 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to the manufacture of wiring sheet assemblies, e.g.
such as are typically used for the mounting of integrated circuit chips.
In particular, the invention is concerned with the manner in which
electrical connections are formed between successive conductive layers of
the wiring sheet assembly which are separated by insulating layers.
BACKGROUND OF THE INVENTION
Multilayer wiring sheets are used conventionally in electronic computer
circuitry. The operating speeds of computers are continuously increasing.
To achieve a high operating speed, high speed signal transmission is
needed. For high speed transmission, signal delay must be minimised at
every part of the system, including the wiring assembly connecting to the
integrated circuitry.
A typical layered wiring assembly has a ceramics substrate on which are
laminated one or more insulating films. The insulating films support and
separate conductive circuit layers, usually of copper. In conventional
manufacture, insulator layers and patterned circuit layers are layered
progressively onto the ceramics substrate, with each conductive layer
being connected to the preceding one at the desired locations through
through-holes of the insulator layers, at each stage of the process.
The number of electrical connections to be made in this sequential
lamination process may be very large, typically thousands per layer. The
best conventional process uses localised metal plating, typically
electroless plating of copper, to plate through the through-holes (or
"vias") and thereby connect an underlying conductor with a connection pad,
adjacent the through-hole, of the upper conductor. The plating process
involves many heat cycles and chemical treatment steps, and the resulting
plated junction is liable both to damage and to peeling from the pad
portions. It is therefore expensive, and the reliability problem makes it
hard to achieve wiring assemblies having large numbers of layers e.g. more
than ten.
Another conventional method uses a conductive solder paste whereof a lump
is placed in the through-hole and fused by heating so that its top flows
over the exposed conductor. The conductivity of these solder pastes is
low, so they are inappropriate for high-speed signal transmission. They
are also liable to cracking, and their adhesion to the conductors is not
good.
SUMMARY OF THE INVENTION
An object of this invention is to provide a novel interconnection method
for wiring sheet assemblies.
Another object of the invention is to provide wiring sheet assemblies
incorporating novel electrical connections.
In this invention, we form an electrical connection between conductors
separated by an insulating layer of a wiring sheet assembly by forming a
through hole overlying one of the conductors, bonding a metal stud,
preferably of a metal having an electrical resistivity not greater than 3
.mu..OMEGA..cm, onto the conductor through the through-hole, and then
pressing the exposed end of the metal stud so that it is mechanically
spread into contact with the conductor at the exposed surface.
Preferably the stud is bonded to the underlying conductor by forming a
metallurgical bond using applied heat, pressure and preferably also
ultrasound. In particular, it is preferred that such a bond be formed by a
wire-bonding technique, in which the stud is formed by first bonding the
end of a wire onto the underlying conductor and then separating the bonded
end from the remainder of the wire to form the stud. This technique has a
great practical advantage, since it can be carried out using wire-bonding
steps which are in themselves conventional, and for which suitable
apparatus is readily available or can conveniently be adapted. The
conventional wire-bonding process, originally developed by Matsushita and
now well known, is used for bonding connecting wires onto chip bonding
pads. For the present process, that method can be adapted to include the
step of detaching the bonded end from the remainder of the wire to form or
partially form the stud.
The pressing of the exposed end of the stud may be carried out by any
suitable mechanical presser e.g. a stamper (stamping in a direction down
onto the stud) or a roller. Stamping has the advantage that it is
conveniently combined with the direction of operation of a bonding machine
which has means for downward pressing for bonding. For example, a stamping
head may be associated with the pressing capillary used in wire-bonding.
The use of mechanical pressing, as opposed to the reflow melting used in
the prior art solder paste process, is advantageous. Firstly it avoids a
heating step, while nevertheless helping to ensure that the through-hole
is filled by the stud so that good conduction is obtained. Secondly, it
becomes possible to form the exposed end of the stud with a flat top. This
contrasts with the rounded dome created by reflow, facilitates the
application of subsequent layers without undesirable voids and
irregularities, and thereby contributes to production reliability.
Thirdly, it presses the flange positively into contact with the uppermost
conductor. If desired, the pressing may incorporate additional bonding
conditions such as heat and/or ultrasound, to form a metallurgical bond to
the upper conductor.
The present method is of course also superior to the solder paste reflow
method in that materials of very high conductivity, generally of
resistivity less than 3 .mu..OMEGA..cm for example gold, copper or
aluminum, may be used. Nevertheless, the technique offers substantial
processing advantages e.g. in the manufacture of non-high-speed circuitry,
even if lower conductivity materials such as tin/lead solder are used.
If desired, the underlying conductor may be plated before the stud is
applied.
