Manufacture of coated spheres

Solid spheres of substantially uniform size and shape and coated with a lower temperature melting material are formed for use in interconnect arrays, solder pastes, Z-axis conduction adhesives, etc. Drops of two materials are merged in flight forming a coating of the lower melting temperature material on the drop of higher melting temperature material.

This invention relates to coated spheres and methods of making same. More 
particularly, it relates to merging of droplets of two liquid materials 
having different melting temperatures to form a coating of the lower 
melting temperature material around a sphere of the higher melting 
temperature material and to uses of such coated spheres. 
Although this invention is applicable to dispensing and merging various 
liquid materials, it is particularly useful in creating electrically 
conductive spheres coated with a layer or coating which may be 
electrically conductive or non-conductive. For brevity and clarity of 
disclosure the invention is described herein with specific reference to 
electrically conductive spheres without limiting the applicability of this 
invention to any particular product. 
In high density electronic manufacturing processes, surface mountable 
microelectronic devices are bonded to a substrate by a solder reflow 
process. The interconnect material between the microelectronic device and 
the substrate is usually a solder bump or ball formed prior to the reflow 
process by any of various prior art processes which include deposition 
through a mask, electroplating, pick-and-place, evaporation, sputtering, 
screen printing, etc. Techniques for forming bumps for electrical 
interconnections are usually costly and require hard tooling. In addition, 
many steps are required when using photoresist masks and expensive 
photolithography equipment is needed. With flip-chip methods, additional 
chip processing steps are required. 
In an effort to improve reliability of the solder joint a number of 
expensive and labor or space intensive steps have been tried which include 
adding a control electrode to the chip surface, stacking solder bumps 
separated by polyamide sheets, casting a copper helix in a solder column, 
etc. 
In the area of solder pastes, existing methods of creating solder balls for 
solder paste require a large quantity of solder. It has been difficult to 
control the size of the solder balls and costly screening steps have been 
required to size the solder balls once they are produced. Extreme care 
must be taken to control the amount of oxide on the surface of the solder 
balls until the solder paste is formulated. 
Z-axis conductive adhesives are generally expensive to manufacture since it 
is expensive to obtain uniformly sized metal particles or balls and the 
random spacing of metal particles or balls within the resin must be 
tightly controlled to prevent conduction in the X-axis and Y-axis 
directions. The percentage of metal particles or balls is limited; the 
minimum bonding pitch is limited; and the selection of polymer resins is 
limited to a film (at room temperature) or a b-staged adhesive since a 
liquid polymer at room temperature would allow movement and settling of 
the conductive metal particles or balls. 
A number of applications and advantages exist for coated metallic balls in 
electronic interconnect and packaging. The production of flux, polymer or 
solder-coated solder balls (or solder-coated copper balls) would be useful 
for producing solder pastes. Higher melting temperature metals such as 
copper allow for stand-offs in the paste for controlled bond lines or 
interconnect heights in the final solder joint. 
Metal or polymer-coated balls may also be used for flip-chip or ball grid 
array applications. The coated balls may be produced and then placed onto 
the device or the balls may be ejected or jetted directly onto the device. 
A solder-coated copper ball placed on a chip or ball grid array package 
will provide a stand-off or controlled distance bond line after reflow. 
Varying sizes of balls may be placed on the device to compensate for 
stresses due to thermal mismatch between the device and the substrate. 
Size variations of the balls required may be determined with finite 
element modelling for each configuration. In addition, multiple coated 
metal balls may be printed to form three-dimensional stacking to 
compensate for coplanarity problems or as stress relief mechanisms for 
microelectronic bonding. For flip-chip bonding, it has been found that a 
stretched joint shape (height greater than width) provides the best 
reliability. The shape and dimensions of a joint or bond line can be 
optimized for maximum performance and reliability. In addition to improved 
electrical performance, improved thermal performance (which is critical 
with increasing heat generation of complex ultra dense semiconductor 
chips) may also be achieved. 
