DOUBLE-CANTILEVER MICRO-RELAY WITH INTEGRATED HEAT SINK
FOR HIGH POWER APPLICATIONS
Fatih M. Ozkeskin1* and Yogesh B. Gianchandani1,2
Department of Mechanical Engineering, University of Michigan, Ann Arbor, USA
2
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA
*Presenting Author: ozkeskin@umich.edu
1
Abstract: This paper reports an electrostatically actuated double-cantilever micro-relay for high power DCapplications. Micro-relays with platinum-rhodium and stainless steel contacts were micromachined from foils and
integrated with micromachined aluminum heat sinks. A stacked structure allowed electrical isolation of the contact
actuation cantilever from the one that provides electrical contact. The heat sinks were used for forced air cooling
with a mini-fan. Micro-relays with 130 V of turn-on voltage and 2.5 mm2 active area accommodate DC current
levels >2.5 A.
Keywords: MEMS Switch, Microrelay, On-device heat sink, Cantilever
INTRODUCTION
Electromechanical micro-relays are widely used for
a number of applications including automotive control
circuitry and test equipments [1]. High current carrying
capacity and small size is frequently sought in such
applications. Lithographic microfabrication techniques
have been explored to address these needs for many
years [2]. For example, an electrostatic micro-relay with
gold contacts was reported to allow 0.4 A of DC current
[3]. Another group reported a magnetically actuated
micro-relay with gold contacts that could switch a 1.2 A
DC load [4]. Although gold has low resistivity and high
resistance to surface oxides, has also low hardness (1-2
GPa) which renders micro-relays with gold contacts
prone to failure at lower DC currents [5].
The choice of contact metals for micro-relays is
inherently limited by lithographic microfabrication
techniques which provide restricted access to metal
alloys and rely more on sputtered thin films. Microelectrodischarge machining (µEDM) provides a
manufacturing method for bulk metal foil based micro
devices with feature sizes down to 5 µm. Metal foil
patterning is possible in serial mode with a wire
electrode, as well as in batch mode with a
lithographically patterned electrode, acting as cookiecutter type tool to machine features in parallel [6].
Platinum-rhodium (Pt-Rh) is a chemically inert
and mechanically robust hard metal alloy that is used
for high quality crucibles. It has higher hardness (~10
GPa) than pure platinum, comparable resistivity and
higher melting point [7]. These properties make Pt-Rh
alloy a good contact metal option for micro-relays with
high current carrying capability.
A µEDM`ed all-metal micro-relay directly
assembled on printed circuit board (PCB) was reported
earlier [8]. This paper presents a substantially more
sophisticated micro-relay, utilizing a double-cantilever
structure with Pt-Rh or stainless steel 316L (SS316L)
bulk foil contacts and micromachined on-device Al
heat sinks to demonstrate high-current handling
capability without compromising device footprint.
DESIGN
The device is a 3-terminal switch; it employs a
double-stacked cantilever structure. A contact
cantilever (400x1400 µm2) bridges signal lines when
pushed down by an actuation cantilever (1200x2100
µm2) that is located above it. A paddle at the distal end
of the actuation cantilever acts as a pull-down
electrode and provides electrostatic actuation when
biased with respect to the ground electrode underneath
(Fig. 1).
The contact cantilever is suspended
orthogonally with respect to the signal line. Two
alignment posts anchor it to the PCB. It has a recessed
region of 2 µm height, located over the signal line to
provide with an initial contact gap. The actuation
cantilever is designed to be directly anchored on the
PCB substrate. A stand-off element which extends from
the tip of the paddle prevents pull-in of the paddle to the
ground electrode beneath it. The gap between the standoff tip and the substrate is designed to be 7 µm, i.e. 3
µm smaller than the interelectrode gap across the
actuation beam paddle and ground electrode to ensure
shorting does not occur. A through hole located near the
anchor on the actuation cantilever prevents contact with
the alignment posts, whereas one in the middle
accommodates subsequent placement of a microsphere.
A thermally conductive sapphire microsphere
(Edmund Optics, 300 µm diameter) is situated on the
contact beam. It fits tightly in a hole in the middle of
the actuation cantilever. The microsphere mechanically
couples the contact and actuation cantilever and acts as
a dielectric material in between, electrically isolating
them. It performs as a thermal conductor to distribute
contact induced heat to the heat sink. It also provides a
single point of contact between contact and actuation
cantilevers alleviating any assembly imperfections.
