Reliability of Spring Contacts in Industrial Environments - Semikron

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Reliability of Spring Contacts in
Industrial Environments
The connection between a power module substrate and a printed circuit board can be established by
spring pressure contacts. This type of contact allows easy assembly without additional soldering. Springs
are used for a variety of current ranges, from sensor currents of a few milliamperes to load currents of
several amperes. Environmental strains by mechanical wear, rapid temperature changes and corrosive
atmosphere are significant stress factors in industrial applications. The reliability of spring contacts under
various harsh environment conditions is investigated. Florian Lang and Uwe Scheuermann,
SEMIKRON Elektronik, Nuremberg, Germany
Spring pressure contacts are often
compared to the reliability of wrap- or
solder-connections. However, the contact
forces are in different ranges for classical
wrap connectors and the springs. Here,
an interconnection between a printed
circuit board (PCB) with driver
components and power connections and
a ceramic substrate (DBC) with dies is
formed by springs. Each spring has two
contact spots. A loop of at least two
springs is used for testing. Due to the
assembly, an individual contact is not
accessible.
The pressure range determines the
choice of contact materials for the
different connectors. Tin and silver plating
are suitable for a contact force of
approximately 2 to 20N, while gold
platings are preferred in the range from 1
to 2N.
Fretting corrosion
‘Fretting corrosion’ is the
phenomenon of the growing, abrasion
and compacting of oxide particles by
repeated micro-movement by vibration.
This process is well known for wrap
connectors of certain contact
combinations.
To simulate a repetitive movement of
the contact partners, as caused by
vibration or different coefficients of
thermal expansion, a set-up was
designed that allows a controlled
movement of a PCB over a spring at a
defined frequency, load and amplitude
(Figure 1).
Test results on the spring contacts
proved to be entirely different from the
published results on wrap connectors.
Displayed in Figure 2 is the change
of contact resistance of two springs
versus the initial value. The contact
resistance decreases initially. This
process is associated with the
cleaning of the contact spot.
Contamination is removed. No
increase of the contact resistance was
detected during 4.65 million
movement cycles. This is attributed to
the high contact forces involved in
the spring contact, as well as the
shape of the springs head and the
contact materials. Good contacts are
Figure 2:
Spring contact
resistance
change of two
pairs of
springs at
4.65 million
cycles, 50µm,
1Hz
Figure 1: Test set-up
for micro-vibration,
the contact resistance
of the system can
be monitored using
four conductor
measurement
defined by showing an increase in
contact resistance of less than 10mΩ
after 100.000 cycles [2].
Temperature shock
Rapid thermal shocks induce stress of
the interface between materials with
different coefficients of expansion. As the
spring pressure connection is not form
locking, thermal movement and potential
wear would be possible. This could lead to
Issue 6 2006
Power Electronics Europe

24 POWER MODULES www.semikron.com
a change of the contact force, orientation
and interface.
To evaluate the development of the
contact resistance the change of the
resistance against the first full cycle is
plotted. The temperature evolution of each
cycle was measured by a soldered
thermocouple attached to the device under
test (DUT).
It was found that some contact
systems were susceptible to degradation
of the contact resistance: As an example
a test system with nickel DBC shows a
rise of the contact resistance due to
oxidation.
Experiments were performed to verify
the beneficial effect of higher currents on
the contact resistance. In literature a
change of the contact resistance is often
attributed to thin surface layers [2]. ‘Drycircuit
conditions’ according to DIN EN
60512-2-1 are limited to a current of up to
100mA and a voltage of up to 20mV to
avoid melting and dielectric breakdown,
respectively.
A test system was prepared with a
material combination that shows a gradual
increase in contact resistance over time.
The current was increased in steps from 1
to 400mA. Figure 3 shows the contact
resistance development. Each step leads to
Figure 3: Test
System:
Influence of
the current
level on the
contact
resistance of
an aged
contact
system using
a copper DBC
Figure 4:
MiniSKiiP II
contact
system -
temperature
shock test
using
an ENIG DBC
at a
permanent
current of
1mA
a significant reduction of the contact
resistance. Testing performed at higher
permanent current levels confirms these
results. Practically no changes in contact
resistance could be found at 6A over 200
cycles. The reliability of springs for load
contacts is thus proven.
Low current levels are typical for sensor
applications. The contact resistance
development of a genuine power module
is displayed in Figure 4. The optimised
material selection leads to a stable contact
resistance. 100 cycles with extreme
temperature swings are considered the
Figure 5:
Temperature
recording using a
soldered
thermocouple
(blue line) and a
temperature
sensor connected
via two springs
(red line)
industrial lifetime requirement. The largest
change in contact resistance across a daisy
chain of eight springs is measured to be
only 100mΩ, even after 200 temperature
cycles.
An extended temperature cycling test
shows the temperature measured via a
temperature sensor connected by springs
and a thermocouple. Figure 5 displays the
temperature measurement for selected
cycles. The temperature evolution of the
thermocouple and the temperature sensor
show a slightly different gradient, due to
the differences in thermal capacity. The
temperature sensor signal was stable for
2000 cycles; for the extreme changes in
temperature this is equivalent to 20 times
the industrial lifetime requirement of a
power module. The soldered connection of
the reference thermocouple failed at 1000
cycles and had to be replaced (see arrow
in Figure 5).
Corrosive atmosphere
Corrosive atmosphere testing
investigates the contact reliability in an
industrial environment. Due to the high
contact forces of the springs, the metallic
contact partners are impervious to outside
contamination. Corrosion products could
not be detected by EDX analysis inside the
contact area. Testing was evaluated by
measuring contact resistance before and
after the test. The change in contact
resistance for various systems was
negligible. No signs of electromigration
could be found in a test with additionally
applied bias.
Intermetallic phases
Tin on copper plating is known to grow
into intermetallic phases with changed
mechanical properties. Those intermetallic
phases can impair soldering due to the
formation of oxide layers that are difficult to
remove with normal fluxes.
The growth of intermetallic phases is
based on a diffusion process, and thus
dependent on temperature. The pressure
Issue 6 2006
Power Electronics Europe