Flux-coated or polymer-coated metal balls may be used in other electronic 
applications. A solder ball coated with a tacky flux may be ejected or 
printed onto a substrate and stored without degradation of the solder to 
be reflowed at a future time. The flux surrounding the solder ball 
maintains an oxide-free surface on the solder ball and provides surface 
activation for reflow of the solder ball when the interconnect joint is 
formed. Flux-coated solder balls may also be used for flip-chip, ball grid 
array and fine pitch surface mount applications. A polymer-coated metal 
ball (solder, copper, gold or nickel alloy) provides the same benefits as 
the flux-coated solder ball (maintaining an oxide-free surface until 
interconnection), but the polymer acts as a mechanical bond to hold the 
metal ball in electrical contact with the two interconnecting surfaces. 
This application is similar to an anisotropically conductive Z-axis 
adhesive but metal balls can be placed only where the interconnect is 
required so a higher density ball can be used to improve electrical 
performance and minimize allowable bonding pitches since concerns of X and 
Y axis conduction between leads is eliminated. The remaining area of the 
device surface may be filled with only the polymer material. Upon 
curing/melting of the polymer under pressure, the metallic particles are 
pressed together forming an electrical interconnect path in the Z-axis 
between the two electrodes. 
It is believed that coated metal balls may be used in any electronic 
application where electrical or thermal conductivity is required between 
two surfaces and/or where a controlled thickness bond line is required. 
For example, solder-coated copper balls may be used to attach heat sinks. 
Metal-coated balls, such as polymer-coated aluminum, may also be used in 
aerospace applications where reduced weight is critical while maintaining 
structural integrity. 
In accordance with the present invention solid coated spheres are produced 
with substantially uniform size and shape for use in interconnect arrays, 
solder pastes, Z-axis conduction adhesives, etc. Liquid-ejecting apparatus 
creates and merges drops of two materials to form a coating of the lower 
melting temperature material around a drop of the higher melting 
temperature material. The first material is maintained in a liquid state 
in a first ejection device. A second material (having a lower melting 
temperature than the first material) is maintained in a liquid state in a 
second ejection device. Generally spherically-shaped liquid drops of the 
first material are ejected from the first ejection device in a 
line-of-flight, the spherically-shaped liquid drops of the liquid first 
material becoming generally solidified spheres of first material during 
flight. Generally spherically-shaped liquid drops of a second material are 
ejected from a second ejection device toward the same line-of-flight. 
Ejection of the generally spherically-shaped liquid drops of a second 
material is synchronized with ejection of the generally spherically-shaped 
liquid drops of the first material so that an individual sphere of the 
second material collides with an individual sphere of the first material. 
The individual sphere of the second material is liquified or maintained 
liquid by the heat of the sphere of the first material and the liquified 
second material flows around the outer surface of the solidified sphere of 
the first material to form a unitary sphere consisting of a central solid 
core of the first material coated with the second material. The resulting 
solid coated spheres may be caught in a container for use in producing 
solder pastes or the like or may be directed onto wettable metal pads of 
electronic devices or the like. Other advantages and features of the 
invention will become more readily apparent from the following detailed 
description taken in connection with the appended claims accompanying 
drawing wherein like reference numerals have been applied to like elements 
and in which: 
FIG. 1 is a simplified pictorial illustration of apparatus for forming 
coated metal spheres in accordance with one embodiment of the present 
invention; 
FIG. 2 is a simplified pictorial illustration of another embodiment of 
apparatus for forming metal coated spheres; 
FIGS. 3, 4a and 4b are simplified enlarged pictorial illustrations of the 
process of forming coated spheres in accordance with the invention; 
FIG. 5 is a simplified enlarged pictorial illustration showing formation of 
coated spheres directly on pads of an electronic device; 
FIG. 6 is a sectional view of a microelectronic device operatively joined 
to a substrate by an interconnect array of spheres; 
FIG. 7 is a sectional view of coated spheres formed directly on pads of an 
electronic device; 
FIG. 8 is a sectional view of stacked coated spheres formed on pads of an 
electronic device in accordance with the invention; and 
FIGS. 9a-9c are sectional views of an electronic device using one process 
of the invention to form bonds to a substrate.

