Chip-on-board Technology
Hybrid Technology
The trend in electronics is to continue to integrate more and more
functions and numbers of components into a single, smaller assembly.
Hybrid circuit technology is a key method of increasing packing
density, usually involving a mixture of active and passive components.
Fig.1 shows a 40 mm x 40 mm package with over 3000 connections.
After many years of
development, hybrid and
chip-on-board or COB
technology has reached
the stage where considerable savings in
space and cost are
achievable. This article
reviews the most important features of these
technologies, together
with the manufacturing
equipment necessary for
their practical use.
Fig. 1:
Hybrid technology in a 40
mm x 40 mm package. 1
During the various manufacturing stages the interconnects and some
of the passive components such as resistors are deposited onto a
ceramic substrate. When designing the layout a number of important
aspects such as track width, proximity of tracks to bonding pads,
bondability, loop heights, heat dissipation and so on must be considered.
In thick film technology the functional interconnects, tracking and
resistors are created by printing various pastes onto the different
levels of the substrate. In thin film technology the interconnects and
tracking are deposited galvanically onto the ceramic substrate,
resistors and other passive components being added using printing
and soldering techniques. Once all passive and surface mount
devices have been mounted the chips are placed onto the substrate
Chip-on-board Technology
using die bonders, the electrical connections between chips and
substrate being made with gold or aluminum wire bonders and the
whole unit being then mounted in a package.
Hybrid technology enables large numbers of chips and miniaturized
passive components to be integrated into a very small area. If standard surface mount technology were used it would occupy up to 20
times the area used with hybrid techniques. Manufacturing hybrid
circuits requires complete familiarity with wafer fabrication technology
as well as chip assembly and bonding techniques. Often smaller
companies do not have this detailed knowledge and it is also relatively
expensive to manufacture hybrids in small batches. Nevertheless
there are a number of applications such as medical technology,
aerospace, military, automotive and communications where hybrid
circuits are indispensable.
Chip-on-Board technology
Over the years considerable effort has been devoted to developing
methods of utilizing the benefits of hybrid technology but at lower cost.
Results show that the tried and tested printed circuit technology is still
the best available for complex circuitry, if certain improvements are
implemented which allow bare die to be placed and bonded easily and
reliably (Fig. 2).
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Fig. 2:
Principle of COB Technology
COB was first used in applications such as digital clocks and watches
where a single chip per board is used. Since then its use has spread
and it is now widely used in areas such as video cameras, pocket
calculators, telephone cards and smart cards. It is also used increasingly in more complex assemblies such as printer modules with upwards of 5000 LEDs, with their associated driver IC's and in sophisticated data processing circuits such as the 32 bit HP 9000 computer
which incorporates 22 IC's and a modem on the board. The historical
development of COB Technology is shown in Fig. 3
Chip-on-board Technology
Simple chip (transistor)
Complex chip (integrated circuit)
Simple hybrid component
Complex hybrid component
Chip-on-Board-Technology
Fig. 3:
Historical Development of COB
Today state-of-the-art multi-chip COB systems with over 100 chips on
a single board have been developed and COB is being used to
produce most of the electronic assemblies in entertainment equipment
in Japan. In some areas COB has already replaced SMD technology.
Cost analyses show that often the DIP packages cost up to three
times as much as the chips that they contain. By eliminating the
packaging with COB technology significant cost savings are achieved,
particularly when producing large batches. Table 1 compares the
space requirements of a chip measuring 3 x 3mm within different
packages. It shows that requirements vary by up to a factor of 20.
There are also significant differences in the height or thickness of the
final product.
Area
Chip
COB
CC 24
SOP 24
mm
3x3
5x5
11.8 x 11.8 15.4 x 10.24 31 x 15.24
mm²
9
25
139.24
157.70
DIP 24
472.44
Table 1:
Space requirements for
different packages
In Europe after a slow start COB technology is becoming more widespread but is still less prevalent than in Japan or the United States
where it is widely used, in particular for applications requiring high
packing density within a small area and the lowest possible package
height.
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Manufacturing COB assemblies
The manufacturing processes for Chip-on-Board modules and hybrids
are very similar, the main differences being in the different basic
materials used and the packaging. With COB technology a PCB rather
than a ceramic is used for the substrate. The bare die are encapsulated or glob topped rather than being assembled in a metal package as
in hybrid technology. Compared to conventional SMT assembly both
COB and hybrid assembly require fewer manufacturing stages as
shown in Fig. 4.
Chip-on-board Technology
COB or Hybrid assembly
Conventional SMD assembly
Wafer
1
Wafer
Test, dice, clean
2
Test, dice, clean
Die bond onto board or ceramic
3
Die bond onto leadframe
Wire bond, test
4
Wire bond
Seal off bond area
5
Mould, Stamp
Add connectors, test
6
Surface treat leads, test
7
Pack and Ship
8
Unpack, prepare for assembly
9
Assemble using SMD or through-hole
machines
10
Solder
11
Cleaning
12
Test
Finished product
Finished product
Fig.4:
Comparison of
manufacturing stages.
