XXIV Conference of International Microelectronics and Packaging Society
- Poland Chapter, Rytro 25-29 September 2000
Current Trends in Flip-Chip Bonding Technique
for Multichip Modules – especially Micro-Jet Printing
Barbara Bober, Andrzej Bochenek,
Bronisława Olszewska-Mateja, Zbigniew Żaluk
Institute of Microsystem Technology, Wrocław University of Technology
ul. Z. Janiszewskiego 11-17, 50-372 Wrocław, Poland
key words: multichip modules, flip-chip technique, bump, micro-jet
Abstract
Recent fast development in telecommunication and computer technique, where electronic
equipment works on very high frequencies, forces design and manufacturing of multi-chip circuits
with low transmission losses. These circuits consist of semiconductor integrated circuits in very
high integration scale and multi-layer passive components. Multi-chip manufacturing applies
mainly wireless flip-chip bonding, which has currently many varieties. In the paper the survey of
flip-chip techniques, giving their main technological features, and the area of their applications is
presented. The particular concern is going to micro-jet technique as the only method enabling in
one technological process to produce multilayer passive circuits with resistive or dielectric and
some optoelectronic components and solder bumps for flip-chip bonding. Advantages and
drawbacks of micro-jet technology and the range of its applications are also underlined.
Introduction
Recently the most important direction of electronic industry development has been
manufacturing of personal equipment that requires many features such as: far-gone
miniaturization, high operation speed, functionality, reliability, creativity, low cost and
environment friendly design. One of the key solutions fulfilling the above conditions is
multichip circuits technology that applies very complex multilayer passive circuits as well as
bare die semiconductor chips, in large or very large scale integration, connected into one
functional module.
The major advantage of multichip modules is ability to apply thick or thin film techniques for
attachment of electronic integrated circuits, manufactured on silicon or gallium arsenide, or
micro-mechanical components.
The multichip technology cannot be realized without specific methods of interconnection,
such as: modified wire bonding, tape automated bonding (TAB) and flip-chip bonding.
Among these three techniques, flip-chip allows for the highest interconnect density and the
lowest packaging profile.
During the last decade there was unusual growth in the research and development of flip-chip
technology. In comparison with traditional wire bonding and TAB, flip-chip enables obtaining
shorter interconnections, lower inductance, higher frequencies, better noise control, higher
density, greater number of inputs/outputs (I/O) and further miniaturisation of electronic
devices.
Multichip modules require substrates with very high density of conducting paths what can be
realized by multilayer constructions with high resolution of interconnections. Such
constructions can be made using the following technologies:
 multilayer laminated printed circuit boards, distinguished by the letter L (MCM-L);
 multilayer ceramic substrates, marked with C, produced by traditional thick film
technology or by the newest technology of high or low temperature cofired ceramic
substrates, HTCC and LTCC respectively;
 multilayer metal-dielectric thin film structures, distinguished by D (MCM-D),
deposited on ceramic or silicon substrate.
MCM-D technology is the most advanced. It enables to reach the highest interconnection
density and transmitted signal frequencies even higher than 10 GHz. The schematic diagram
of MCM-D is presented in Fig. 1. The multilayer thin-film circuit is laid on multilayer thickfilm substrate. Such a complex construction allows to combine advantages of two module
types, MCM-D and MCM-C, and to produce substrates even with few dozen of levels, what
significantly increases the packaging density and shortens transmission lines. Additionally,
the following assembly methods are shown in Fig. 1:
 wire bonding,
 flip-chip bonding,
 tape-automated bonding.
Fig. 1. Schematic drawing of a multichip module with connected thin- and thick-film
circuits
In the paper the main attention is paid to the flip-chip technology, pointing at many
construction and technological solutions applied in industrial production, and at a great
number of suggested techniques being under intensive investigations aiming at their
application. The variety of different solutions is demonstrated in Fig. 2.
Fig. 2. Block diagram of various assembly techniques in flip-chip technology
Rigid bumps are usually made of Au, Ag or their alloys and also of NiAu double layer. Due
to their low flexibility there is necessity to match the temperature coefficients of expansion of
connected materials (the semiconductor die and the substrate). If not so, in the joint area too
high thermal stresses would appear. Another sufficient method can be the application of
conductive adhesives with very high flexibility for joining die bumps with pads on the
substrate.