FIG. 1 illustrates one embodiment of apparatus (generally designated by 
reference character 10) for producing coated spheres in accordance with 
the invention. Apparatus 10 creates and merges drops or droplets of two 
materials resulting in a coating of the lower melting temperature material 
around the drop or droplet of the higher melting temperature material. For 
best results, the higher melting temperature should have a melting point 
at least about 25.degree. C. higher than the lower melting temperature. 
For purposes of illustration, copper and solder will be discussed herein as 
the materials used in making coated spheres in accordance with the 
invention. It will be appreciated, however, that many other materials may 
be used in various combinations. 
In the embodiment of FIG. 1, apparatus 10 comprises a preload reservoir 12 
for initially receiving and holding the copper 14 (or other higher melting 
temperature material) in a liquid state. Heating element 16 is operatively 
positioned with respect to preload reservoir 12 to maintain the copper 14 
liquid. Thermocouple 18 (or other temperature monitoring device) is 
operatively coupled to control power source 20 so the copper 14 is 
maintained liquid. An ejection chamber 22 is connected to the outlet of 
preload reservoir 12 through conduit 24 and valve 26 to allow liquid 
copper 14 to flow from the preload reservoir 12 into ejection chamber 22. 
Depending on the material used, the preload reservoir may not be 
necessary. 
Heating element 28 is operatively positioned in ejection chamber 22 and 
thermocouple 30 is operatively coupled to power source 32 to control 
heating element 28 to maintain the copper 14 liquid. The ejection chamber 
22 is pressurized through inlet port 34. Inert gas from gas source 36 
forces liquid copper 14 through filter 38 into ejection device 40 in 
preparation for operation. Ejection chamber 22 is pressurized during 
operation with an inert gas to eliminate oxygen from the atmosphere above 
the liquid copper 14. 
Heater 42 surrounds ejection device 40 and controls the temperature of the 
liquid copper 14 within ejection device 40. Heater 42 is operatively 
connected to power source 44. Programmable controller 46 provides 
activating signals to drive electronics 48 whose output causes ejection 
device 40 (which is a continuous type ejection device) to form drops 50 of 
liquid copper 14 from the liquid copper stream 52. The drops 50 are 
ejected from orifice 54 under pressure from gas source 36. 
Ejection device 40 is preferably an electro-mechanical transducer and may 
be a piezoelectric, electro-magnetic or other electro-mechanical source 
which causes the liquid copper stream 52 to break into drops 50 of liquid 
copper 14 in response to an excitation signal from drive electronics 48. 
If the material used in making the drops has a melting point above the 
active range of the transducer, the transducer must be removed from the 
hot zone and mechanically coupled to the molten fluid tube of the ejection 
device. For example, a piezoelectric transducer should be kept below 
300.degree. C. to function properly. If copper (which has a melting point 
of about 1000.degree. C.) is to be ejected, the transducer is removed from 
the hot zone and mechanically coupled to the molten fluid tube. If 63/37 
solder (which has a melting point of about 185.degree. C.) is ejected, the 
transducer may remain in the hot zone and function properly. 
The portion of the apparatus 10 which provides liquid to ejection device 60 
to form the lower melting temperature material for coating the copper 
spheres or balls has been omitted from FIG. 1 for clarity. It will be 
appreciated that the necessary components are substantially the same as 
shown for making the higher temperature melting balls. For illustrative 
purposes, heater 62 is shown operatively connected to a power source 44. 
Programmable controller 46 provides activating signals to drive 
electronics 48 whose output causes ejection device 60 (which is also a 
continuous type ejection device) to form drops 70 of liquid solder from 
the liquid solder stream 64 ejected from orifice 66 under pressure from a 
gas source (not shown). 
