ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 541 (2005) 286–294
www.elsevier.com/locate/nima

Application of Cu-polyimide ﬂex circuit and Al-on-glass pitch
adapter for the ATLAS SCT barrel hybrid
Y. Unnoa,, T. Kohrikia, Y. Ikegamia, T. Kondoa, S. Teradaa, N. Ujiiea, K. Harab,
Y. Iwatac, T. Ohsugic, R. Takashimad, I. Nakanoe, R. Tanakaea

KEK-High Energy Accelerator Reasearch Organization, Institute of Particle and Nuclear Studies, 1-1 Oho,
Tsukuba City, Ibaraki-ken 305-0801, Japanb
Institute of Physics, University of Tsukuba, Ten’nodai 1-1-1, Tsukuba 305-8571, Japanc
Department of Physics, Hiroshima University, 1-3-1 Higashi-Hiroshima 739-8526, Japand
Kyoto University of Education, Fukakusa, Fushimi-ku, Kyoto 612-0863, Japane
Department of Physics, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
Available online 2 March 2005

Abstract
We applied the surface build-up Cu-polyimide ﬂex-circuit technology with laser vias to the ATLAS SCT barrel
hybrid to be made in one piece from the connector to the electronics sections including cables. The hybrids, reinforced
with carbon–carbon substrates, provide mechanical strength, thermal conductivity, low-radiation length, and stability
in application-speciﬁc integrated circuit (ASIC) operation. By following the design rules, we experienced little trouble inbreaking the traces. The pitch adapter between the sensor and the ASICs was made of aluminum traces on glass
substrate. We identiﬁed that the generation of whiskers around the wire-bonding feet was correlated with the hardnessof metallized aluminum. The appropriate hardness has been achieved by keeping the temperature of the glasses as low
as room temperature during the metallization. The argon plasma cleaning procedure cleaned the contamination on thegold pads of the hybrids for successful wire bonding, although it was unsuccessful in the aluminum of the pitch
adapters.
r 2005 Elsevier B.V. All rights reserved.
PACS: 85.40Xx; 81.15.E; 29.40
Keywords: Hybrid; Flex circuit; Al-on-glass; Pitch adapter; Wire bonding

1. Introduction
Corresponding author. Tel.: +81 29 864 5791;

fax: +81 29 864 2580.
E-mail address: unno@post.kek.jp (Y. Unno).

ATLAS detector is under construction and will
be placed at the large hadron collider (LHC) at theEuropean Organization for Nuclear Research0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2005.01.068

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Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 286–294(CERN) [1]. Among various detector components,a charged particle tracking system, named Semiconductor Tracker (SCT) and composed of silicon
microstrip detectors, will be positioned near theinteraction point and inside the 2 T solenoidal
magnetic ﬁeld [2]. The SCT is made of the barrel
and the endcap sections where 2112 and 1976
detector units are disposed on cylinders and disks,
respectively.
The detector unit in the barrel section is the
ATLAS SCT barrel module. It is composed offour silicon microstrip sensors, two on the top andtwo on the bottom side, together with theelectronics for providing electrical power to andreading out signals from the sensors [3]. The pairof sensors, on the top and the bottom, is connected
strip by strip with Al-wire bonding to make 768
strips on the top and another 768 strips on thebottom side. These strips are then connected to 12
application-speciﬁc integrated circuits (ASIC,
ABCD3T [4]): 6 for the top and 6 for the bottom
on an electronics carrier the hybrid.
Due to a large number of particles generated at
the collisions, the silicon microstrip sensors will bedamaged by radiation and biased up to 500 V after
10 years of operation. The power dissipation of thesensors will be 2 W total and that of electronics
will be 5 W. The heat, especially of ASICs, has tobe released to the cooling element efﬁciently. The
hybrids must have good heat conduction and little
radiation length.
We designed the hybrids for the ATLAS SCT
barrel modules by applying Cu-polyimide ﬂexcircuit technology. We fabricated 2600 hybrids tomake 2112 barrel modules.