An aluminum heat sink (500x500x500 µm3) is
located above the actuation beam. A through hole at
the base of the heat sink, concentric with the middle
hole on the actuation beam, supports the microsphere
and provides a heat dissipation path from contacts to
the heat sink fins.
Duralco 4703) (Fig. 2c). The flatness of the cantilevers
was maintained by monitoring beam height with a
displacement sensor (Keyence LK-G). The sapphire
microsphere was inserted through the perforation in
the actuation cantilever and attached to the contact
cantilever underneath by thermally conductive epoxy
The heat sink was placed on top of the actuation
cantilever, surrounding the microsphere, and was
secured by thermally conductive epoxy (Fig. 2d).
Fig. 1: Exploded view of double cantilever-PCB
assembly. Gold alignment posts hold the contact beam
over gold-coated copper traces on PCB. The actuation
beam pushes the contact beam via thermally conductive
microsphere. Heat sink is integrated atop. The 2.5 mm2
active area is defined by the actuation cantilever size.
(a)
FABRICATION/ASSEMBLY
Cantilevers and Heat Sink
Contact cantilevers were µEDM’ed from both
stainless steel SS316L and Pt-Rh (80:20) 50 µm thick
stock foil. Cantilevers were machined down to 30 µm
final thickness; two perforations of 300 µm diameters
were positioned in anchor regions for attachment to the
PCB via alignment posts.
Actuation cantilevers were µEDM’ed from
hardened Al (3003) using 250 µm thick stock foil. The
paddle section, middle section and anchor sections
were reduced in thickness to 30 µm, 120 µm and 230
µm, respectively. Similar to contact cantilever, 300 µm
diameter perforations were machined on anchor and
middle sections for subsequent alignment and
microsphere assembly.
The heat sinks were µEDM’ed from Al (3003)
using 500 µm thick stock foil. A perforation (300 µm
diameter) was machined at the base of the heat sink to
accommodate microsphere. Thermally conductive
sapphire microspheres (300 µm diameter) were used.
Printed Circuit Board
A standard 1.6 mm thick FR-4 constituted the PCB
substrate (Advanced Circuits). Metal interconnect
traces of 90 µm thickness Cu were chosen for high
current ratings (up to 4 A for 300 µm wide signal
lines). In such PCBs, a 4 µm thick Ni was used as
adhesion layer on Cu base and 0.15 µm thick outer
gold layer was selected as contact surface. Through
holes formed by vias were positioned for subsequent
assembly of the cantilevers.
Assembly
Gold wire posts (400 µm height; 300 µm
diameter), were tightly fitted into PCB vias of the same
diameter (Fig. 2a). Conical tips allowed easy insertion
and assembly. The contact cantilever was assembled
onto the posts and held in place by thermally
conductive epoxy (Aavid Thermalloy) (Fig. 2b).
Subsequently, the actuation cantilever was assembled
on top of contact cantilever and attached to the
substrate using high temperature epoxy (Cotronics
(b)
(c)
(d)
Fig. 2: Assembly sequence showing: (a) floating anchor
with alignment posts inserted. (b) Contact cantilever
aligned on top of posts and anchored with epoxy. (c)
Actuation cantilever directly placed on PCB substrate.
Stand-off tip prevents electrostatic pull-down. (d)
Microsphere inserted into the hole on top beam, secured
with epoxy. Heat sink placed by aligning the center hole
around the microsphere, fixed with epoxy.
ELECTRICAL PERFORMANCE
The test circuit (Fig. 3a-b) used separate power
supplies as inputs for the cantilever actuation voltage
(VG) and line current (ION). The current source was
limited to 10 V compliance. The voltage VISO was
monitored for isolation and current leakage in case of
pull-in or ground electrode short. Under normal
operation, there was no current flow across RISO; hence
VG and VISO were the same. The voltage VFLOAT was
tracked for possible current leakage to the electrically
floating anchor. The signal line voltage VS was
measured for varying line current to determine on-state
resistance RON. RON was also characterized for increased
VG. Tests were conducted in 0.2 Torr nitrogen ambient
and were repeated for both Pt-Rh and SS316L contact
cantilevers.
(a)
position anymore when the DC-bias is removed. Onstate resistance was sharply increased after around 1.3 A
and 1.8 A and device failures occurred at 1.8 A and 2.6
A for SS316L and Pt-Rh contacts respectively.
