"Area Bonding Conductive Epoxy Adhesive Preforms for Grid Array and
MCM Substrate Attach"
by
Justin C. Bolger, Merix Corp., 77 Charles St., Needham, MA 02194
and Ken Gilleo, Adv. Products Div., Alpha Metals Corp., Jersey City, NJ
Abstract
A new type of 2-axis epoxy tape adhesive, called
an area bond conductive (ABC) adhesive, has been
developed under ARPA contract to replace tin-lead solder
for surface mounting grid array components. The ABC
adhesive tapes contain discrete regions, or Idots", on pitch
down to 0.2 min, of an electrically conductive epoxy
within a high strength, high Tg , epoxy dielectric phase.
The adhesives are supplied as die cut preforms, which
match the size and bond pad pattern of the component to
be attached. The preforms cure at 160-175 ° C and require
no pressure during cure, in yield a shock resistant, voidfree area bond to any FR4 or other board surface. This
paper presents bond strength, conductivity, dielectric
strength, humidity and thermal shock results for daisy
chain test circuits and other components attached to FR4
boards.
Introduction
ARPA contract #DAAH01-93-C-R322 to the group
consisting of Merix, Auburn University and the
University of Arizona, is intended to develop and
demonstrate new polymeric materials to reduce the
cost, improve the manufacturabilily and permit the
customization and repair of MCM-L's.
One of the materials being developed is a new type of
z-axis adhesive, called an area bond conductive (ABC)
epoxy adhesive preform. The ABC preforms consist of
a high strength, thermally conductive epoxy tape
adhesive, within which is formed columns or regions of
a second, electrically conductive, epoxy adhesive. These
adhesive preforms are intended to replace C-4 solder
bumps for mounting flipped chips, and to replace solder
paste and solder balls or columns for BGA, CGA,
substrate and component attach.
These adhesive preforms are now being made, in
sample quantities, at Merix. The Alpha Metals Corp.,
now the largest supplier of solder paste and solder halls
to the I.C. industry, has the exclusive license to
manufacture and supply these preforms, in commercial
quantities. The Alpha Advanced Products Division also
supplies die attach adhesives and polymeric solder
alternatives for board assembly and direct chip attach.
Discussion of prior work
Area array packaging is increasing rapidly in volume,
both for attaching flipped I.C. chips, and for surface
mounting BGA's, OMPAC's, and COA components (1).
IBM's development of the C-4 solder bump process, 20
years ago, led to large volume use of area array bumped
flipped chips. Recently, Motorola, Amkor, IBM,
Compaq and others have begun volume production of
I.C. packages with solderable bond pads arrayed in a
grid on the bottom surface, to achieve smaller size,
higher lead count and other performance advantages
over QFP's or other fine pitch perimeter bonded
packages.
Although these BOA'S are now in volume
production, some problems remain (I) (2) (3). These
include "pop corning" (destructive release of absorbed
moisture during solder mounting) and solder joint
fatigue. Although BOA reduces thermal mismatch by
mounting an FR-4 package on an FR-4 board, the die
which is attached to the BOA circuit hoard appears to
restrain movement, causing solder joint fatigue in the
The ABC preforms are made by a new stencilling
process, from art-work showing the size and location of
the bond pads on the part to be bonded. The process
can print down to 0.1 mm dots on 0.2 mm pitch. For
cost reasons, the preforms are produced in sheet form,
yielding many preforms per sheet, Fig. 1. In production,
the sheets would contain multiple rows of 8 or more per
row, which would then be slit and kiss cut to provide the
strips shown in Fig. 2.
die area (2). These fatigue problems frequently require
the additional use of an underflow epoxy encapsulant to
improve reliability.
Conventional z-axis conductive adhesive films, which
contain randomly distributed quantities of small (10 to
25 p dia) metal plated polymeric balls, have been
considered for solder replacement in area array surface
mounting, but give low electrical conductivity relative to
solder. And became these z-axis films generally give
final bond thicknesses less than 10 to 25 p, they have
almost no tolerance for non-planarity and provide
limited relief of interfacial stress.
Fig. 3 shows a 50X Polaroid enlargement of the
individual conductive dots plus a cross sectional
drawing. Present standard ABC preforms are 50 to 75
p thick in the (black) dielectric epoxy regions with dot
heights from 100 to 150p . Other, thicker preforms can
be made, if desired, for non - planarity tolerance or stress
relief purposes. The preforms also contain a fiberglass
mesh, 30 p thick, for dimensional stability and to permit
kiss cutting into the strips of Fig 2.
In contrast to the previous random particle z-axis
adhesives, the new preforms described in this paper are
customized for each component to be mounted.