2. Design and construction of the hybrid
The Cu-polyimide ﬂex-circuit technology has
been applied in industry to numerous commercial
products, such as digital cameras. The vendor [5]had the capability to make an object as large as
300 mm  300 mm and multiple layers from single
to any number of layers. The size and ﬂexibility inlayer construction allowed realizing the hybrid in
one piece from the connector to the end ofelectronics. The electronics section is made of four287

layers and cable section of two layers. The
electronics sections were reinforced with a carbon
material to be rigid and to have good thermal
conductivity. The carbon substrates, together withthe organic material of insulator, polyimide,
enabled the hybrid to have low-radiation length.
2.1. Circuit diagram
The circuit diagram of the ATLAS SCT barrel
hybrid is shown in Fig. 1. The chart is speciﬁc for
the ASICs used however, common general features
can be seen in the chart: multiple bus lines runningalong the hybrids for multiple ASIC chips. There
are also communication lines connecting adjacentchips and connecting alternative chips for bypassing a chip in case of failure. There are 17 bus lines
and 16 communication lines at a cross section.
2.2. Cu-polyimide flex-circuit construction
The fundamentals in the ﬂex-circuit design are
the layer construction. From the circuit diagram of
the ATLAS SCT barrel hybrid, the minimum
number of layers required and designed at theelectronics section was four. The metal layers were,from the top, L1—traces to the chips, L2—bus
and communication traces, L3—power planes foranalog and digital circuits, and L4—analog anddigital ground planes. Since the section of four
metal layers was not ﬂexible enough, the cablesection was reduced to two metal layers.
A cross-section of the ﬂex-circuit at four layers
is shown in Fig. 2. The two Cu layers in the
middle, sandwiching a polyimide sheet of 25 mm
thick, formed a core layer. The Cu sheets werejoined to the polyimide sheet without an extra
adhesive. This double-sided core layer was extended to the cable sections. Two single-sided
layers were added, one on the top and the other on
the bottom, glued to the core layer with anadhesive. The basic thickness of Cu sheets was
1
3 oz (12 mm thick). The top and bottom Cu layers
were covered with relatively thick Cu layers after
metallizing the via and through-holes. In order toreduce the amount of Cu in the top and thebottom layers, both Cu sheets were etched to a half
thickness before the through-hole metallization.

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Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 286–294

Fig. 1. Excerpt of circuit diagram of the ATLAS SCT barrel hybrid. A section of only the ﬁrst three chips is shown in the ﬁgure.

Fig. 2. A cross-section of Cu-polyimide hybrid with the surfacebuild-up technology. A cross-section of via is also shown
between the top and an inner layer.

In order to make connections between the top or
bottom and the inner layers, holes (called ‘‘via’’)
were made while processing the top or bottom
single-sided layers. The vendor uses a laser for
drilling vias. The diameter of the via was 150 mm,
and the diameter of the metal area around the via,called ‘‘land’’, was 300 mm. Constructing a layerover-layers with via holes is called ‘‘surface buildup’’ technology. The connections between the top
and the bottom layers through the inner layers
were also made with through-holes realized withmechanical drilling. The diameters of the throughhole and its land were 300 and 500 mm, respectively. The narrow land size of the vias allowed tolay out the multiple bus and communication tracesin high density, thus reduced the width of hybrid
and ultimately the radiation length.
2.3. Flex-circuit design rules

The top and bottom layers, after the through-holemetallization, were etched to a mesh of 50%
opening, effectively reducing the thickness by half.

The vendor provided its design rules of ﬂexcircuits [6]. The design of ATLAS SCT barrel

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Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 286–294

hybrid followed their critical design rules. However, we asked for tighter tolerances for somepatterns that they could provide ultimately: theminimum metal trace width of 100 mm and theBreakdown voltage [V]Breakdown voltage [V]
50% safety factor included

1000

Polyimide(25µm)/Cu(35µm)
No cover-film
Ref. Mektron data sheet
100
0.1

1
Conductor gap [mm]10Fig. 3. Breakdown of high voltages in two conductors as afunction of conductor gap, without a cover ﬁlm. The line with
an open circle is the measurement and the line without markers
is the recommended values that include a 50% safety factor.