The technique may be used to connect one patterned wiring layer to another
through an intervening insulator layer. It may also be used with advantage
to connect the wiring layer of a lowermost insulating layer through to the
connector terminals of a substrate of the wiring assembly, typically a
substrate of ceramics or silicon. These terminals, commonly of tungsten,
may be plated before the stud is applied.
Embodiments of the invention are now described by way of example, with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a multi-layer wiring board assembly;
FIG. 2a-f illustrates the stamping of a stud microbump electrical
connector;
FIG. 3a-c shows a wire-bonding method for initially forming a stud;
FIG. 4a-b illustrates modes of mechanical pressing of the studs;
FIG. 5 is a plan view showing the spread exposed end of a stud on a bonding
pad;
FIG. 6a-e illustrates forming of electrical connections through a duplex
Cu-clad insulator sheet, and
FIG. 7 shows schematically, in perspective, a thin-film assembly board with
an LSI mounted thereon.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows in section part of a multilayer wiring board comprising a
stack 2 of wiring sheets superimposed on a stiff substrate 1.
The substrate 1 is of a generally conventional type. Its main layer 11 may
be of e.g. ceramics, glass ceramics, silicon, glass-epoxy,
glass-polyimide, epoxy reinforced with organic fibre (e.g. aramid),
polyimide reinforced with organic fibre, or the like. Ceramics,
glass-epoxy and glass-polyimide are preferred because they have a good
combination of heat resistance, flexibility, moisture resistance,
electrical resistance and dimensional stability. In this particular
embodiment, a 3 mm mullite layer 11 was used. Terminal pins 12 for making
electrical connections, e.g. to signal, power source, ground etc, extend
through the layer 11. The top off each terminal pin 12, at the upper
surface of the layer 11, has a connecting pad 13.
The terminal pins 12 are preferably of tungsten. To improve bonding to the
tungsten, the connecting pads 13 are preferably plated. In this
embodiment, a 1 .mu.m nickel layer and a 0.5 .mu.m gold layer were used.
The wiring sheet stack 2 consists essentially of metal wiring patterns: 4,
each forming a desired circuit pattern, formed on arid separated by
insulating layers 3 of high dielectric constant. In the embodiment shown,
each wiring pattern 4 has been fabricated onto the top surface of a
respective one of the insulating layers 3 so that they form together an
integrated wiring layer.
The embodiment shown has five wiring layers. However the number may vary
e.g. from 1 to 20, and usually at least 2.
The insulation layers 3 preferably have a thickness between 0.1 and 500
.mu.m, more usually 5 to 100 .mu.m. A variety of suitable materials is
well known in the art. For example, polyimide, polyamide, polyester,
polysulfone, polyparabanic acid, polyhydantoin, polyetheretherketone,
polyaddition-type imide, epoxy resin, phenolic resin,
poly-p-hydroxystyrene polymer, fluororesin, silicone or phosphazene resin,
or composites of the mentioned polymers with reinforcing materials such as
glass fibres, aramid fibres or the like, are known suitable materials.
Among these, polyimide is particularly preferred since it has a good
combination of heat resistance, flexibility, electrical insulation and
adhesive strength. Among polyimide films, a combination-type film using a
condensation-type aromatic polyimide (or a precursor of such a polyimide
e.g. a polyamic acid) with a polyaddition-type polyimide (e.g. an
N,N'-substituted bismaleimide) is particularly preferred.
Materials for the conductor patterns or wiring are also well known. Copper,
aluminum, and gold are all suitable; copper is used most frequently. The
wiring pattern is preferably formed on the preformed insulation layer (or
substrate) e.g. by a metal plating method after patterning with a resist,
or by an etching method for etching metal foil from a metal foil-clad
insulating layer at parts other than parts intended to constitute the
circuit, and which are protected by a resist.
The wiring patterns 4 of adjacent wiring layers are connected to one
another through the insulating layers 3 by means of metal studs 5
extending through through-holes or vias 31. In a typical assembly, in
which the area of each layer might rankle from 100 to 500 cm.sup.2, there
might be from 2000 to 100,000 such electrical connections per layer.
The forming of electrical connections through through-holes is per se
conventional, and consequently techniques for forming the through-holes 31
are well known. The preferred technique uses an excimer laser. Usually,
the diameter of the holes is between 30 and 70 .mu.m. The holes 31 are
made at locations which overlie, or will overlie when laminated on, a
portion of the conductor on the layer beneath, while their tops open at or
through bonding pads 41 of the exposed upper wiring pattern 4.
