# INTRODUCTION

[CDF]{.smallcaps} is a collider experiment that is running at the
Tevatron. The core of the CDF detector is an 8 layer silicon micro strip
tracker. There are over 722K active strips with pitches that range from
25 to 140 μm. Wire-bonds are widely used for signals, commands and power
connections between the various components of the modules. Long after
the end of the detector commissioning phase a non-negligible rate of
permanent failures (14 failures in 704 modules) was recorded. A strong
time correlation between the occurrences of the failures and anomalous
trigger conditions was promptly noted. An investigation started in order
to understand these trigger dependent failure modes and possibly reduce
or remove their rate of occurrence. The silicon detector was installed
in the CDF experiment in January 2001. A general description of the
system can be found elsewhere \[1\]. A schematic view of the CDF silicon
is shown in fig.1. Once installed it is not accessible for maintenance
and a direct investigation on the faulty modules is not possible. The
investigation had to be pursued with spare parts left over after
construction \[2\]. The emulation on the bench of the anomalous trigger
conditions was not sufficient to reproduce the failure modes until the
modules were placed in a 1.4 T magnetic field. Oscillating Lorentz
forces on the wire-bonds induced by non-DC currents were found to excite
resonance movements of the *Al* wires. During this motion fatigue stress
is accumulated at the wire-bond heel until the wire breaks and the
electrical connection is lost.

![](media/image1.pct)

Fig. 1. Schematic view of the CDF silicon system. To be noticed that all
of the components in the blue box in the bottom of the figure are not
accessible after the installation in the CDF experiment. Wire-bonds are
widely used on the portcard and on the hybrids for electrical
interconnections.

This paper presents the symptoms associated with the failure modes, a
comprehensive description of the current consumption on the power lines
that failed and the consequent Lorentz forces on the wire-bonds.
Furthermore, a qualitative and quantitative analysis of the general
behavior of wire-bonds in a magnetic field with non-DC current in the
1-50 KHz range is described.

# Description of the failures 

The command and data flow within the CDF silicon system is described in
fig.1. Commands are generated in the VME crates and routed to the
tracking volume. Here they are received by the portcard \[3\] and fanned
out to 5 ladders (single independent readout unit in the system) where
the SVX3d \[6\]\[7\] read-out chips integrate the charge, digitize it
and send the data out through a 8 bits parallel bus. The data are then
sent from the portcard to the VME through a parallel optical link from
now on called DOIM (Dense Optical Interface Module)\[5\]. For the
failure mode described here the possibility of the *Al* wire fusing due
to excess current has been ruled out by introducing a short-circuit (and
so maximum current draw) and verifying that the tripping mechanism of
the power supplies always reacts before any permanent damage occurred in
the wires. Possible aging effects (e.g. electron migration) inducing the
loss of electrical continuity were ruled out by accelerated aging tests
performed at high temperature with spare parts.

The CDF detector is designed to run with a level 1 trigger (L1A) rate up
to 50 KHz. At each one of these triggers the silicon system is read out
and its data are used in the second trigger level to select events with
displaced tracks. All the failures described here appeared during
trigger conditions where the L1A were issued according to well defined
synchronous conditions. The distribution of the time interval between
L1A in the experiment is usually very broad due to the natural
randomness of the physics processes being investigated. During specific
tests trigger were issued according to the repetitive beam structure so
the silicon system was read out with fixed intervals between a L1A and
the next one. These conditions were induced to explore the maximum
sustainable rates for the experiment.

The failures experienced in the CDF silicon detector can be grouped into
two categories, affecting two pieces of the detector, the jumper and the
DOIMs. Both groups share the time behavior of the current consumption
and its correlation with the trigger rates of the experiment.

## The DVDD jumper failure

The Double-Sided silicon Detectors (DSSD) in the SVXII modules \[3\] are
readout by a daisy chain of SVX3d chips. The connections between the Phi
and the Z hybrid for commands, power and data is established by a
ceramic substrate (jumper) with pass-through via holes that is glued on
the sensor's edge and wire-bonded to both hybrids. The jumper provides
electrical continuity from one side to the other. A simple schematic of
this interconnection is shown in fig.2. Both the analog (AVDD) and
digital (DVDD) power lines as well as their return ground paths are
connected with two redundant wire-bonds while single wires are used for
the commands and data lines.

Twelve of the fourteen failures described in this paper share the same
symptoms:

-   Lack of data from the Z chips

-   Decrease in the DVDD power consumption

-   Increase on the AVDD power consumption

![](media/image2.png){width="2.93125in" height="1.0083333333333333in"}

Fig.2. Simple schematic of the jumper connection via wire-bonds.