Fig. 4: Experimental results for change of on-state
resistance with actuation voltage (ION =1 A, 0.2 Torr
nitrogen). Increased VG yields higher contact force
hence lower contact resistance. Pt-Rh accommodates
lower resistance.
(b)
Fig. 5: On-state resistance RON for up to 2.6 A of line
current (in 0.2 Torr nitrogen). VG was kept at 150 V.
Failures due to microwelding occurred at 1.8 A and
2.6 A for SS316L and Pt-Rh devices respectively.
Fig. 3: (a) Circuitry for testing; VISO and VFLOAT are for
current leakage monitoring. ION and VS are to
determine on-state resistance (RON). (b) Voltage
transients associated with actuation.
Typical turn-on voltage for micro-relays was 130
V and switching times were around 10-15 ms. Fig. 4
shows on-state resistance RON with increasing gate
voltage for a constant line current of 1 A. The contact
resistance was initially large when the actuation
occurred around 130 V, and then reduced with higher
actuation voltage due to larger contact force on the
signal line. On-state resistance was around 1.5 Ω and
1.25 Ω lowest for SS316L and Pt-Rh contact
cantilevers respectively. The resistance did not exhibit
further diminution after around 165 V of VG.
Fig. 5 shows on-state resistance with increasing line
current for a constant actuation voltage of 150 V. ION
was increased to the point of failure where excessive
contact heats caused contact and signal line metals to
first soften then microweld into each other to the point
where the micro-relay would not restore its up-state
THERMAL PERFORMANCE
Thermal tests included evaluation of micro-relays
with Pt-Rh contacts under unforced and forced air
cooling conditions. Finite-element modeling was used
to estimate the temperature distribution at 1 s into the
on-state. The solution demonstrated effectiveness of
the heat sink in suppressing contact area temperature
(Fig. 6), when combined with forced cooling using a
commercially available mini-fan (Sunon UF3A3).
Experimental measurements of the contact area
temperature were performed on the completed Pt-Rh
devices in 0.2 Torr, 300 K nitrogen ambient and for 1-s
on-state times. Perfluoroalkoxy insulated copper wire
thermocouples were used to measure temperature.
Contact area temperatures were monitored by increasing
line current ION and were compared to simulation.
The thermal behavior of Pt-Rh micro-relays is
shown in Fig. 7. The use of forced cooling suppressed
contact temperatures by more than 20%. Additionally,
devices with forced cooling exhibited higher current
handling (2.8 A). Experimental results were overall in
good agreement with simulations. Table 1 benchmarks
this work to DC MEMS relays as well as solid-state
relays. While RON is larger for this work, the
compactness is very competitive and the current
handling is adequate for several applications.
(a)
CONCLUSION
A micro-electrodischarge machined doublecantilever micro-relay for high power DC applications
was presented. Double cantilever structure allows
isolation of actuation and contact elements. A micromachined heat sink directly assembled on-device
reduced contact heating for higher current ratings.
Fabricated micro-relays with Pt-Rh contacts
demonstrated up to 2.8 A of current rating under
forced air cooling and 0.2 Torr nitrogen ambient. The
micro-relays have an active area of 2.5 mm2 and
provide high current rating per area compared to solidstate and MEMS counterparts.
ACKNOWLEDGEMENT
(b)
This study is supported in part by Defense
Advanced Research Projects Agency, Microsystems
Technology Office (DARPA MTO) contract #
W31P4Q-09-1-0009.
REFERENCES
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Fig. 6: FEA for 1s on-state time. Temperature
distribution for (a) unforced (b) forced cooling (Pt-Rh
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upward flow)
Fig. 7: Steady state contact temperature TSS for 3terminal Pt-Rh switches subjected to unforced and
forced cooling in 0.2 Torr nitrogen. Experimental data
was compared with simulations. Heat management
with upward forced cooling (compatible with Sunon
UF3A3 mini-fan, 0.22 m/s air flow) yields lowest TSS.
Table 1: Device benchmarking to solid-state and
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Active area (mm²) 1.15 9.6 1302 (pk.) 303 (pk.)
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1.2
45
10
1.8
2.8
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N/A
N/A
0.72
1.12
On resistance (Ω) 0.014 0.022 0.005
0.1-0.15
1.5
1.2
Switch time (ms)
20
2.5
0.002
0.0001
10-15
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