As can be seen in FIG. 1, each metal connecting stud 5 is a single piece
which entirely fills the through-hole 31, has a bottom end 52 abutting and
bonding against the conductor below in face-to-face relationship and a top
end 53 which is radially spread out to form a flange 54 overlapping in
contact with the bonding pad 41 of the upper wiring 4.
Most of the studs 5 abut squarely against the flat surface of the
underlying wiring. Some of them, however, bond partly or entirely against
the head 53 of a similar stud in the layer beneath.
The preferred material for these studs 5 is a metal having an electrical
resistivity less than 3 .mu..OMEGA..cm, particularly if the circuitry is
intended for high-speed operation. Gold, an alloy of gold with one or more
of indium, silicon, germanium and antimony, or silver, copper or aluminum,
are all suitable materials having excellent conductivity, and thereby low
heat generation at the pad-stud junction. They also reduce signal
transmission delay while achieving good product reliability.
Methods for forming the studs 5 are now described.
FIG. 2(a) to (f) shows steps in forming and connection of a bonding pad 41
of the wiring on a first insulating layer 3 through to the connector pad
13 of one of the substrate terminal pins 12. The wiring layer is a 25
.mu.m polyimide sheet having an 18 .mu.m copper cladding 4' on one surface
(FIG. 2(a)). The copper cladding was photoetched according to known
processes to form the wiring pattern, including a bonding pad 41 (FIG.
2(b)). An excimer laser of 248 nm wavelength was used to drill through the
polyimide at the locations for connection through to the substrate
terminal pins, forming through-holes 31 (FIG. 2(c)).
Then, a conventional alignment apparatus was used to register alignment
marks on the substrate layer 11 and insulating sheet 3, and thereby
superpose the substrate 1 and sheet 3 with the substrate connector pads 13
aligned with the through-holes 31 of the sheet 3. Sheet and substrate were
then fixed together by conventional means to hold them in position. FIG.
2(d) shows the ensuing alignment. Then, as shown in FIG. 2(e), a metal
bump or stud 5' was positioned in the through-hole 31 and bonded to the
underlying connector pad 13, which had previously been prepared by nickel
and gold plating, to form a metallurgical thermocompression bond using
heat, pressure and ultrasound. A gold bump 5' was used in this embodiment.
Then, the studded bump 5' was stamped from above causing its body to form a
shank 51 wholly filling the through-hole 31, and its exposed end 53 to be
spread into a flange 54 in overlapping contact with the bonding pad 41 of
the upper wiring (FIG. 2(f)). The stamper used was connected to a heater
and ultrasonic source, so that the flange 54 adhered strongly to the top
bonding pad 41.
Bonding conditions for the face-to-face bonding at the bottom of the stud
typically use a temperature of 150.degree. to 400.degree. C. in
combination with ultrasonic energy, in a manner known per se for wire
bonding.
The partially-formed bump or stud 5' may be introduced into the
through-hole by various means. For example, lumps of suitable metal may be
printed into the holes. By far the most preferred method, however, is to
form and bond the studs directly into the holes 31 using a wire-bonding
method, since wire-bonding techniques are suitable for use with these
materials, they achieve bonds of a good strength, and the technology for
accurate location of a very large number of wire-bonding operations is
already available.
So, as indicated in FIG. 3, a partially-formed stud 5' can be deposited
using a wire-bonding head 6 having a guiding capillary 61 through which
the end of a wire 64 projects, a releasable clamp 63 for releasably
gripping the wire 64 above the capillary opening, and a movable electrode
62 which can be positioned adjacent the end of the wire 64 in a known
manner, to form a softened ball 65 at the wire end by electrical arcing.
FIG. 3(a) shows the formed ball.
FIG. 3(b) illustrates withdrawal of the electrode 62, release of the clamp
63 and pressing of the ball 65 against the prepared bonding region by the
longitudinally-drivable end of the capillary 61, which (as is conventional
in wire-bonding methods) is adapted to apply heat and ultrasound energy as
well as mechanical pressure. The ball is accordingly flattened and bonded
onto the underlying conductor.
Then, as shown by FIG. 3(c), the clamp 63 grips the wire 64 again and
pupils sharply upwardly to separate the bonded ball, now the part-formed
stud 5', from the rest of the wire. Alternatively, this can be done by a
sideways shift of the capillary 61 relative to the bonding site. The
bonding head 6 then moves to the site of the next connection, in
accordance with a control program.
Most preferably, the bonding machine is also provided with a stamping head
57, of the same general nature and positional controllability as the
bonding capillary 61 but having a flat stamping face 68 as shown in FIG.
4(a). This is preferably connected to the heating and ultrasound sources.