The symptoms have been successfully reproduced without any ambiguity on
the bench with spare modules by pulling the DVDD jumper wire-bonds. The
digital power consumption of the chips is strongly dependent on the
activities of its digital part. Fig.3 shows the current behavior through
the DVDD jumper connection during a DIGITIZE-READOUT sequence that is
triggered by a CDF L1A (the jumper wire-bonds were replaced by a
discrete wire in order to use a Tektronix A6302 current probe). It can
be noticed how the current quickly changes by more than 100 mA within a
few micro seconds while the readout switches from the Phi to the Z side
of the module. The amount of time spent in the high current state is
directly proportional to the occupancy on the Z side.

![](media/image3.pct){width="2.6555555555555554in"
height="1.6965277777777779in"}

Fig.3 Current through the DVDD jumper connection (Digital power supply)
versus the time during L1A-DIGITIZE-READOUT sequence (this plot is for a
six chip ladder). Each division is 10 μsec on the X-axis and 50 mA in
the Y-axis. At about 33 μsec from the left border the SVX3 chips start
the digitization. The current linearly increase for about 5 μsec. At the
end of the digitization the Phi chips are readout first. In the center
of the graph we have the transition when the Phi chips read-out is
completed and the Z chips start to send out the data. The current
through the DVDD jumper connection rises by 100-150 mA. A plateau is
reached within a few μsec. As long as there are data on the chips to be
readout the current stays at this value until read out is completed and
then slowly goes back to the steady state value.

## The DOIM failure

The data from the SVX3d chips are received on the portcard where the
DOIMs (one per module so five per portcard) send them to the VME
buffers. Two of the 14 failures described in this paper share the same
symptoms:

-   Lack of data from the whole module

-   Decrease by 1/5 of the DOIM power consumption within the portcard.

Once again the symptoms have been successfully reproduced on the bench
with spare parts by pulling the power wire-bonds between the portcard
and the DOIM.. The power consumption of a single DOIM is about 350 mA.
The current amplifiers drive either the laser diodes (for bit HIGH) or a
dummy resistive load (for bit LOW). The matching of the two loads is not
perfect and therefore implies current pulses of a few mA between idle
state and during data transmission. Out of 18 spare packages that were
tested the maximum current variation detected was 7 mA. The limited
number of tested parts cannot rule out larger variation on some of the
504 installed in the CDF system. The current variation is not always in
the same direction. Direct measurements of the current swing on those
installed in CDF are not reliable due to the fact that the same power
supply (and therefore the same ammeter) feed five on a portcard.

# Lorentz forces and resonant behavior 

The geometry of the DVDD jumper bond is shown in fig.4 as measured with
a coordinate measurement machine at the Slicon Detector Facility at
Fermilab. The magnetic field is directed orthogonal to the plane of the
wire as if entering the plane of the paper. As a consequence the *Al*
wire is exposed to a distributed force while current is flowing through
it. In CDF the magnetic field is 1.4 T, which together with a current of
200 mA results in a Lorentz force of 28 milligrams per millimeter of
wire length. This is a couple of orders of magnitude smaller than what
is required to pull a healthy wire-bond to failure. The movement induced
in the wires from these forces has been estimated to be on the order of
a few micrometers and therefore hard to detect with optical tools.

![](media/image4.png){width="1.6034722222222222in"
height="1.2520833333333334in"}
![](media/image5.png){width="1.6034722222222222in"
height="1.2583333333333333in"}

Fig.4 Geometry of the DVDD jumper bonds Z and Phi side from left to
right. On each picture X=0 correspond to the bond foot on the jumper.

A Finite Element Analysis (FEA) of the system resulted in the natural
resonant frequencies listed in TABLE I. In fig.5 a graphical view of the
natural modes of the oscillation is shown with the relative resonant
frequencies as simulated for a bond with the same shape as the Phi side
DVDD jumper connection. Further details on the results of the FEA can be
found in \[8\]. The first three natural resonant modes of the wires are
well inside the operational range of the experiment.