Soft bumps are made from low-melting solders, commonly lead-tin alloys. Due to plastic
deformation and bumps material creeping, thermal stresses disappear in the joint area.
However, during the periodical and frequent temperature changes the fatigue failures of
bumps can take place.
Elastic bumps are usually produced as polymeric studs with metallic coating. Assuring
constant pressure by applying thermally shrinking adhesives the rigid interconnection with
substrate pads is reached. Due to their elasticity they are resistant to the thermal dilatations of
substrates [1-Chapter 1].
There are five methods of bumps manufacturing mainly used, applying:
 ball thermo-ultracompression bonding,
 electrolytic or electroless plating,
 printing technology,
 vacuum film evaporation or sputtering,
 solder-jet dispensing.
One of the major benefits of flip-chip technique is that all interconnections are manufactured
in one go. It remarkable well speeds up the process of electronic circuits production.
There are the following bonding methods applied:
 thermocompression,
 soldering,
 using adhesives.
Further classification of flip-chip assembly techniques can be demonstrated by detailed
description of various methods of bumps formation and ways of their interconnection.
Exploitation of conventional ball thermocompression technique for bumps
manufacturing
The technical solution is based on the fact, that after the ball joint is formed on the conducting
pad of a semiconductor device, the wire loop connection is not made, but wire breaking
follows directly the first step of welding (Fig. 3.).
Fig. 3. Conventional stud bumping
In this way so-called stud bumps can be formed onto any kind of semiconductor structures,
even onto those assigned for wire bonding. Die metallization made of aluminium films have
not to be additionally prepared (cleaning, activating or thickening with another contact layer).
The thermocompression process assisted with ultrasounds removes the aluminium oxide film
and gives good contact of Au ball with Al pad, and good joint strength. Main advantage of
this method is simplicity of bumps forming, leading to application of commonly used ball
thermocompression bonding with no bonder modification. There is the difficulty that the gold
wire breaks nonrepeatedly above the ball. The wire endings with uneven heights h (Fig. 3)
make difficult the further assembly with the contact pads on the substrate. This problem can
be solved in a couple of ways.
The first consists of using, instead of pure gold wire, the gold wire doped with 1.52% Pd
which gives better uniformity of the wire endings [1-Chapter 15, 2]. Our investigations [3]
show that Pd addition significantly diminishes the recrystallization area of the wire near the
ball during its melting. The wire breaks near the ball for there appears smaller zone of its
softening.
The stud bumps formed in the described way with short and uniform endings are particularly
suitable for further assembly by thermocompression, aided at the same time with an insulating
adhesive. It is based on the piercing of the thermally softened adhesive film by the bumps [1Chapter 15]. The adhesive film shrinks when setting after cooling and then the additional
pressure onto the bumps appears from the substrate, what increases the reliability of obtained
contact interconnections (Fig. 4).
Fig. 4. Schematic process of thermocompression bonding across the isolating adhesive film
It is very important to select suitable kind of adhesive which being nonhygroscopic, will not
increase its volume, because increase of the adhesive volume causes disappearance of the
contact between the bump and the substrate.
The second way of obtaining uniform wire endings consists of electric discharge application
at the moment of wire breakdown due to melting at the controlled height. The modified
capillary is used containing a small sidewall electrode [1-Chapter 11] (Fig. 5). Application of
laser beam for melting the strained wire at certain height gives successful results (Fig. 6).
Fig. 5. Thermocompression capillary
with sidewall electrode for
melting the wire by electrical
discharge
Fig. 6. Schematic view of producing
uniform endings of stud bumps by
laser beam application
Fig. 7. Flexible bumps attachment by fluxless soldering and using conductive adhesive
The produced stud bumps with elongated heights are connected with conducting pads of a
passive circuit by fluxless soldering or by using conductive adhesives (Fig. 7). The elongated
stud bumps, due to their flexibility are resistant to the thermal dilatation of substrates.
Another way of obtaining stud bumps with repeated heights is explained in Fig. 8 [4]. It is
based on straining the wire by the capillary on the ball joint. After the ball connection is
completed, the capillary is raised and the die is moved horizontally against the capillary,
together with the sample holder, by the distance D of 0.51.5 times the wire diameter. In the
second step of bonding, the capillary comes down and strains the wire pressing it to the ball.
