radiation while the component 10 is being held mechanically 
under pressure against the substrate 11 as the component 10 is placed on 
the substrate 11 and with the thicknesses of the layers of adhesive 16 at 
the operable conductive dimensions at which conductive paths are completed 
through the particles within the adhesive composite. The assembly can then 
be transferred to an oven to subsequently cure the anisotropically 
conductive adhesive layers 16 in the absence of further external pressure. 
The required pressure will be maintained on the layers of adhesive 16 as 
they are being cured by operation of the previously-cured second adhesive 
layer 17. 
The nonconductive adhesive 17 will normally constitute a thermosetting 
adhesive that can be cured below the curing temperature of the 
anisotropically conductive adhesive layers 16. The nature and amount of 
energy utilized for setting the nonconductive adhesive will be dependent 
upon the specific choice of adhesive. Numerous adhesives are commercially 
available that meet the physical requirements of both described 
submethods. 
The nonconductive adhesive might be cured by application of any compatible 
form of energy, including ultraviolet radiation or convective heat. 
Similarly, the anisotropically conductive adhesive, which is typically 
comprised of a thermosetting binder, can be heated by any desired 
radiation or heat source, such as an oven. The details of curing both 
types of adhesives are well known and need not be further developed herein 
in order to enable those skilled in this field to practice the invention 
as described. 
In compliance with the statute, the invention has been described in 
language more or less specific as to methodical features. It is to be 
understood, however, that the invention is not limited to the specific 
features described, since the means herein disclosed comprise preferred 
forms of putting the invention into effect. The invention is, therefore, 
claimed in any of its forms or modifications within the proper scope of 
the appended claims appropriately interpreted in accordance with the 
doctrine of equivalents.