Conductive adhesive areas are located only at bond pad
locations and final thickness can be optimized and
controlled between 25 and 250 p for planarity and
stress relief purposes. This paper will describe preforms
for attaching area array packages with grid pitch down
to 1 mm. Future work under the ARPA contract will
focus on preforms with pitch down to 0 2 mm for area
bonding flipped chips.
The two epoxy adhesive phases differ primarily in
filler type. The conductive dots contain 85% by wt. of
silver flake for high electronic conductivity. The region
between the dots contains a high loading of oxide fillers
for high thermal conductivity plus high electrical
resistivity. Both epoxy regions are solvent free, Bstaged, non-tacky solids, with a melting point of about
70 ° C before cure. Both regions cure, with no volatiles
or outgassing, to yield high T g , high strength adhesive
bonds. T depends on cure time and temperature.
g
After 1 hour
cure at 160 ° C, T E is about 150° C. After 1
°
hour at 175 C, Tg is about 16d° C.
Description of product and assembly process
Figs. 1 to 6 show several area bonding conductive
(ABC) epoxy preforms. These are similar chemically
and in form to the epoxy tape preforms which have been
used for 20 years to bond hybrid substrates into military
hermetic packages. In each case., an epoxy tape
adhesive is die cut into customized shapes, "preforms",
which match the size and shape of the part to he
mounted. The preforms are normally sold in strips on a
Mylar film carrier (Fig. 2), But whereas the previous
mil-spec adhesives were insulating materials, intended
only for high strength bonding, the present ABC
preforms also contain a matrix of electrically conductive
epoxy adhesive "dots". These conductive dots are
Assembly Process. The ABC preforms are applied,
cured and handled using the same methods as for
previous (non-electrically conductive) tape adhesive
preforms. Figure 4 shows how the BGA or other
component is first tacked to one of the preforms on the
Mylar carrier film of Figure 2. This is normally done by
passing the carrier film across a heater block surface at
100-140 ° C, to warm the preform to about 80 ° C. When
the BGA is pressed against the warm preform for 1-2
seconds at 2-4 psi, the preform bonds to the BGA
surface.
arranged in a customized pattern which corresponds
exactly to the bond pad pattern on the bottom of the
BGA or other component to be surface mounted.
The BGA's may then be left attached to the carrier
strip for shipment and/or storage, or may be lifted off
the strip and tacked down on a heated mounting board
by contacting for 2-3 seconds at 80 ° C and 2-4 psi
pressure. The board is then oven cured for 30-60
minutes at 160 175 ° C. There are no volatiles released
during the process and no pressure is required to hold
the SGA's during cure.
The figures show two different ABC preform sizes.
The larger preform has a 11 x 11 = 121 dot pattern on
100 mil pitch. These were used for the humidity, bond
strength, and other reliability tests discussed below. Fig.
1 shows higher density 15 x 15 = 225 dot preforms on
1.5 mm pitch which were made to fit present standard
1.5 mm BGA's and OMPAC's.
-
2
Electrical Conductivity. Conductivity was measured
using the daisy chain circuit patterns of Fig. 5. The
circuit patterns were printed on FR-4, Kapton, alumina
or glass substrates using a high T g , thermosetting epoxy
PTF ink. When any two substrates were properly
aligned and bonded with one of the 11x11 dot preforms,
all 11 dots in each row were connected in series. The
solid line at the left edge of the circuits in Fig. 5 is not
contacted by the preform dots, and is used to factor out
line resistance. Typical values for the daisy chain rows
in Fig. 5 ranged from 1.6 to 2.0 ohms at 25 ° C and from
2.0 to 2.5 ohms at 130 ° C. Subtracting the circuit line
resistance of about 1.2 ohms gave an average dot
resistance of 0.03 to 0.06 ohms at 2.5 ° C and 0.04 to 0.08
ohms at 130 ° C. This is an order of magnitude higher
conductivity than for previous random particle z-axis
adhesives, and is comparable to conductivity through
present BGA or CGA solder joints.
Repair Procedure. The ABC adhesives form high
strength bonds at ambient temperature, but soften when
heated above T g . One inch square BGA's can be
removed from board surfaces using present (1) torsion
or lifting tools when (locally) heated to 180-200 ° C. This
causes cohesive delamination, within the adhesive. If
there were no glass mesh within the preform, this
delamination would leave a hard-to-remove epoxy
residue on both surfaces. But the ABC preforms are
made such that the thin (30 it) glass internal mesh is
directly against the release film side of the preform, Fig.
3. Bond failures above T galmost always occur just at
the board-preform interface, leaving a relatively clean
bond pad surface for the replacement BOA. When,
occasionally, delamination occurs on the other side of
the fiberglass mesh, the mesh allows the residue on the
board to be peeled away, above T g , again leaving a
reasonably clean surface for replacement.