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Printed circuit boards or PCB's are made from a number of different
materials such as phenolic resins, polyurethane, polyamide resin,
silicon, epoxy, Teflon and more. Teflon exhibits particularly good
resistance to high temperatures whereas polyurethane is used when
the product is subjected to extreme temperature variations as in
automotive electronics. In applications where minimal thermal
expansion is required at very high temperatures Teflon is preferred.
The printed circuit tracks used in COB can be normal copper tracks
but the bond pads on the board require special preparation. Usually
they are built up from a copper base covered with a 2 to 4 micron
layer of nickel on which a 0.1 to 0.2 micron layer of gold is deposited.
Today bond pads of less than 100 microns with a pitch of less than
100 microns are state of the art.
Chip-on-board Technology
minimal unevenness
no distortions
heights maintained to +/- 0.01mm
surface roughness maintained to +/- 1 to 2µm (depending on
wire diameter)
optimum bond surfaces:
Al-Wire
Gold Wire
Copper foil
Copper foil
Nickel (2 µm)
Nickel (2 µm)
Gold (0.1 µm)
Gold (1 to 2 µm)
The bond surface must be completely free of contaminants.
Table 2:
Printed circuit board
prerequisites for COB
The chips are bonded to the board using silver epoxy pastes which
are cured and degassed at around 150°C. The problem of conducting
away the heat generated by power devices is solved by bonding the
chips onto a metalic plate integrated into the board. During final
assembly the cooling plate will be fixed to cooling fins or the main
casing.
Electrical connections between the chip and the PCB are usually
made with gold or aluminum wire. Aluminum wire bonding has the
great advantage of being executed at normal room temperature but
takes around three times as long as gold wire bonding. Nevertheless
aluminum wire bonds are more reliable when the product is subjected
to high temperatures or large temperature variations. To bond reliably
with gold wire requires a bond temperature of at least 120° C. At
higher temperatures many of the materials used for printed circuit
boards go soft and the bond pads pull away from the base material.
When bonding with gold wire onto aluminum bond pads on the chips
there is a danger of the bonds degrading if the end product is subjected to elevated operating temperatures. The degradation is caused
by the growth of Kirkendall pores and can lead to the bond pulling
away from the bond pad completely. The decision on which wire to
use, gold or aluminum, is made depending on the specific application
and the ambient temperature under which the product must operate.
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After wire bonding the chips are coated or encapsulated using various
processes. Silicone compounds that harden at room temperature,
epoxy compounds and other materials are used. Additionally the chip
can be covered with a plastic or metal cast. Finally the COB element
will be mounted in a package and electrically connected using
soldering, bonding or crimping techniques.
Aluminum wire bonding
When high bond quality is required aluminum wire will normally be
used. Bonding speed is slower when compared to gold ball bonding
but the end product is often cheaper since the surface treatment of the
materials is less expensive.
Chip-on-board Technology
Fig. 5a and 5b show two views of a memory module COB product
bonded with aluminum wire.
Fig. 5a
Fig.5b
Two views of a COB
technology memory
module
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Aluminum wire bonding is a friction welding process. In this process
two pure metals are pressed together at a pre-defined pressure and
then, using a transducer, ultrasonically vibrated until the friction-bond
occurs. The amplitude of the ultrasonic vibrations is usually 1 to 2
microns.
The welding process is divided into three distinct phases: firstly the
surface cleaning phase, secondly the breakdown of the oxide layers
and thirdly the joining together of the pure metals. The metals are
pressed together until there is less than one atomic lattice distance
between them and the resulting weld is characterized by high quality
and extreme stability.
Chip metalization is usually aluminum or an aluminum alloy 0.8 to 2
microns thick and is particularly suited for aluminum wire bonding. The
ideal bond pad on a PCB consists of a layer of copper covered with a
2 to 4 micron layer of nickel on which a 0.1 to 0.2 micron layer of gold
Chip-on-board Technology
has been deposited. The gold surface protects against impurities and
chemical reactions that could occur during the manufacturing process.
It does not effect the bonding process as it is pushed aside during the
cleaning phase and the friction weld is made between aluminum and
nickel. Evaluations show that aluminum-nickel bonds are best where
stability, reliability and good conductivity are required, particularly at
elevated temperatures.
When the PCB layout is designed a number of aspects such as pad
size, pitch and so on must be considered. To avoid problems during
bonding it must be ensured that the PCB's are not mechanically
distorted and that the top surface is plane.
Aluminum wire bonding is a pure ultrasonic welding process done at
room temperature. It is absolutely essential that the board is held fast
during the welding process to prevent movement or vibrations which
could destroy the bond area. This is best achieved by holding it in a
vacuum chuck during bonding. It is also essential that the adhesion of
the copper tracks to the board in the area around the bond pads is
optimized since a lateral movement of 1 micron will adversely effect
the bond process.