Ejection device 60 is an electro-mechanical transducer as described above 
which causes the liquid solder stream 64 to break into drops 70 of liquid 
solder in response to excitation signals from drive electronics 48. 
The space surrounding orifices 54 and 66 and the drops 50 of liquid copper 
and the drops 70 of liquid solder between ejection devices 40 and 60 and 
container or catcher 68 is filled with a relatively inert gas such as 
nitrogen or helium to eliminate oxygen from the path traveled by the 
drops. As shown, a gas source 72 provides a flow of inert gas through 
housing 74 which encloses the path traveled by the drops and the container 
or catcher 68. The housing 74 may be used to provide a positive gas flow 
from gas source 72 to outlet 76. Thus oxides and/or contaminants removed 
from the space will flow out through outlet 76 along with the inert gas 
and will not be deposited on the drops. The inert atmosphere also greatly 
enhances formation of generally spherical drops 50 and 70 of liquid copper 
and liquid solder, respectively. 
Ejection devices 40 and 60 are physically aligned and electronically driven 
so that an ejected drop 50 of copper will meet in flight and merge with an 
ejected drop of solder to form a solder-coated copper ball 80. As the drop 
50 of copper is ejected, it cools very rapidly and is essentially solid by 
the time it merges with drop 50. However, copper drop 50 retains 
sufficient heat energy to liquify solder drop 70 (or maintain it liquid) 
so that the solder will flow around drop 50 and completely coat the 
outside surface of drop 50, forming solder-coated copper ball 80. The 
resulting solidified solder-coated copper balls 80 are collected in 
container or collector 68. Coated balls can be produced in sizes with 
diameters ranging from about 20 .mu.m to about 1000 .mu.m. Accordingly, 
chamber or housing 74 should be long enough to allow the solder-coated 
copper balls 80 to solidify before reaching catcher 68. 
Ejection devices 40 and 60 are precisely controlled to produce a stream of 
drops 50 and 70, respectively, which are spherical in shape and are 
reproduceably precise in diameter. In the preferred embodiment, ejection 
devices 40 and 60 have orifice openings of about ten (10) to about five 
hundred (500) .mu.m in diameter and are excited with signals from drive 
electronics 48 having a frequency between about two thousand (2,000) to 
about one million (1,000,000) hertz. Changes in the amount or volume (and 
the diameter) of the drops are controlled by the excitation frequency 
provided to ejection devices 40 and 60 and the sizes of orifices 54 and 
66. Small changes in diameter (and volume) of the drops require only 
changes in excitation frequency. Large changes require changes in the 
sizes of orifices 54 and 66. 
Apparatus 10 produces spherically shaped quantities of liquid copper and 
liquid solder which are merged in flight. The copper droplets cool in 
flight, allowing the copper droplet to solidify before merging with the 
solder droplet. As the solder droplet is merged with the hot, clean 
(oxide-free) copper droplet, the solder droplet melts (or remains melted) 
and reflows to wet the entire outer surface of the copper sphere, forming 
a solder-coated copper ball. Other metal/alloy combinations are possible 
so long as there is enough difference in melting temperatures to allow the 
higher melting temperature metal to solidify while retaining enough heat 
to maintain the lower melting temperature sphere liquid so that the lower 
melting material can coat the higher temperature melting sphere upon 
merging. Suitable examples of appropriate metal balls are copper, gold, 
aluminum, nickel and alloys thereof. Other metal combinations are 
possible, such as an aluminum-coated copper balls and high melting 
temperature solder (as low as about 100.degree. C.) coated with low 
melting temperature solder (up to as high as about 350.degree. C. or 
higher). Other materials such as polymers and fluxes could be merged with 
metal balls to form coated balls. 
The size of the spheres or balls which can be produced ranges from a few 
microns to hundreds of microns in diameter. The ejection process allows 
production of very uniformly sized balls (less than 5% variation) and, by 
appropriately changing process parameters, any size of ball within the 
range can be produced. It is also possible to control the coating 
thickness by controlling the relative sizes of the drops prior to merging. 