It stamps down on the partly-formed stud 5' to force it to fill the hole
31 and also to spread its head to form the flange 54, which is thereby
bonded firmly against the upper conductive terminal 41. The flat stamping
surface 68 ensures regularity and flatness of the tops of the studs,
improving contact reliability and uniformity in a multi-layer structure.
It will be understood that the volume of metal to be applied for each stud
should be predetermined in relation to the layer thickness and
through-hole area, so that excess metal is available for spreading to form
the top flange 54. Typically, a metal volume 20 to 30% greater than the
volume of the through-hole 31 is used.
With wire-bonding methods, when a wire of 20 to 40 .mu.m diameter is used,
stud diameter might be e.g. 60 to 120 .mu.m and stud height 40 to 500
.mu.m, more usually 40 to 100 .mu.m. Of course, this is determined in
dependence on the layer thickness and through-hole size.
FIG. 4(b) shows schematically how, in an alternative embodiment, the tops
of the part-formed studs 5' are pressed between rollers 69 of a roll press
after all of the studs for a given layer have been bonded on in a
part-formed state.
FIG. 5 shows in plan view the surface of the layer after the stud
through-connection has been made at a bonding pad 41. The circular
through-hole 31 is in register with a corresponding etched hole through
the bonding pad 41 of the wiring 4, and is occupied by the shank 51 of the
metal stud 5. The flange 54 at the top of the stud spreads to overlap the
bonding pad 41, but without extending beyond the pad edges. The thickness
of the spread flange will usually be between 3 and 40 .mu.m. However,
usually it should not be thicker than the underlying insulating layer 3,
otherwise there may be problems in applying subsequent layers.
By repeating the process described above in relation to one or more
subsequent wiring layers, an assembly as shown in FIG. 1 can be created
for any desired number of layers. It will be understood that the
insulating layers 3 need not necessarily be Cu-clad, pre-etched and
predrilled before application to the underlying structure. The method is
applicable for any mode of formation of the structure e.g. methods in
which insulating layers are formed in situ from a liquid precursor and
wiring is then deposited on their surfaces.
By way of comparison, we compared the metallurgical bonding strength of
studs formed as described above, using gold wire and a wire-bonding
technique, with soldered bumps of the conventional type. In the tests, a
test substructure consisted of a silicon substrate, a 10 .mu.m polyimide
layer and a surface gold/nickel plating. Gold bumps of diameter between 80
and 100 .mu.m were bonded between this stiff substructure and a flexible
polyimide sheet also having gold/nickel plating. The flexible sheet was
peeled perpendicularly away from the substructure, and the peeling force
measured by a strain gauge. We found that the measured bonding strength
between the layers ranged from 40 to 70 g per bump, compared with 5 to 15
g per bump for a conventional solder connection.
FIG. 6 shows the method applied to a duplex Cu-clad polyimide sheet,
comprising a central polyimide layer 3' having similar lower and upper
copper foil claddings 14', 15' on its two surfaces. Photo-etching was used
to preform wiring patterns 14, 15 on the surfaces (FIG. 6(b)), followed by
drilling of through-holes 131 at predetermined locations, corresponding to
holes in bonding pads 151 of the top wiring layer 15, but overlying
continuous portions 141 of the lower wiring layer 14 (FIG. 6(c)). The
holes 131 are drilled by excimer laser through a masking window 17. To
improve bonding strength on the contact surfaces of the wiring patterns,
these surfaces are selectively plated with nickel and gold plating 16.
Gold studs 5' were then stamped into the through-holes 131, by a suitable
method as described above (FIG. 6(d)), and then stamped to flatten their
upper ends into good contact with the upper bonding pads 151 and thereby
form effective stud connectors 5.
FIG. 7 shows the present techniques applied to multilayer tape automated
bonding (TAB). On an underlying Cu-polyimide sheet 100 a grounding layer
101, power source layer 102 and signal layer 103 are formed successively.
An LSI 110 is bonded down onto the connection terminals of the signal
layer by conventional means. Conventionally, bonding between the
respective layers of such constructions has been by batch bonding with a
heating tool, or by a single bonding process. Metallic connections have
been previously established by plating or printing solder lumps or pieces.
In the present embodiment, wire-bonding techniques as described above are
used to form electrical connecting studs and thereby eliminate printing or
plating processes. The wire-bonded studs can then be directly stamped to
form the interconnections, enabling a reliable multilayer TAB to be made
at low cost.
Thus, the present invention is applicable not only to wiring board
construction in thin film multilayer high-density packaging modules for
use in mainframe computers, but also to other applications such as
large-scale printed boards, multilayer TABs and the like.
* * * * *
|
|
 |
|
 |
Description |
|
