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.
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’ 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).
Figure 1. Test set-up for micro-vibration - the contact resistance of the system can be monitored using four conductor measurement
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.
Figure 2. Spring Contact Resistance Change of Two Pairs of springs at 4.65 million cycles, 50 μm 1 Hz
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 defined by showing an increase in contact resistance of less than 10mΩ after 100.000 cycles .
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 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 . ‘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.
Figure 3. Test System: Influence of the current level on the contact resistance of an aged Contact system using a Copper DBC
Each step leads to 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.
Figure 4. Contact System - Temperature Shock Test using an ENIG DBC at a permant current of 1mA
The optimised material selection leads to a stable contact resistance. 100 cycles with extreme temperature swings are considered the 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.
Figure 5. Temperature recording using a soldered thermocouple (Blue Line) and a temperature Sensor connected via two springs (red line)
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 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.
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 contact system used in power modules has the storage time of PCB boards, however thus the growth of intermetallic phases have to be considered.
Figure 6. Test system - first temperature cycle of corroded intermetallic phases as PCB surface after assembly for a series of 8 springs plus electric path
To validate the reliability of a contact after long periods of time an extreme aging of a contact system was tested using the following parameters: Storage of an immersion tin PCB and a tin lead hot air levelling PCB at 150°C for 90 hours (unpopulated test of PCBs); storage in corrosive atmosphere (0.4 ppm H2S, 0.4 ppm SO2, 0.5 ppm NOx, 0.1 ppm Cl2, 25°C, 75% RH, 21 days); and temperature cycling with a permanent current load of 1mA.
The first step ensures that intermetallic Cu6 Sn5 - phases have grown completely through the metallisation layer. the second step exposes the open intermetallic phases to an extremely aggressive mix of corrosive gases. Then this extremely aged PCB was assembled to a new MiniSKiiP module. Temperature cycling with permanent voltage drop monitoring was performed on the assembly. The contact resistance drops quickly to a stable value , once different coefficients of thermal expansions cause some micro-movement of the contact partners.
The resistance increases during continous cycling . Not suprisingly, end of life is reached more quickly on a pre-aged and corroded PCB than on a new system, yet the pressure contact was still functional while a soldering attempt failed after the corrosive atmosphere test.
Extensive testing has proven the reliability of the spring pressure contact system in a variety of conditions associated with an industrial environment while retaining the ease of assembly. This is achieved for both high current load contacts and low current sensor contacts.
 P. van Dijk, M. van Meijl: Contact Problems due to Fretting and their Solution, AMP Journal of Technology, Vol. 5, June 1996, Page 14ff.
 P. Slade: Electrical Contacts - Principles and Applications, Marcel Dekker, New York, 1999
 Anders Electronic: Oberflächen [Online], http://www.andus.de/leiterplatten/Oberflaechen/oberflaechen.htm [21/03/2006].
 E. Hornung, U. Scheuermann: Reliability of low current electrical spring contacts in power modules, microelectronics Reliability of Low Current Spring Contacts for Lead-Free Applications, Power Electronics Europe 4/2004, pp. 34 - 36.
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