Apparatus 90 of FIG. 2 uses drop-on-demand ejection devices. These are 
shown ejecting or printing solder-coated copper balls 80 directly onto 
wettable metal pads 96 on an element 99 such as a microelectronic device 
or the like. Apparatus 90 otherwise is essentially the same as shown in 
FIG. 1. However, in the embodiment of FIG. 2 an X-Y table 100 is provided 
on which the element 99 is mounted. The X-Y table 100 is equipped with an 
X-axis positioning servo 102 and a Y-axis positioning servo 104. 
Programmable controller 46 is operatively connected to the X-axis 
positioning servo 102 and the Y-axis positioning servo 104 and provides 
programmed control signals thereto to move the X-Y table 100 to a 
particular desired location and/or a predetermined sequence of locations 
with respect to ejection devices 92 and 94. A robotic arm 106, controlled 
by control signals from programmable controller 46, may be provided to 
control the position of the element 99. 
A magnified illustration of the process of forming solder-coated copper (or 
other material) balls 80 is shown in FIG. 3. Upon excitation of ejection 
device 40 by drive electronics 48, drops 50 are formed from the liquid 
stream 52. At the programmed time, ejection device 60 is excited by drive 
electronics 48 to form drops 70 of solder from the liquid solder stream 
64. By the time drop 70 of solder contacts drop 50, drop 50 is essentially 
solid but still contains sufficient heat to cause the drop 70 of solder to 
reflow and completely coat the outside surface of drop 50. The coated 
solidified balls are collected in collector 68. 
FIGS. 4a and 4b illustrate formation of solder-coated copper (or other 
metal) balls having different ratios of component metals. In FIG. 4a the 
thickness of the solder coating is much greater than that formed in FIG. 
4b as is readily apparent from the relative sizes of drops 50 and 70. As 
previously noted, the size of each drop may be varied as desired by 
changing the sizes of orifices 54 and/or 66 and by changing the excitation 
frequency applied by drive electronics 48. 
Solder pastes, which are generally composed of powdered solder alloy 
dispersed in a relatively small volume of a carrier vehicle, are sometimes 
used for attaching microelectronic devices to printed circuit boards and 
other electronic substrates. Although solder pastes may comprise different 
components, the pastes generally are comprised of a solder powder (which 
is a distribution of solder particles); flux to promote wetting of the 
metal to be soldered by removing oxides and contaminates from the surfaces 
to be joined; and viscosity control agents comprising rheological polymers 
to control the rheological properties which influence the deposition 
characteristics of the solder paste. Solvents may be included to aid in 
the flux activation process and to improve shelf life of the flux. It will 
be appreciated that the invention may be employed to produce solder-coated 
solder balls (low melting temperature solder coating a high melting 
temperature solder) and/or solder-coated copper balls which could be used 
in making solder pastes. Coated balls of different diameters may be made 
of the same metal alloys or of different metal alloys. The higher melting 
temperature balls allow for standoffs in the paste for controlled bond 
lines or interconnect heights in the final joint (see FIG. 6). The coated 
ball diameters may be chosen to increase or to decrease the total weight 
percent of solder in the solder paste; to improve the application thereof; 
to allow precise modification of the final alloy composition; etc. 
The solder in solder-coated balls can be an alloy of at least two metals 
selected from the group consisting of tin, bismuth, nickel, cobalt, 
cadmium, antimony, indium, lead, silver, gallium, aluminum, germanium, 
silicon, gold, etc. Solder alloys may be eutectic alloys but that is not 
required. The carrier or vehicle in the solder paste can comprise several 
components which may include (but are not limited to) rosin or derivatives 
thereof, organic solvents, thixotropic agents, active hydrogen-containing 
compounds, diluents, polymers, fluxes, etc. 