289

minimum distance between the traces (i.e., gap) of
80 mm.
Unlike usual commercial products, the hybrid
was required to connect a high voltage of 500 V.
The vendor had a datasheet of the breakdown ofhigh voltages as a function of conductor gaps, as
shown in Fig. 3. The curve with open circles is themeasured breakdown voltages and the line withoutmarkers is the recommended values including a
50% safety factor. From the curve, a gap distanceof 700 mm from other traces was chosen for the
traces carrying the 500 V high voltage.
There are detailed design rules to adopt in orderto reduce the risk of breaking the traces. These areshown in Fig. 4 and reproduced from the
document in Ref. [6]. The top three patterns show
the so-called ‘‘tear-drop’’ designs where narrow
traces join larger patterns. The rule is to smooththe transitions with a larger radius (R) to avoid
abrupt transitions of metals. The bottom two
ﬁgures are to reinforce the possible bending points.
These design rules are important as the ﬂex circuits
are bent, often unintentionally. Besides, CAD

R

R

R

Cover film
Bending point

Cu sheet

Bending point

Base film

Reinforcement plate
Overlap >0.5 mm
Fig. 4. Design rules for reducing the risk of trace breaking. The left patterns shall be modiﬁed to the right patterns. The top three are to
avoid abrupt transitions and the bottom two ﬁgures are to reinforce possible bending points.

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programs for rigid printed circuit board may draw
patterns as in the left side.
2.4. Mechanical and thermal design
The electronics sections of the Cu-polyimide
ﬂex-circuits were reinforced for mechanical andthermal reasons by gluing the ﬂex-circuit to asupport substrate. Mechanically, ATLAS SCT
barrel hybrid was designed to ‘‘bridge’’ over thesilicon sensors so as to inﬂuence them minimally.
The electronics sections were also to withstand thewire bonding in the hybrid. Thermally, the heat of
the electronics was to be transported efﬁciently
from the ASICs to the cooling element.
In order to fulﬁll the above requirements, amaterial called ‘‘carbon–carbon’’ with the carbon
ﬁbers running along the direction of the hybrid
was adopted as support substrate. The unidirectional carbon–carbon material had Young’s
modulus as strong as ceramics and the thermal
conductivity of 700 W/m/K longitudinally. The
details of the design can be found elsewhere [7].
2.5. Cu-polyimide ATLAS SCT barrel hybrid
The overall design of the hybrid is shown in
Fig. 5. Two electronics, two cables, and one connector section were made into one continuous piece,
thus eliminating vulnerable external connections

between the sections. Two electronics sections weredisplaced by 1.67 mm vertically to match the stereo
angle of 40 mrad of the top and the bottom sensors
when the hybrid was wrapped around. Due to theuse of organic and carbon materials, it was light inweight (6.2 g+1.0 g (ASICs)), and low in radiation
length (0.30%+0.06% (ASICs) Xo, averaged overthe module area of 128 mm  64 mm). The carbon
substrates were not only providing good mechanical strength and thermal conductivity but also
reinforcing the ground plane as they were connected electrically to the ground plane with aconductive epoxy. The thick ground planes of Cu
and carbon substrates helped to make the hybrid
robust against electrical instability in the ASICs.
The cost of one hybrid was relatively inexpensive
due to the low cost of Cu-polyimide ﬂex-circuit.
A section of the ﬁrst three chips corresponding
to the circuit diagram in Fig. 1 is shown in Fig. 6.
The top shows the ﬁrst (L1) and the bottom, thesecond (L2) layer. The bus traces were laid in L2 ina pitch of 300 mm where the metal and gap widths
were 200 and 100 mm, respectively. In L2, thecommunication traces along which digital signals
are transmitted, were positioned under the LSI
chips. The LSI chips were shielded from thecommunication traces through the ground plane
including the chip pad areas. In a section of thechip pads, there were bunches of through-holes
connecting the ground planes in the L1 layer andFig. 5. The overall design of the ATLAS SCT barrel hybrid. The sections where LSI chips are mounted are the electronics sections
with four layers and reinforced with carbon–carbon substrates. The leftmost section is the connector section with four layers forreinforcement reason. The remaining sections are the cables with two layers.

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291

the main ground planes in the L4 layer. The
through-holes were then ﬁlled with conductive
epoxy when the ﬂex-circuit was glued to thecarbon–carbon substrates. The through-holes also
helped to transmit the heat of the chips to thecarbon–carbon substrates.
The electrical continuity and short circuit in
the ﬂex-circuits were tested in the ﬂex vendor
before delivery. About 5% of hybrids were set
aside due to various damages in the assemblingstages.