To minimize the possibility that one or more
individual dots in a matrix will not make contact, the
process for making the preforms minimizes deposition
of dielectric epoxy on top of the dots. The dots are
made (Fig. 3) to project above, and to have a much
higher viscosity than the surrounding dielectric epoxy, to
maximize pad contact. And an optical final inspection is
used to reject preforms with undersized or missing dots.
Thermal Stability and Solder Reflow. The cure
temperature of 160-180 ° C in Fig. 4 will avoid the
cracking and vapor release problems (pop-corn effect)
now observed with solder paste mounted BGA's.
Components attached with the ABC preforms may be
exposed to reflow temperatures if other components are
later soldered to the same board. This re-heating
should not cause a problem, because the cured ABC
adhesives retain enough hot strength to hold
components in place at reflow temperatures, and
because TGA data in air show no significant weight loss
or adhesive decomposition below 300 ° C.
Dielectric Strength and Silver Migration. Insulation
between neighboring dots was measured by applying a
DC voltage between adjacent lines of the assembled
circuits of Fig. 5. Dielectric strength was tested after
initial cure and also after water immersion and after
pressure cooker or 85/85 humidity tests. Typically, after
initial cure, adjacent lines showed over 10 6 ohms
resistance and allowed no detectable current flow at
V =120 volts.
Results and discussion
Bond strengths. Oxide filled adhesives are stronger
in shear or tension than the same epoxies with a high
loading of silver filler. The silver filled dots in these
ABC adhesives (Fig. 3) occupy only about 5% of the
volume, the remainder being the higher strength
dielectric epoxy. Hence, the standard shear strength
tests used for conductive adhesives, such as MIL 883B,
Method 5011, can not be used with the ABC preforms
because die fracture occurs before adhesive failure in
shear. For example, 1.2x1.2 cm silicon dies, bonded to
alumina substrates, failed by die fracture at about 30 kg
shear force, leaving the adhesive and bottom half of the
die intact. Tensile shear was therefore measured by a
variation on ASTM D-1002, using 7" wide, 60 mil thick
aluminum strips with a 1" overlap, bonded with the
preforms of Fig. 5. After 1 hr cure at 165 ° C, overlap
shear strength averaged 1650 lbs (750 kg) at 25 ° C.
Because the silver filled epoxy regions are entirely
encapsulated within a high Tg , low moisture absorption
epoxy matrix, silver migration within the adhesive would
not be expected to occur in a properly made, void free,
joint. This was found to be true. Daisy chain circuits,
Fig. 5, were tested after removal from the humidity
chambers, while still saturated with water, by applying
the Mil Std 20 volts D.C. between adjacent daisy chain
lines. Present Mil specs require no shorting after 20
minutes at 20 volts. No migration failure was observed
after 60 minutes at 20 volts for any of the 12 line pairs
tested.
3
Humidity and Pressure Cooker Tests. Test samples
were made with two substrate combinations using the
11x11 = 121 dot, 100 mil pitch, ABC preforms. For one
set of 12 samples, Fig. 5, a daisy chain circuit on a 66 mil
thick FR-4 board was attached to a second circuit on 2
mil thick Kapton. A second combination (10 samples)
attached a 2.5 cm x 2.5 cm x 10 mil thick FR-4 square to
the 66 mil FR-4 board. The laminated parts were first
tested after initial assembly per Fig. 4 and then again
after 50, 100 and 200 hours in humidity.
refs. 2 and 3. After 10 cycles, all samples were inspected
and tested for conductivity. No cracking or
delamination was observed for any sample. All daisy
chain lines on each assembly remained conductive,
generally with less than 25% change from initial
conductivity.
While these reliability results have been encouraging
to date, it is clear that considerable additional testing is
needed. Future pressure cooker tests will use boards
and substrates which will not themselves degrade in
steam, to provide a test of the adhesive itself. Longer
term, 85/85 humidity tests and thermal cycling tests, are
also now in progress using the standard 1.5 mm pitch
BGA packages and daisy chain boards shown in Fig. 6.
Half of the samples were exposed to 85 ° C/85% RH.
After 200 hours, none of these samples showed
significant change or degradation. All daisy chain lines
remained conductive and within 25% of initial
resistivity. All boards continued to show >10 6 ohms
between adjacent lines. The 85/85 tests were therefore
discontinued in favor of a more severe test in pressure
cooker, at 15 psig/121 ° C.
Conclusions
A new type of area bonding conductive (ABC) epoxy
adhesive was developed to replace solder for surface
mounting grid array chip carriers and other
components. These adhesives are supplied as die-cut
preforms, which match the size, shape and conductor
pattern of the component to be attached. The preforms
are applied by a conventional assembly process to yield
high strength, shock resistant, void free area bonds to
FR4 or any other board surface. Electrical conductivity
of the individual epoxy dots is about equal to present
solder ball joints.