It will be appreciated that solder pastes could be made which include 
coated balls of only one diameter or with various combinations of coated 
balls of different diameters within the capability of the present 
invention. Because the diameters of the coated balls provided by the 
present invention are precisely controlled, the diameters of the coated 
balls can be chosen to develop solder pastes which are tailored for 
specific applications not previously obtainable. For example, balls of 
various sizes may be used and balls coated with flux or solder may be 
used. In the preferred embodiment, the solder paste is formed using a 
carrier vehicle and solid coated spheres wherein the solid coated spheres 
comprise at least about 80% by weight of the paste. 
FIG. 5 illustrates formation of solder-coated copper (or other material) 
balls 80 directly on wettable metal pads 96 on an element 99 such as a 
microelectronic device or the like to provide an interconnect array. As 
described above with reference to FIG. 2, the element 99 is moved by 
programmable controls so the solder-coated copper (or other material) 
balls 80 are deposited at the correct locations. When the element 99 is 
properly positioned, the solder-coated copper (or solder) balls 80 still 
contain sufficient heat to permit them to stick to the wettable metal pads 
96. It will be appreciated that the wettable metal pads 96 and/or the 
element 99 could be heated if necessary. 
It will be appreciated that the process illustrated in FIG. 5 may also be 
used to form flux-coated metal balls (flux-coated solder or flux-coated 
copper, etc.) or polymer-coated metal balls (polymer-coated solder or 
polymer-coated copper, etc.). A solder ball coated with a tacky flux may 
thus be printed in place on a substrate and stored for future reflow 
without degradation. The flux surrounding the ball maintains an oxide-free 
surface and provides surface activation for reflow when the interconnect 
joint is formed. Flux-coated solder balls may be used for flip-chip, ball 
grid array, fine pitch surface mount applications and the like. 
A polymer-coated metal ball (solder, copper, gold, nickel, alloy, etc.) may 
provide the same benefits as a flux-coated solder ball (maintaining an 
oxide-free surface until interconnection) but the polymer coating may also 
provide a mechanical bond to hold the metal balls in electrical contact 
with the two interconnection surfaces. This application performs similarly 
to anisotropically conductive Z-axis adhesives. The metal balls, however, 
are easily be placed only where the interconnect is required so a higher 
density of balls may be used to improve electrical performance and 
minimize allowable bonding pitches. 
FIG. 6 is a magnified illustration showing a microelectronic device 98 
operatively joined to substrate 97 using an interconnect pattern of 
spheres or balls of higher melting temperature conductive material coated 
with a lower melting temperature material (such as solder-coated solder 
balls or solder-coated copper balls 80, etc.) after the solder reflow 
process has been completed. The solder-coated copper balls 80 (or 
solder-coated solder balls) are generally of the same overall size. It 
will be appreciated that the temperature for the reflow process is 
controlled so that only the solder coating reflows and not the copper (or 
higher melting temperature solder). Use of copper provides additional 
advantages which include controlling bond-line thickness and providing 
lower electrical resistance than solder so the electrical performance of 
the connection or joint is improved. 
FIG. 7 illustrates the arrangement of various sizes of spheres of higher 
melting temperature conductive material with a lower melting temperature 
material such as solder-coated copper balls 80 (or solder-coated solder 
balls) on element 99 (substrate 97 or a microelectronic device 98) to 
provide an interconnect pattern of solder-coated copper (or solder) balls 
80. Varying sizes of balls 80 may be placed between substrate 97 and a 
microelectronic device 98 to compensate for stresses due to thermal 
mismatch between the substrate 97 and the microelectronic device 98. Size 
variation and placement of the solder-coated copper (or other material) 
balls 80 may be determined with finite element modelling for each 
configuration. The various solder-coated balls 80 may be deposited 
directly on the wettable metal pads 96 of element 99 (substrate 97 or a 
microelectronic device 98) as desired. 