3. Al traces-on-glass pitch adapter

Fig. 6. Metal patterns in the ﬁrst (L1) (top ﬁgure) and thesecond (L2) (bottom ﬁgure) layers at the ﬁrst three LSI chips
corresponding to the section in Fig. 1. The L1 layer is meshed
except for the chip pads and traces.

The pitch of the ASIC chips input pads was
48 mm, while that of the silicon microstrip sensors
was 80 mm. In order to make the wire bondingstraight, a pitch adapter (PA) was designed andplaced in front of the chips on the hybrid as shownin Fig. 5. The pitch in the ASICs was too ﬁne tointegrate the PA in the L1 layer. The PA wasproduced in thin aluminum (1 mm thick) on a glass
substrate (200 mm thick). The dimensions of the
PA and the aluminum trace patterns are shown in
Fig. 7.

Fig. 7. The dimensions of the pitch adapter (top ﬁgure) and the trace patterns (bottom ﬁgure). The traces were aluminum of 1 mm
thickness and the substrate was a glass of 200 mm thickness.

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3.1. Whiskers in PA wire-bonding
Although the PA is a simple construction, purealuminum on a glass substrate, this has been themost difﬁcult object in the whole hybrid fabrication. The difﬁculty was associated with wirebonding. In some products of the PAs, a splashof metal, ‘‘whiskers’’, was observed around the feetof wire bonding, as shown in Fig. 8. In the ﬁgure,the wire was Al with 1% Si of a diameter of 25 mm
and wire bonding was made with a supersonicwedge-bonding machine. In a batch of PA, thewhiskers were narrow like hairs, and in otherbatches thick as seen in Fig. 8. In some PAs, thewhiskers were seen at the sides of and in other atthe ends of the feet. The length of whiskers varied
from little to a few tens of microns, and varied
depending on the bonding machines.
Although the whiskers were also observed in thewire bonding on the ASIC chips or on the silicon
microstrip sensors, their amount and length weregenerally less. For a comparison, silicon wafers
were metallized with aluminum simultaneouslywith the glass substrates. The wire bonding on the
Al-metal on silicon wafers conﬁrmed less generation of whiskers in amount and in length.
Changing Al-metallization on PAs from purealuminum to Al+1% Si did not make muchdifference. Other metallization, e.g., Al+0.5% Cu,
gave a better result and the metallization of
Al+0.5% Cu elsewhere has given a satisfactory

Fig. 8. Whiskers observed around the feet of the wire bonds ona pitch adapter made of Al-on-glass substrate.

result [8]. However, we have not adopted
Al+0.5% Cu metallization mainly because ofavailability of the process to us in the industry inJapan and a potential corrosion issue when the Cu
content is high [9].
After numerous investigations, it appeared thatthe generation of whiskers was most plausibly
correlated with the hardness of metallized aluminum. The window of hardness for having a
reduced amount of whiskers seemed narrow.
When it was too soft, it generated whiskers; when
it was too hard, it became difﬁcult to make thewire bonds stick. The window seemed also
dependent on machines in wire bonding. Unfortunately, we failed to measure the hardness of a thinmetal, such as 1 mm thick, reliably.
The difference of the ﬁnish of the metallized
surfaces: uniform in the silicon wafers and nonuniform in the glass substrates implied that thetemperature is a critical factor. In the metallizationchamber, a wafer or a substrate had been held with
its fringe to the metal blocks. The non-uniformity
in a glass could be caused by the low thermal
conductivity of glass, which induced higher
temperatures toward the center of the glass. The
generation of whiskers indicated that the temperature had to be as low as room temperature.

Fig. 9. Improved aluminum metallization where few whiskers
were generated.

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Having understood the fundamental factors, themetallization vendor modiﬁed the process anddelivered much improved and consistent products
over the production batches. As shown in Fig. 9,
few whiskers were generated when the process ofmetallization was improved.
The yield of PA due to the whiskers had
increased from about 75% to about 95% after
the improvement. Because of improper ﬁnish of
traces due to dusts and scratches, the overall yieldof the PA was the yield due to the whiskers times
about 80%.