Next, the capillary is raised again and the wire breaks closely to the ball. In this way the
uniform bumps height determined by the flattened ball level is obtained. Such stud bumps are
particularly useful for assembling with anisotropic adhesives.
Fig. 8. Consecutive steps of thermocompression bonding with wire straining
Stud bumps manufacturing by the ball thermocompression bonding is the simplest method.
However, because of the fact that each bump has to be formed separately, this technique can
be used in small series production. Additionally, it can be suitable in the case, where for
various reasons, semiconductor chips planned earlier to be assembled by wire bonding, have
to be attached by flip-chip method. Typically, the pitch of stud bumps made of gold wire of
25 m in diameter is 250 m. It is determined by the ball dimension (3 times wire diameter)
and the standard capillary size (Fig. 9a).
In the paper [5] the innovation of wire welding process in the aim of decreasing significantly
the pitch size of wire connections is presented. The capillary with the special shape was
applied leading to the development of a new welding method called “encapsulated wire
bonding” (Fig. 9b). According to the authors’ statement, using this new method, without
diminishing the commonly used wire diameter (25 m), the in-line pitch of 55 m was
achieved. Additionally, the strength of the ball connections was even greater than the strength
of the bonds produced in the standard wire bonding process. The “encapsulated wire bonding”
is going to be particularly useful for ultra fine stud bumps manufacturing and will allow for
significant increase of interconnects density in flip-chip assembling.
Fig. 9. Schematic view of thermocompression bonding: a) standard, b) “encapsulated”
Solder bumps produced by evaporation and printing
The conducting pads on semiconductor devices are made in the form of multilayer (e.g.
Al+TiW+Au) that, among other functions, assures good wettability by solder. The layers of
lead and tin are deposited by evaporation (usually through mechanical masks) on the bond
pads. The solder content is controlled by the layer thickness. In order to increase the solder
volume, the evaporated area is suitably greater than the bond pad and reached onto unwettable
surface (Fig. 10a). In the next step, the chip is heated up and after melting of Pb and Sn films,
blending takes place, followed by contraction of the produced alloy into spherical shape
caused by surface tension forces (Fig. 10b).
Fig. 10. Schematic manufacturing of soft bumps: a) evaporation of Sn, Pb layers, b) spherical
bump formation by solder melting
The bumps formed on the chip as described above can be connected with the passive substrate
in two ways. The bumps made of low-melting solder (37/63 PbSn, Tm= 183C) are placed on
the conducting substrate pads and heated up to the temperature above the melting point of the
solder. The soldering process is conducted most frequently fluxlessly in a reducing
atmosphere or by the method, called plasma assisted dry soldering (PADS) [6-10]. After these
connections were made, the chip is lifted up in the controlled manner so that the melted solder
gains hyperbolic shape and solidifies in this form (Fig. 11a). The hyperbolic shape of the joint
ensures significantly its greater resistance to the thermal load fatigue than the cylindrical or
barrel one. The detailed description of strains in solder bumps gives Lau in [11].
Fig. 11. Schematic view of interconnection of solder bumps with the substrate: a) by melting
the bump material, b) by melting the additional solder
In the case of interconnecting the silicon chip and the substrate with similar thermal
expansion coefficients, the method with two solders can be applied (Fig. 11b). Then spherical
bumps are made of the solder with higher melting temperature (e.g. 95/5 PbSn, T m  300 C),
and the contact pads on the passive circuit are coated with the lower melting alloy (e.g. 37/63
PbSn, Tm = 183 C). The bumps do not undergo melting but are only wetted by additional
solder. In both cases for the high density and precision of layers deposition the bump pitch
can be 150 m. In circumstances, where the greater distance between the pads is tolerated
(250400 m pitch) the solder bump forming process is realized by printing solder pastes on
the passive substrate. Contact pads on the semiconductor chip are produced by evaporation of
metals so as to ensure good wettability by the solder.