FIG. 8 illustrates the arrangement of solder-coated copper (or solder) 
balls 80 which are stacked on top of each other on the wettable metal pads 
96 of element 99. To form the structure of FIG. 8, a first layer of 
solder-coated copper (or other material) balls 80 is deposited on the 
wettable metal pads 96. When the solder-coated copper (or other material) 
balls 80 have cooled sufficiently, another layer of solder-coated balls 80 
is printed directly onto the first layer. When the solder-coated balls 80 
of the second layer have cooled sufficiently, another layer of 
solder-coated balls 80 is printed directly onto the second layer, etc., 
until the desired height of solder-coated balls 80 is obtained. The 
subsequent layers of solder-coated balls 80 may be formed while the 
previous layer is still warm so that when the subsequent solder-coated 
balls 80 contact the previous solder-coated balls 80 the subsequent balls 
stick to the previously formed balls. This arrangement of tall thin stacks 
of solder-coated copper (or other material) balls 80 may be placed between 
substrate 97 and a microelectronic device 98 to compensate for coplanarity 
problems or as a stress relief mechanism for microelectronic bonding. It 
has been found that for flip-chip bonding a stretched joint shape (height 
greater than width) provides the best reliability. It will be appreciated 
that the stacks of solder-coated balls 80 need not be of the same height. 
For example three layers of solder-coated balls 80 may be formed around 
the outer portion of element 99 and two layers of solder-coated balls 80 
formed in the center portion, etc. 
The shape and dimensions of a joint or bond line can be optimized for 
maximum performance and reliability. In addition to improved electrical 
performance with solder-coated copper balls, improved thermal performance 
(which is critical with increasing heat generation of complex ultra dense 
semiconductor chips is also achieved. 
FIGS. 9a-9c illustrate polymer-coated balls which are stacked on the 
wettable metal pads 96 of element 99 to form a conductive adhesive bond 
between a microelectronic device 98 and a substrate 97. In prior processes 
of forming conductive adhesive bonding, the polymer used to hold randomly 
dispersed conductive metallic particles is in the form of a film at room 
temperature. Only small numbers of randomly dispersed conductive particles 
are mixed in the film to avoid conductive metallic particles touching each 
other in the X and Y directions since conductivity in only the Z direction 
is desired. It is obviously very difficult to control the random 
dispersion of the conductive particles in making of the film. 
As shown in FIG. 9a, a first layer of polymer-coated metal balls 108 is 
printed directly onto the wettable metal pads 96, followed by another 
layer and additional layers, if desired, etc. It will be appreciated that 
the apparatus of FIG. 2 may be used to produce such layers using polymer 
and copper, etc. 
As illustrated in FIG. 9b, the same polymer 109 has been dispensed (by 
conventional apparatus through dispenser nozzle or tip 110) to fill the 
areas between the wettable metal pads 96. Conventional processing is then 
used to place the microelectronic device 98 on the polymer 109 and 
polymer-coated metal balls 108 and apply heat and pressure, forming the 
structure of FIG. 9c. It will be appreciated that conductivity occurs only 
in the Z direction between the wettable metal pads 96 and the desired 
conductive areas on the microelectronic device 98 through the metal balls 
112. 
It will be appreciated that the present invention permits formation of 
spheres of one material coated with a lower melting temperature material. 
The lower melting temperature material can be a polymer, flux, solder, 
glass, wax, metal or mixture, solution or alloy of metals, while the 
higher melting temperature material can be almost any material which has a 
higher melting point and can be ejected in drops. In the presently 
preferred applications, materials such as aluminum, lead, tin, copper, 
gold, nickel, bismuth, gallium, silicon, cobalt, cadmium, antimony, 
silicon, germanium, tellurium, indium and mixtures, solutions or alloys of 
two or more of such metals may be used. The resulting coated balls can be 
used in electronic applications where electrical or thermal conductivity 
is required between two surfaces; where a controlled thickness bond line 
is required; where conductivity needs to be limited to the Z direction 
only and/or where reduced weight is critical while maintaining structural 
integrity, etc. Although the invention has been described with particular 
reference to presently preferred embodiments thereof, it will be 
appreciated that various modifications, alterations, variations, etc., may 
be made without departing from the spirit and scope of the invention as 
defined in the appended claims.