4. Other issues in hybrid fabrication

293

lowered and spread wider. Without contamination, the pull strength in the gold pads of thehybrid and the aluminum pads of the PA wasabout 10 g with a spread (one standard deviation)of a gram or so.
The residues on the surface of the hybrids werecleaned with argon plasma cleaning, e.g., 500 W
power and a few 10 seconds. Although the residues
on the PAs surface could be cleaned with theplasma cleaning, the surface of the aluminum
became unbondable afterward such that the wirebonds did not stick. The reason for this has been
under investigation. Because of the unbondability,
the residues on the PAs surface were cleaned withalcohol-dipped cotton tips.

4.1. ASIC chip replacement
5. Summary
The ASIC chips were attached on the chip padsof the L1 layer with an electrically conductive
epoxy adhesive. The epoxy adhesive had silver
powders for electrical conduction and could be
cured in room condition, e.g., reaching 90% of
ultimate hardness after 24 h at 23 1C [10]. The
electrically conductive epoxy helped not only tohave the ground plane as close to the chips as
possible but also to transport the heat of the chips
to the hybrid with its higher thermal conductivity.
The epoxy was softened at a temperature around80 1C and the chips could be removed by twistingthem with a tool.
4.2. Surface contamination and plasma cleaning
The surface of the ﬂex-circuit was cleaned withargon plasma cleaning at the vendor. When the
ﬂex-circuits were glued on the carbon–carbon
substrates, with heat pressing at 120 1C and 4 kg/
cm2, the surface of the ﬂex-circuits was protectedwith a ﬁlm with weak adhesion. Similarly, when
the PAs were glued on the hybrids, while no heatpress was applied, the surface of the PAs wasprotected with the same ﬁlm.
Occasionally, we observed surface contamination by residues of adhesives coming from theprotection ﬁlm. The residues were only visible witha microscope; however, the quality of wirebonding was degraded: the pull strength was

We applied the Cu-polyimide ﬂex-circuit technology that is widely used in commercial products
to the ATLAS SCT barrel hybrid. The surfacebuild-up technology with laser vias provided
ﬂexibility in the layer construction and allowed
high-density layout of multiple traces. The ﬂexibility in layer construction and availability of alarge size of ﬂex-circuit enabled the ATLAS SCT
barrel hybrids to be constructed in one piece fromthe connector to the far-end section of electronics
including the cable sections. The high-density
layout enabled the width of the hybrid to be
narrow and with less material. Reinforcement withthe carbon–carbon substrates at the electronics
sections gave strong mechanical construction,
good thermal conductivity, and stability in the
ASIC operation. By following the design rules ofthe vendor, little trouble in breaking the traces was
experienced.
The pitch of input pads of the ASICs required
pitch adapters between the sensors and the ASICs
to be made of aluminum traces on glass substrates.
The condition not having whiskers around thewire-bonding feet required an appropriate hardness of metallized aluminum. The range of hardness seemed to be narrow but it was achieved bykeeping the temperature of glass as low as room
temperature and as uniform over the entire glass
area, during the metallization.

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Argon plasma cleaning process cleaned thecontamination on the gold pads of the hybrids
for successful wire bonding. The surface of thepitch adapters was cleaned separately with alcohol-dipped cotton tips as the argon plasma
cleaning made the aluminum of the pitch adapter
unbondable.

Acknowledgements
The work was supported partly by the Grant-inAid of Japan Society for Promotion of Sciences,No. 11207204. We thank T. Takada and his teamat Seiko Precision Inc. [11] for assembling the
hybrids, and especially for the results of the wirebonding and the argon plasma cleaning. We also
thank the ATLAS SCT barrel module collaboration, convened by A. Carter and Y. Unno, and the
ATLAS SCT collaboration, led by M. Tyndel, forvaluable inputs for designing the hybrid anddebugging the problems.

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pp. 115–119, and references therein.
[5] Nippon Mektron Ltd., 1-12-15 Shiba-Daimon, Minato-ku,
Tokyo, Japan.
[6] Flexible Interconnection System, Nippon Mektron Ltd.,
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[7] T. Kondo, et al., IEEE Trans. Nucl. Sci. NS-49 (2002)
3278.
[8] M. Ullan, et al., CNM Insitituto de Microelectronica deBarcelona, used for the ATLAS SCT endcap modules,
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[9] G. Harman, Wire Bonding in Microelectronics, Materials,
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