Table I

Calculated resonance frequencies

  ------------ ------------------------ ----------------------------------
  Bond type    Frequency \[KHz\]        Plane of motion w.r.t. the plane
                                        of the wire

  Z            14.2                     Perpendicular

  Z            37.2                     Coplanar

  Z            42.0                     Perpendicular

  Z            83.0                     Coplanar

  Z            84.7                     Perpendicular

  Phi          20.4                     Perpendicular

  Phi          39.8                     Coplanar

  Phi          55.2                     Perpendicular

  Phi          103.6                    Coplanar

  Phi          111.5                    Perpendicular
  ------------ ------------------------ ----------------------------------

![](media/image6.png){width="3.515972222222222in"
height="4.745138888888889in"}

Fig.5 Graphical view of the oscillating modes for the Phi side DVDD
jumper wire-bond geometry. The black lines (straighter) represent the
shape of the bond at rest while the blue lines show its position during
oscillations.

Small test boards with wire-bonds of similar geometry as the ones of
fig.4 were built to verify the results of the FEA. A pulse generator and
a current amplifier were used to deliver current sine waves or current
pulses through the wires. The current and the frequencies were tunable
with 1 mA and 10 Hz precision respectively. Magnifying optics and a
40,500 frames per second fast video equipment were used to monitor the
motion of the *Al* wires. The FNAL Technical Division calibration magnet
was used to generate the 1.4 T magnetic field. The orientation of the
bonds with respect to the magnetic field was the same as for the CDF
silicon modules. Frequency scans were performed with sine waves of 100
mA amplitude. Resonant behavior was detected at frequencies close to the
ones predicted by the FEA at 14 and 26 KHz. The discrepancy can be
attributed to a slightly different shape of the bonds under test with
respect to the ones simulated. In the case of current pulses a resonant
frequency *f* could be exited with a train of pulses at frequency *f*,
*f/2*, *f/4* etc. A full frequency scan up to 50 KHz was difficult to
perform on each bond. The wires break at the heel in time scale of
minutes, corresponding to 10^5^-10^6^ cycles, that is comparable to the
time to perform the scans. In fig.6 the broken surface of a pulled bond
foot is compared to one of a bond foot that broke due to fatigue stress
accumulated at the heel. These pictures are in good agreement with
previous work \[10\].

![](media/image7.png){width="1.6666666666666667in" height="1.25in"}
![](media/image8.png){width="1.6666666666666667in" height="1.25in"}

Fig.6 SEM picture of broken bond heels. On the left the pure ductile
surface of a bond heel that has been broken with the usual pull-test is
shown while the one on the right broke due to fatigue stress induced by
the resonant vibrations and shows a brittle surface.

The motion of the wires was recorded in short movies with the fast video
camera. The images were digitized on a frame-by-frame basis. The
position of the wire versus frame number was plotted in ten bin
intervals and a sinusoidal fit gave the amplitude of the motion. A
wire-bond was fed with a train of 40 current pulses with a period of 108
μsec, a duty cycle of 12 % and amplitude of 70 mA. In fig.7 the
amplitude of the motion versus frame number is plotted. During the first
20 frames no pulses are released. At frame 20 the wave function
generator is triggered to start the train. Only 3-4 pulses are necessary
to start the motion of the wire. The amplitude of the motion grows
linearly until frame 200 when the last pulse is delivered. The amplitude
of the motion saturates at several wire diameters after 50-80 pulses
depending on the driving force (pulse amplitude and width). The free
exponential decay of the motion was used to calculate a dumping ratio of
0.0076 ± 0.0004. This value is very similar to the 0.01 used during the
finite element analysis.

![](media/image9.png){width="2.8916666666666666in"
height="3.0027777777777778in"}

Fig.7 Amplitude of the motion versus frame number when a train of 40
current pulses is sent to the wire as described in the text.

The DOIM wire-bonds lie on a plane that contains the magnetic field. Due
to the different height of the two substrates (there is a step up of
\~300 micrometers between the portcard and the DOIM bonding pads) and
the loop height there is still a component of the wire that is
orthogonal to the magnetic field. Even in this case Lorentz forces can
excite resonant vibrations. Furthermore the forces are now moving the
wires from side to side. With this configuration we expect the amplitude
of the motion to be larger than in the case of the DVDD jumper bonds. In
fig.8 the amplitude of the motion versus the frequency of the pulses is
plotted for the same wire-bond. The filled squares are measurements
taken with the jumper orientation and the current pulses as previously
described in this section, while the empty squares are measurements
taken after rotating the wire under test by 90 degrees in order to
emulate the DOIM orientation.

![](media/image10.png){width="3.51875in"
height="2.4055555555555554in"}Fig.8 Amplitude of the motion versus
frequency of the current pulses. Filled squares are for the jumper
orientation while empty squares are for the DOIM orientation. The lines
are joining the measured values and are there to guide the eye. The
characteristics of the current pulses are described in the text.