Solder bumps produced by ball thermocompression technique
The authors in [12, 13] manufactured bumps by thermo-ultracompression method applying
SnSbAg solder alloy wire of 44.8 m in diameter. The electrical discharge melting of balls on
the wire was conducted in Ar+H2 reducing atmosphere. The characteristic feature of this
method is that the solder balls with increased hardness are rubbed by ultrasonic vibration,
what makes possible to obtain the metallic connection directly with the Al pad on the chip in
spite of its natural oxidation. The bumps produced in this way are transformed later by
melting into spherical bumps (Fig. 12). It is worth emphasizing that this method does not
require additional preparation of Al metallization.
Fig. 12. Manufacturing steps of solder bumps by thermo-ultracompression method
In the paper [14] the bumping technology applying the wire made of 98/2 PbSn soft solder is
presented. In this case the Al metallization is covered with Ni-Au layer by electroless plating
for obtaining good wettability by the solder. The thermocompression process is applied there
to join initially the bump material with the contact pad. This technology for its greater
complexity connected with traditional metallization process of contact pads, becomes too
expensive for low quantity production. The positive side of this solution is decreased risk of
the thermal strains arising in the joint for the great deformability of soft bumps and their
material creeping.
Generally, it is necessary to underline that the technology of wire solder bump formation
gives wide flexibility in the solder content choice. The multiple solder compositions
promising good weldability, wettability, suitable plasticity and environment friendly process
can be applied without any complications. Usually alloys based on Pb or lead free based on
Sn, In and Bi are used. The minimal bumps pitch depends on the capillary diameter and
practically reaches the range of 200300 m.
Solder bumps produced by plating method
Electrolytic plating through photoresist masks is used to produce solder bumps on
semiconductor wafers with electronic devices. Before plating the active wafer surface is
metallized, usually by sputtering or evaporation (e.g. NiAu or TiWAu) to provide a current
path to the individual bond pads. The plating deposits consecutively the metallic layers being
constituents of the solder alloy. Next, the resist is removed and the conducting paths outside
the bump areas are etched away. After that, melting of the plated layers gives spherical solder
bumps. This is low cost method in the case of mass production, as all bumps, even on many
wafers can be manufactured simultaneously in one plating process.
On the contrary to the wire solder bumps formation, plating is more constrained in multicontent solder alloys formation because of complexity of the process increasing with any
additional alloy constituent. Usually plating is reduced to two-component alloys. However,
the essential advantage of the described method is obtaining the smaller bump pitch, for there
the factor connected with the welding capillary does not exist.
In [1-Chapter 15], the solder bumps formed by this method with 80/20 AuSn alloy reaching
20 m pitch are described.
Hard bumps produced by plating method
Two methods of bump formation can be distinguished: maskless, electroless plating and
electroplating through photoresist masks (Fig. 13).
Fig. 13. Schematic view of plated bumps manufacturing on silicon wafers: a) maskless and
electroless plating, b) electroplating through photoresist masks
In the first case the aluminium bond pads are activated by zincate pre-treatment, then covered
with Ni and thin Au layer that prevents oxidation and ensures good wettability of bumps by
the melted solder. The bumps growth during the plating process is multidirectional, so the
bumps diameter is limited by their heights. The electroless Ni-Au plating is the cheapest way
of bumps formation. In [1- Chapter 15] the authors consider the possibility of forming bumps
with 7 m in height onto 5 m in diameter Al pads with 26 m pitch.
The bumps produced in the described way can be soldered or connected with the passive
circuit by a conductive adhesive. They can also be applied to assembling with anisotropic
adhesives. Because of the fact that these bumps are relatively low, in the case of substrates
thermal deformation, mainly shear strains arise in interconnections (Fig. 14a). To minimize
these strains, it is recommended to use elastic joining materials (e.g. anisotropic adhesives
with elastic particles).
Fig. 14. Schematic explanation of possibility of the bond degradation due to the thermal
deformation in the case of: a) low bumps, b) high bumps
Another way of reducing thermal strains is underfilling the space between the die and the
substrate with epoxy resin, which, in the great extent, adopts the strain and takes it off the
joint.
In the second method (Fig. 13b) the bumps are produced in the shape of metal posts called
WIT (wire interconnect technology) by electrolytic plating through a thick photoresist mask.