The surface homogeneity of the PCB tracks is another factor. If the
thickness of the nickel layer varies and goes below 0.5 microns the
bond quality is unstable and bondability may be reduced to zero. The
roughness of the copper surface in the bond area should be controlled
to less than 2 microns, this being the maximum deviation that can be
compensated for by the ultrasonic vibrations.
Gold wire bonding
Unlike aluminum wire bonding, gold ball wire bonding can not be done
at room temperature. A temperature of at least 120°C is necessary to
achieve acceptable bond quality. There are the same pre-requisites as
with aluminum bonding: the PCB must be plane in order to avoid
surface temperature variations and loss of ultrasonic power during the
bond process.
The bond surface is built up from a 1 micron thick nickel layer covered
with a 1.5 to 2 micron thick gold layer. The manufacturing costs of the
PCB are higher than for aluminum wire bonding because of the
amount of gold used but the bonding speed is about three times
faster. This is because the low temperature used for aluminum bonding requires more ultrasonic power and a very precise work holder to
clamp the PCB, which has an effect on the overall throughput. Also
the loop speed is influenced by the type of tool used - wedge with
aluminum wire and capillary with gold wire.
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COB and packaging technology
The chip is separated from the outside world by a protective package
which also provides the electrical connections.
Standard packages are available commercially in a limited number of
forms and the number of connecting pins is also standardized. This
can mean that one is compelled to select a much larger package if
only one extra electrical connection is required which increases the
Chip-on-board Technology
size and cost unnecessarily. Chips with over 100 pins usually require
expensive packages and sometimes the package geometry makes
bonding more difficult which can result in damaged die. Special ASICs
are usually produced in only small quantities with a corresponding
increase in the difficulty of selecting a suitable package. The greatest
difficulties lie in manufacturing custom specific packages with the
largest possible pin counts.
COB technology offers the best solution in this case as a specific
board design with the requisite number of interconnections can be
generated in a very short time using today’s technology.
See fig. 6a and 6b
Fig. 6a
Fig. 6b
Chip-on-Board packaging
technology
The number of bond connections and the interconnections required
can be properly accounted for allowing very small runs of 10 or more
to be manufactured economically. After wire bonding the chip and all
the bonds are encapsulated as discussed earlier in this article. The
resulting package is perfectly matched to the requirements and cannot
easily be copied which is often an advantage with small runs of ASICs
where intellectual property needs to be protected. A further benefit is
that passive components and/or other chips can be integrated into the
same package.
The advantages of this packaging method is firstly the miniaturization
that would not be possible with standard packages that are often 10 or
20 times larger that the die themselves. Secondly the cost of the
standard packages with high pin count ASICs is often higher than the
cost of the die themselves.
Wire bonders for COB
Our research shows that 90% of the products being manufactured
with COB technology require a board size of 100mm x 100mm and
less than 100 die per board. Wire bonders for COB must therefore
fulfill the following minimum requirements:
Bond area of 100mm x 100mm, minimum
Storage of at least 200 reference images for pattern recognition
Programmable focus to accommodate multiple chip heights
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A fine touch-down mode to allow for uneven board surfaces
COB_E2.doc
Up to 4 programmable light sources to process chips with
different surface contrast
Wire control facility to detect wire loss
Large Z-axis movement to allow bigger components such as
capacitors to be accommodated
The capability to bond at 60° when wedge bonding
Fig.7: Wire-bonder for COB
Chip-on-board Technology
A vacuum chuck to hold the board
Sufficient clearance around the bond tool to allow bonding in
deep packages
A flexible transport system able to handle 25mm to 150mm
board edge lengths
Die Bonders for COB
Among the important features of die bonders for COB are:
Bond area of 100mm x 100mm, minimum
Storage of at least 100 reference images for pattern recognition
with ink dot and edge recognition capability
Programmable focus to accommodate multiple heights
Programmable light sources to accommodate chips with
different contrasting surface
Chip input from wafers, waffle packs and Gel-packs
Programmable eject system for at least four different die sizes
At least four automatically selectable die collets
The ability to print or dispense adhesive as required, selectable
and interchangeable under program control
A flexible transport system able to handle 25mm to 100mm
board edge lengths
Figures 7 and 8 show wire and die bonders from F & K Delvotec
suitable for COB applications.
Conclusion
The requirements for COB boards outlined above must be
considered at the design stage. Once these fundamental
techniques have been mastered and are under control, COB
technology offers a number of benefits in handling, packing
density, reliability and in reduction of manufacturing costs when
compared to SMD assembly.
The author
Dr. Farhad Farassat has worked for Delvotec since 1977 and is
president of F&K Delvotec Bondtechnik GmbH. Dr. Farassat holds
many European and International patents and is widely recognized as
one of the leading experts in bonding technology.
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References
-
COB, the technology of the future?: Dr. Farassat, Fellbach 1990
-
Trägersubstrateinfluß beim Bonden von COB: Dr. Lang, NIM Berlin
der SLV Hannover,
-
Miniaturization and the increased availability of bare chips is
accelerating the adoption of COB Technology in Japan: Havao
Nakahara
Fig. 8: Die-bonder for COB