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CDF Silicon DetectorWire-Bond Failures Induced by Resonant
Vibrations
Reid Mumford
Johns Hopkins University
CDF Collaboration
CDF Silicon Detector – p. 1/18
Component FailuresAfter commissioning, Several failures occurred under anomalous trigger conditions.
Two types of failures.
DVDD Jumper - loss of z information from detector
DOIM - loss of all information from the module
Anomalous trigger conditions
“Torture Tests”: tests to explore dead-time at high rate.
high occupancy mode in SVX3 chips
Both trigger conditions cause the system to be read-out at a fixed rate
CDF Silicon Detector – p. 2/18
DVDD Jumpers
Layers of SVX are double-sided w/ bonds connecting z to
�
hybrids
Bonds supply power and move data from z channels during readout.
Bonds are in a plane orthogonal to the magnetic field in CDF.
CDF Silicon Detector – p. 3/18
DOIMsD¯
ense O¯
ptical I¯nterface M
¯odule
Laser diode package that moves data from detector to VME crates in parallel (9-bit bus).
Power connection wire-bonds are in a plane parallel to magnetic field.
Vertical components however are perpendicular to magnetic field.
CDF Silicon Detector – p. 4/18
The CDF Detector
CDF Silicon Detector – p. 5/18
DOIM and Jumper Currents
DVDD Jumpers have
��
up to 150mA
� �� � occurs when readout shifts from
�
to z chips
DOIMs have
��
of no more than
40-50mA (top trace)
Current swings come from switching be-
tween laser diode and dummy loads
CDF Silicon Detector – p. 6/18
Bond Specifications
� � � � in length
� �� � � in diameter
Composition: 99% Al 1% Si
Different profiles for DOIM,
�
and
z-side bonds
Not encapsulated
CDF Silicon Detector – p. 7/18
Lorentz Forces
Ma
gn
etic
Fie
ld
Current
CurrentForceForce
DVDD DOIM
� ������ �� � � � � � ��� � � � �� � � � � � � � � � �� � � � �� ���
CDF Silicon Detector – p. 8/18
ResonanceResonance enhances fatigue due to Lorentz forces
Visually scanned for resonance in bonds in a 1.4T test magnet
Resonances observed with driving currents of 10mA - 150mA.
CDF Silicon Detector – p. 9/18
Characteristic Frequency
Initial calculations for 2mm SiAl bond predicted 15kHz
Many bonds tested w/ 40mA sinusoidal driving current (push and pull)
Fundamental frequency in agreement with calculations
1st harmonic was also observed
Typically 1kHz resonant width
Arbitrary Measurement Number0 5 10 15 20 25 30 35
Dri
vin
g F
req
uen
cy (
kHz)
0
5
10
15
20
25
30
35
40
45
50
Resonant Frequencies w/ SineHigh Resonance
Low Resonance
Resonant Frequencies w/ Sine
CDF Silicon Detector – p. 10/18
More Realistic CurrentsBonds driven with a more realistic current pulse (100mA,
� � � �
) exhibit more resonances.
Resonant frequency
�
can be excited with pulses at
�
,
� � �
,
� � �
, etc
Width of resonance is between 1 and 200Hz.
Differently shaped bonds imply different frequencies.
Arbitrary Measurement Number0 20 40 60 80 100 120
Dri
vin
g F
req
uen
cy (
kHz)
0
5
10
15
20
25
30
35
40
Resonant Frequencies w/ PulseResonant Frequencies w/ Pulse
CDF Silicon Detector – p. 11/18
Foot Fatigue
On the time-scale of minutes, a resonating bond will fail
At 10kHz this is about
�� � � � � �
cycles.
Breakage occurs at the bond’s foot
Breakage is due to stress fracture that forms during vibration
CDF Silicon Detector – p. 12/18
Fatigue vs. Pulling
Failure due to fatigue looks very different from a pulled bond
Similar to a failure mechanism described in a paper by Raymond T. Fitzsimmons and C. E.
Miller (IEEE Transaction on Components, Hybrids and Manufacturing Technology, Vol. 14,
No. 4, December 1991)
Fatigue Failure Pull Failure
CDF Silicon Detector – p. 13/18
Fast Video Analysis
A camera with a 40500Hz shutter rate was used to record the motion of bonds.
Single frames have been digitized and a quantitative analysis performed
CDF Silicon Detector – p. 14/18
Excitation and Damping
Bonds were excited w/ a limited number of
� � � �
,
�� � �
current pulses
The amplitude was measured as a function of the number of pulses
The damping ratio was also measured from the decay of the free oscillations to be roughly
��� � �
Free Oscillation
� � � � �
0 200 400 600 800 1000
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8 / ndf 2χ 26.98 / 70
offset 0.01843± 0.03918
amplitude 0.1283± 3.586
damping 0.000423± -0.007632
phase 0.06057± -0.5621
40 kicks: Amp vs Frame / ndf 2χ 26.98 / 70
offset 0.01843± 0.03918
amplitude 0.1283± 3.586
damping 0.000423± -0.007632
phase 0.06057± -0.5621
CDF Silicon Detector – p. 15/18
Mitigation
3-4 current pulses at the bond’s characteristic frequency are enough to excite motion.
Efforts have been made to reduce Lorentz forces.
Reduce current swings by changing SVX3 chip settings
Reduce duration by lowering noise occupancy
Minimize time at resonance; trigger inhibit on resonances detected directly with FFT.
Banned “Torture Tests”
CDF Silicon Detector – p. 16/18
Encapsulation
The foot of the bond was encapsulated w/ Sylgard 186 Silicon Elastomer
Encapsulant thickness is no more than
� � � �
Amplitude of resonant vibrations was reduced by more than an order of magnitude
Unable to break encapsulated bonds even when driving for several hours with large currents
( � � � � � �
)
CDF Silicon Detector – p. 17/18
ConclusionLast fall the CDF experiment faced a crisis due to internal unrecoverable
failures on the silicon detector
The source of the problem has been understood to be a simple physics
mechanism
The understanding of this problem should be applied to the construction of
future silicon detectors
Counter measures have been studied and applied to the CDF experiment.
Since the implementation, no other failures have occurred
Resonance cannot be avoided but the accumulated stress and fatigue on
bonds has been minimized by
Reducing the strength of the Lorentz force
Reducing the time spent at resonance
An encapsulation method for future applications has been successfully
tested on a small number of samplesCDF Silicon Detector – p. 18/18