It is unidirectional metal deposition, so the bump diameter is not restricted by its height. In [1Chapter 13] the WIT posts produced from copper (Cu) on the silicon chip with Al pads coated
with additional adhesive layer are presented. These particular WITs were approximately 10
m in diameter and 47 m tall with 30 m pitch. However, in the aim of receiving proper
conditions for their soldering, 50 m pitch was applied. Such slim metal posts are
characterized by pretty good elasticity what makes them more resistant to thermal
deformation. Yet in the solder connection, unfavourable loads appear due to the posts
bending, that causes breaking loose WIT in the soft solder (Fig. 14b). Taking this into
account, the solders with increased strength and diminished plasticity should be applied there.
Small dimensions of conducting pads taken up by the WIT result in reduction of the
connection capacitance by an order of magnitude over any other method of chip attachment
available today. Small solder volume and long distance of the solder from the active surface
of the integrated circuit result in at least 3 orders of magnitude improvement with regard to particle radiation dosage on the active devices at the circuit surface.
Rigid bumps cofired with the substrate
The possibility of making bumps of conductive pastes dedicated for printing is considered in
[15]. Multilayer glass ceramic substrates (LTCC) are placed on base plates made of special
ceramic, which has small thermal expansion coefficient and is easy to crush. A plate of the
same ceramic with holes spaced according to the circuit contact pads layout is placed on the
top surface of the LTCC circuit. After lamination the holes are filled with a conductive paste
(e.g. Ag) and then the whole structure is fired in the temperature depending on the applied
glass ceramic green sheets. During the firing LTCC ceramic shrinks reasonably, even by more
than 10%. This shrinkage could cause significant and difficult to control change of bump
pitch, which must be compatible with contact pads of the semiconductor structures to be
connected at a later stage. Such shrinkage takes place, but only in z direction, as both top and
bottom ceramic base plates do not allow for shrinkage in x, y directions.
Fig. 15. LTCC structure with cofired bumps before firing process
After firing, base plates are crushed and their remains removed. Bumps formed in the top
plate holes have the same pitch as the primary pitch of the holes. Using this method, the
pitches of 500 m and 250 m were practically obtained. Research works in this area head
mainly towards cheap technology of producing bumps by printing technique.
Flip chip assembly using anisotropic adhesives [1]
Usage of adhesives in flip chip technique is common. Conductive adhesives are used as well
as dielectric underfill resins. An example of such connection is shown in Fig. 16.
Fig. 16. Schematic view of classical interconnection with adhesive
Application of anisotropic adhesives is the fundamentally different solution, which idea is
presented in Fig. 17.
Fig. 17. Schematic view of interconnection with anisotropic adhesive filled with Ag balls
Flat rigid bumps are made on the passive substrate and on the semiconductor structure.
Between their surfaces an anisotropic adhesive is introduced, which has dielectric properties
in free state, but becomes a conductor being squeezed between bumps. This property has been
achieved by filling resin with sphere-like silver particles of 10 m in diameter, covered by
thin insulating layer made of plastic dielectric – Fig. 17. The isolated balls floating in resin do
not conduct electric current. However, when crushed between bumps, they provide electric
contact because the plastic insulation is squeezed out of their surface in the contact area.
While external pressure is applied, thermal polymerisation of the resin and creation of the
permanent electric contact take place. Stability of the contacts made in this way can be
obtained when the resin does not extent its volume neither by thermal expansion nor by
hygroscopicity. This condition is difficult to be fulfilled in practice.
Fig. 18. Schematic view of interconnection with anisotropic adhesive filled with elastic balls
Further improvement of this solution is application of plastic springing balls, covered first
with thin silver layer and then with thin layer of plastic dielectric. The balls prepared in this
way subjected to the pressure are elastically deflected and lose insulation – Fig. 18. Elastic
strain ensures permanent contact even at clear increase of underfill resin volume resulting
from thermal fluctuation. In order to ensure equal distances between bumps, there were
additionally introduced spacer balls, made of hard dielectric, with diameters adequately
smaller than elastic balls [16].
