Wire Bond Encapsulation for the CMS
Forward Pixel Upgrade
Sam Higginbotham
Prof. Matthew Jones
Purdue High Energy Physics
Introduction
The pixel detectors used in the CMS experiment at
CERN will be replaced by an upgraded detector
system in 2016. Modules consisting of a pixel sensor
and 16 readout chips are being assembled at Purdue
with electrical connections to the support circuits
made using aluminum wire bonds. We have
developed a process to encapsulate these wire
bonds in a silicone compound to provide mechanical
protection and to prevent electrolytic corrosion.
Presented here are the techniques developed for
depositing this viscous compound with a precision of
100 μm.
Precision Dispensing
Purdue takes advantage of the gantry’s precision by
fixing a 150 micron inner diameter dispensing tip to
an EFD dispensing pressure multiplier mounted on
the gantry head. To interface with the hardware, a
LabVIEW program performs the vector algebra to
systematically deposit encapsulant with enough
degrees of freedom to account for variance of parts
and positions.
Figure 5: ROC to HDI encapsulation results
Benefits of Encapsulation
Module Assembly
Purdue is responsible for delivering at least 500
modules for the phase-1 upgrade of the CMS
detector. These modules consist of an array of silicon
pixels (sensor) that are bump-bonded to silicon read
out chips (ROCs) and glued to a high density
interconnect (HDI) circuit. There are 560 wire bonds
that make electrical connections between the ROCs
and the HDI on each module.
HDI
Sensor
ROCs
Figure 1: Forward pixel detector module stack up.
A Devoltek F&K 6400 ultrasonic wire bonder is used
with 38 μm wire to make the wire bonds. Sylgard
186, a silicone based elastomer, is used to
encapsulate the wire bonds. Sylgard is a very viscous
polymer before curing, which after curing has
flexibility, high shear strength, and excellent
dielectric properties.
Equipment
To encapsulate at the 100 μm precision, Purdue uses
an Aerotech AGS10000 robotic gantry system which
is capable of 1 μm positioning precision over large
distances. An Edmund Optics machine vision camera
with 2560 x 1920 resolution is used to index the wire
bonds to the gantry.
For the CMS Forward Pixel detectors, there are three
main reasons for encapsulation:
• Mechanical protection
• Prevention of electrolytic corrosion
• Resonance damping
Figure 3: Token bit manager encapsulation
The Token Bit Manager (TBM) is a custom integrated
circuit on the HDI that is responsible for coordinating
readout of data from the ROCs. Wire bonds on the
TBM are placed at Fermilab and are encapsulated at
Purdue. The geometry of the chip poses a challenge
for encapsulation because of the fine pitch of the
wires. We prefer to encapsulate only the feet of the
wire bonds which achieves the main objectives while
avoiding encapsulant encroaching on unwanted
places. For example, encapsulant seeping into the
gap between the sensor and a ROC has been seen to
slightly alter the electrical properties of the pixels Figure 6: Wire bond resonance from Lorentz force
that are in contact with the encapsulant.
and wire bond breaking at the heel.
The motion of the dispensing tip is piecewise linear
and the LabVIEW program is used to acquire points
along its path from the absolute coordinate system
of the gantry. With the precise optics of the gantry, a
3D point can be measured with the XY positions
based on the image and the Z position from the
focus of the Camera. In conjunction with the
acquired positions of the wire bond feet, a CAD
model of the part is used to deposit encapsulant in 8
sets of 35 bonds in a single operation. Movements
such as retracting, shown below, are used to ensure
an even glue deposition.
5 mm
0.75mm
Figure 2: Indexing the last wire bond of a line.
6.5 mm
The camera’s precision allows the operator to gather
positions of wire bonds in any configuration in all
three dimensions. To deposit the encapsulant at high
pressure, an EFD Ultimus V pressure control
dispenser with pressure multiplier is used.
The encapsulant provides mechanical protection for
the wire bonds, ensuring the longevity of the part
once it has been installed at the center of the CMS
detector, where access is impractical.
The encapsulant also prevents water and other
electrolytic catalysts from accelerating the entropic
corrosion process [1].
Figure 4: Needle retract movement (not to scale)
Forced harmonic oscillations in the wire bonds can
result from currents on some wire bonds in the
presence of the 3.8 Tesla magnetic field used in CMS
[2]. Periodic currents at a resonant frequency can
result in large amplitude mechanical vibrations which
could eventually result in bond failures, as shown in
Figure 6 [3]. The encapsulant damps these
resonances, preventing large amplitude vibrations
from developing.
Conclusion
We have developed a process for the selective
encapsulation of wire bonds used in the phase-1
upgrade of the CMS forward pixel detector. Using an
Aerotech robotic ganrty system, Purdue can
encapsulate a module with 100 μm precision in
approximately 20 minutes. Encapsulation provides
mechanical
protection,
prevents
electrolytic
corrosion, and damps mechanical vibrations. This
process will be used throughout 2015 in the
production of approximately 500 sensor modules.
References
1.
2.
3.
D.R. Sparks, Chemically-accelerated corrosion tests for
aluminum metallized ICs. Thin Solid Films 235 (1993) 108-111.
S. Chatrchyan, et al. (CMS collaboration.) The CMS experiment
at the CERN LHC, JINST 3 (2008) S08004.
G. Bolla, et al., Wire-bonds failures Induced by resonant
vibrations in the CDF silicon detector. IEEE NSS 3 (2003) 16411645.