All of the circuit assemblies (6 FR-4/Kapton and 5
FR-4/FR-4 samples) inspected after 15 psig/121 ° C
steam exposure showed severe degradation of the
substrates, but not of the adhesive used to join the
substrates. After 100 hours, the Kapton circuit films
had hydrolyzed and had begun to flake apart, leaving an
intact preform which was still bonded to the FR-4
board. After 200 hours, the FR-4 boards softened and
lost insulation properties. Volume resistivity for the
FR-4 boards decreased from an initial value above 10 7
ohms/sq to less than 500 ohms/sq when measured
(while still saturated) after 200 hours.
The cure temperatures of 160-175 ° C avoid pop-corn
effects, and no external pressure is required during cure.
Adhesive thickness can be varied from about 50 to 200 p
for planarity and stress optimization. This paper
showed encouraging bond strength, conductivity,
humidity and thermal shock results using the preforms
to attach Kapton or FR4 daisy chain circuits to 1'R4
boards. Additional longer term tests, using standard
grid array chip carriers on FR4 daisy chain boards, are
still in progress. These long term results will be
presented at the conference in March.
Although these substrate failures made it difficult to
evaluate the ABC epoxy adhesive, it was clear that the
epoxy did not soften or blister or suffer any adhesion
loss at either 85/85 or in pressure cooker. At the end of
the test, when the FR-4/FR-4 samples were dried and
tested electrically, all daisy chain lines remained
conductive, and there was no electrical contact between
any adjacent lines.
References
Thermal Shock Tests. The cycle used was 30 minutes
in a 150 ° C air oven, followed immediately by 30 minutes
in a -20 ° C cold chamber, followed by inspection at
room temperature. The test pieces were similar to the
FR-4/FR-4 humidity samples described above (2.5 x 2.5
cm x 10 mil thick FR-4 squares, bonded to the 66 mil
FR-4 board using the 100 mil pitch preforms) except
that a silicon die, 0.480" square, had previously been
attached to the top surface of the 10 mil FR-4 piece.
This sample geometry was intended to duplicate the
geometry and thermal shock conditions described in
(1) Tuck, John. "BGA's: A Trend in the Making',
Circuit Assembly, Dec. 1993.
(2) Ejim T.I. et al. "Attachment Reliability of BGA
Packages" VLSI Conf., Yorktown Hts, N.Y., Oct. 1993.
(3) Mawer, A., et al. 'Calculation of Thermal Cycling
and Fatigue Life of Plastic BGA Packages", p. 413-22,
Nepcon East 1993 Proceedings.
4
'Figure 1. Two preforms (top
right) with 15 x 15 conductive dots,
on 1.5 mm pitch, cut from sheet of
4 units, left. The conductive
areas correspond exactly to the
bond pads on the EGA chip
carrier, lower right.
For production
Figure 2.
purposes, large sheets of epoxy
adhesive arc slit into strips and
then kiss-cut to provide the
individual preforms in strips or
rolls, pre-indexed on a Mylar
release film carrier. EGA to be
surface mounted at lower left
Figure 3. The 50X Polaroid, top,
shows the individual silver filled
(85% silver by wt.) conductive
'epoxy dots. The black area is a
'high strength, T g = 160 ° C, oxide
filled, electrically insulating, epoxy
adhesive. Dielectric thickness can
be varied from 30 to 200 A, with
conductive dot height from about
100 to 300 p
4/
.2"
Dot Height
4-5 mils
,/,/'
(.7
— 2-3 mil
Dielectric
2 mil Mylar
Carrier Film
Figure 4. Assembly Method. The
preform is first tacked to the
BGA or other component surface .
by contacting for 1-2 seconds at
80-100 ° C. The BGA's may then
be left on the carrier strip and
stored for later mounting, or
removed and attached to the
motherboard surface by tacking in
place and then oven curing at 160170 ° C. No pressure is required
during cure.
80C
1 -2"eitt:'
I
transfer
ABC Preform on Carrier Film
111111211211EMM-
Surface Mount
80C
3 sec
111111111111011
Cure at 160C
-
Mother Board
Figure 5. Daisy chain pattern on
circuit boards used for pressure
cooker and other reliability tests
with the 100 mil pitch preforms.
Various substrate combinationsi
were used. This photo shows one
circuit formed on FR-4 board .
(left) and the second on Kapton
(top center). After connecting the
two circuits using the 11 x 11 dot
preform (bottom center) all 11
dots in each row are connected in
series (right).
Figure 6.
Standard 1.5 mm
BGA's before (bottom right) and
-after attachment to a FR-4 daisy,
chain test board, using the 15 x 15
dot, 1.5 mm pitch, ABC preforms
(top right).