A variation of this technique consists of application of anisotropic conductive films (ASF)
instead of anisotropic conductive adhesives (ACA). Properly prepared film used in the form
of pad additionally simplifies assembly process [17]. In this technique good flatness of bumps
surface is required. There is however the possibility of easy correction of bumps surface, e.g.
golden ones, by initial squashing on flat model plates. Development of this technique and
improvement of anisotropic adhesives (films) may cause that flip-chip assembly becomes
exceptionally simple technological operation.
This technique allows now to obtain pitches of 80100 m with elastic balls of 7-10 m in
diameter. It is connected with the necessity of using quite big contacts, being multiple ball
diameters in size. Only such condition ensures creation of electric contact in each junction.
Micro-jet technique in flip-chip assembly [18, 19, 20]
Micro-jet technique is the newest technology entering the production of VLSI electronic
devices. It uses well known, especially in computers industry, ink-jet printing method. The
method consists of very precise dosage of very small (picoliters) droplets of certain substance,
using specialized, computer controlled jet (Fig. 19). In this way conducting paths, joining
columns (vias), and insulating layers can be formed as well as bumps for flip-chip assembly
on semiconductor structures, passive substrates and printed circuits.
Fig. 19. Schematic view of a demand mode micro-jet printing system
Very high flexibility of deposition in precisely defined places on substrates is obtained
through computer control of the volume of disposed droplets (change of the volume 1 to 8
times) as well as dispensing frequency (up to 1000 droplets per second), and also precise,
programmed substrate movement in x-y plane, closely connected with the dispensing
programme.
Research works in MicroFab Technologies, Inc. lead to improvement of “heads printing on
demand” operating in temperatures up to 300 C for deposition of solders. There is also
research conducted on technologies of various polymeric compositions deposition, conductive
and dielectric ones, as well as other materials used in CSP and MCM electronic circuits
manufacturing. Building machines in a form of a set of printing devices one can deposit
sequentially consecutive layers and components of a circuit. Fig. 20 presents schematic
diagram of a simplest example of such process.
Complex circuits may be produced in a production line built of several printing stations.
Transitions between stations can be used for operations not connected directly with printing,
like e.g. thermal or UV polymerisation of deposited resin layer. In such a way very complex
multilevel circuits can be produced, including such elements as: vertical columns joining
levels, horizontal conducting paths, resistive tracks, insulating layers, capacitors, polymer
waveguide joints, polymer microlenses, polymer elements with ferrite particles, encapsulating
layers, solder bumps etc. Auxiliary materials can be deposited, like flux pastes, conductive
adhesives etc.
Fig. 20. CSP manufacturing method
Joining columns as well as conducting paths and bumps are made of solders, but columns and
paths are made of the solder with higher melting temperature than bumps, so that they do not
melt during flip-chip assembly process. Solder bumps can be produced as very tall columns
(25 m in diameter, 500 m in height) which allow for joining substrates not matching
thermally. Resulting pitch is even smaller than 50 m. Deposition rate is high enough to
consider this technique useful for mass production. Currently droplets disposing frequency
above 1000 per second is being obtained, and substrate movement when “printing in fly” is 60
mm/sec. Making 400 bumps per second is no problem at all. Accuracy of components
deposition reaches 10 m. There is still research conducted, heading towards improved
printing efficiency and precision. Basic advantage of this technique is very high flexibility in
programming of the technological process. It is reduced to another data set entry to the
computer programme. This plays superb role in the case of preparing and investigation of
prototype solutions. This technique can be also used for manufacturing circuits with similar,
but slightly different parameters, creating their wide range without expensive modifications of
technological processes. Contactless material deposition is less expensive, as neither masks
nor screens are required. The process is environment friendly for there are no extra processes
and chemical wastes.
Conclusions
Different flip-chip assembly methods were described. The main trends of flip-chip technology
development were pointed out and many ways of their realization presented. None of these
methods is particularly favoured. Certain application and requirements determine the choice.
It is recommended to take into account such circumstances as: production volume, cost,
operating conditions, complexity, and miniaturization scale. It is worth to note that
information about operating conditions of an electronic circuit helps in choosing proper
assembly technique ensuring good interconnection reliability.
Continuous progress in electronic industry forces either further improvement of existing
interconnection systems or search for completely new solutions. The paper shows how to
design and manufacture interconnection systems what may help to make advances in
electronics packaging and to obtain benefits in cost, performance, quality, size and weight.
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