For this second set of measurements the amplitude of the current pulses
was diminished from 70 to 14 mA. With the current lowered by a factor of
five the amplitude of the oscillation at its peak frequency is still a
factor of two bigger than what is measured with the jumper orientation.
This explains how the DOIM wires can break in the same way as the jumper
wires even if pulsed with much smaller current amplitudes.

Four SVXII spare modules were placed in a second test magnet with a
larger bore \[9\] in order to physically accommodate the 30 cm long
devices. The synchronous trigger conditions were emulated in a 1.4 T
magnetic field by forcing the DAQ into a fixed frequency mode of
operations. This setup did not allow any optical monitoring of the
motion of the wires. The symptoms associated with the failures
previously described were the only available feedback. A slow and
time-consuming frequency scan was necessary to reproduce the first
failure. The DVDD jumper wire-bonds broke at their heel on the Phi bonds
at a L1A frequency of \~10 KHz. The frequency of the failure does not
match any listed on table I but it is consistent with ½ of the first
mode and ¼ of the second one. The location of the breakage is consistent
with the position of maximum stress as seen in the FEA.

Wire-bond encapsulation can prevent this failure mode. Different
Coefficients of Thermal Expansion (CTE) of the various substrates used
for these applications could also induce fractures on encapsulated
bonds. For future applications this was addressed by depositing a
silicon elastomer (Sylgard 186 silicon Elastomer from Dow Corning) only
at the feet of the bonds. Due to surface tension the encapsulant adheres
to the *Al* wire surface up to a height of 50-100 μm. These wire-bonds
were tested with the same procedures as described above. The amplitude
of the vibration was measured to be more than one order of magnitude
smaller than without the encapsulant. The bonds were pulsed for hours
with 0.5 A pulses at resonant frequencies without any failure.

# Operational response in the experiment

360 SVXII jumpers are used in the SVXII detector. Their wire-bonds are
very similar in shape to the ones in fig.4 but cover a broad
distribution due to variations during the construction process. Similar
considerations apply to the 504 DOIM that are used. In order to mitigate
the occurrence of the failures described above the current consumption
on the affected power lines has been minimized by reducing the current
output of the bus drivers on the SVX3d chips (this is a remotely
programmable setting). The readout thresholds have been raised in order
to further minimize the noise occupancy and therefore the readout time
(i.e. the width of the current pulses). Administrative procedures have
been implemented to avoid the use of the silicon detector in any test
that might enhance the probability of running with fixed trigger rates.
Finally a trigger inhibit on potential resonant behavior detected with
Fast Fourier Transform (FFT) of the silicon readout command has been
commissioned and implemented in the DAQ with a minimum impact on the
downtime of the experiment.

# conclusions

The symptoms of the failures in the SVXII modules were reproduced on
spare modules in a controlled environment by emulating the experiment
running conditions at the time of the failures. Even if this does not
guarantee that the failure mode is the same it is a direct proof that
wire-bonds can break according to this mechanism. In the assumption that
the mechanism for the failures in the CDF experiment was due to the
resonant vibration excited by Lorentz forces, counter-measures have been
applied to minimize the failure rate. No other failures have been
detected in \~1 year since their implementation. The failure mode can be
prevented in future applications by encapsulating the bonds at their
heel.

# Acknowledgment

The authors would like to thank B. Kephart and M. Tartaglia from the
Technical Division at FNAL for the help with the calibration magnet,
Greg Selberg from the silicon Detector Facility at FNAL for the optic
and mechanics and M. Lindgren and R. Roser for all the support from the
CDF Operation Department.

# References

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    CDF"**.** Nucl. Instrum. Meth. A383:13-20, 1996.

2.  G. Bolla "Testing and quality insurance during the construction of
    the SVXII silicon detector"*. Nucl. Instrum. Meth., vol. A473, pp.
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3.  M. Garcia-Sciveres et al., "Readout electronics architecture and
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4.  Y. Gotra et al., "The port card for the silicon vertex detector
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7.  T. Zimmerman et al., "SVX3: a dead-timeless readout chip for silicon
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8.  More details about the FEA analysis can be found at the following
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    [www-cdf.fnal.gov/upgrades/silicon/TASK-Force/FEA.html](../www-cdf.fnal.gov/upgrades/silicon/TASK-Force/FEA.html).

9.  More details about the Magnet used for these tests can be found at
    the following
    URL:<http://www-cdf.fnal.gov/upgrades/TOF/doc/notes/cdf-2573/node36.html>.

10. T. Raymond et al. "Brittle cracks induced in AlSi wire by the
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