Ironwood BGA elastomer contact DC Measurement Results prepared by Gert Hohenwarter 11/7/2014 GateWave Northern, Inc. 1 Table of Contents TABLE OF CONTENTS....................................................................................................................................................... 2 OBJECTIVE..................................................................................................................................................................... 3 METHODOLOGY.............................................................................................................................................................. 3 Setup........................................................................................................................................................................... 4 Test procedures........................................................................................................................................................... 6 Current carrying capability........................................................................................................................................ 7 Pulse testing............................................................................................................................................................. 10 GateWave Northern, Inc. 2 Objective The objective of these measurements is to determine the DC performance of an Ironwood BGA elastomer contact. Measurements are to determine current carrying ability. Methodology A four terminal (Kelvin) measurement setup is used that includes a computer controlled voltage source capable of delivering 10 A. The voltage developed across the contact is measured with a HP 3456A DMM and yields a V-I record. A 4 terminal setup (Kelvin measurement) setup is used and the DMM is operated in compensated mode to remove the effects of thermo-electric voltages due to dissimilar metals. For the current handling tests the temperature rise in the center of the pin is measured with a 0.003” diameter thermocouple as drive current levels are gradually increased. GateWave Northern, Inc. 3 Setup The BGA elastomer contact is installed in a small block which is mounted on an Au covered brass base plate (see Fig.1 and 2). Figure 1 Test setup The current/voltage probe consists of a copper post with suitably shaped surface. This surface is Ni and Au plated. The post has two connections, thus allowing for a four terminal measurement with very low residual resistance (about 1 milliOhm). It should be kept in mind that in this setup the spring probe presses against two surfaces that are very well heat sunk. GateWave Northern, Inc. 4 Figure 2 BGA elastomer contact mounting plate example Au over Ni plating was applied to all metal surfaces. Material type and thickness specifications were identical to those used for PCBs. The DUT with its plate is mounted in a test stand with XYZ adjustment capability: Figure 3 Test stand This setup has a micrometer screw that allows repeatable adjustments in the Z direction. Also included is a transducer that converts Z position to an electrical signal for the data acquisition. GateWave Northern, Inc. 5 Test procedures During I-V testing, the z value is adjusted to nominal operating position and drive current is increased in steps of 0.05 A up to the maximum tolerable level. The dwell time for each current step is 1 s for V/I curves. Once the data are available, they are processed to reveal the resistance, power dissipation and temperature as a function of drive current. Pulse load testing is performed by providing a current pulse of 0.3 seconds length followed by a pause long enough to facilitate 10% and 1% duty cycles. Current levels are ramped to the maximum value and temperature rise determined is determined at each step. The current handling capability is then determined for the allowable temperature rise. The thermal response time constant is a result of determining both rise and fall-times and averaging the two values. GateWave Northern, Inc. 6 Measurements Current carrying capability The measured current–voltage relationship of an Ironwood BGA elastomer contact is recorded for gradually increasing drive current: V and R as a function of drive current I 100 V[mV] / R [mOhms] 90 V R 80 70 60 50 40 30 20 10 0 0 2 4 6 I [A] 8 GWN 404 Figure 4 Voltage and resistance as a function of drive current There are no aberrations in the response. Of interest is also the power dissipation in the contact: GateWave Northern, Inc. 7 P as a function of drive current I 0.6 0.5 P [W] 0.4 0.3 0.2 0.1 0 0 2 4 6 I [A] 8 GWN 404 Figure 5 Power dissipation as a function of drive current dP/dI as a function of drive current I 0.35 0.30 dP/dΙ [W/A] 0.25 0.20 0.15 0.10 0.05 0.00 0 2 4 6 I [A] 8 GWN 405 Figure 6 Derivative power dissipation as a function of drive current Power dissipation follows a square law up to a current value of 5 A. GateWave Northern, Inc. 8 ∆T as a function of drive current I 30 ∆T [deg C] 25 20 15 10 5 0 0 2 4 6 I [A] 8 GWN 405 Figure 7 Temperature rise as a function of drive current The temperature rise above ambient temperature increases as drive currents increase. At 5.4 A that value has reached 20 degrees C. d(∆T)/dI as a function of drive current I 18.0 16.0 d(∆Τ)/dΙ [deg C/A] 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 0 2 4 6 I [A] 8 GWN 405 Figure 8 Derivative temperature rise as a function of drive current GateWave Northern, Inc. 9 Pulse testing During pulse testing current is turned on for 0.3 seconds and then set to zero for the remainder of the cycle time (3 seconds for 10% and 30 seconds for 1%). As peak current increases so does the temperature rise. Because of the thermal response time of the contact, it does not, however, reach the full temperature value as in DC testing. Hence the current carrying capability is higher under pulse conditions. A graph shows temperature rise as a function of drive and duty cycle: dT (I) 50 DC 50% 10% 1% 25% 45 40 dT [C] 35 30 25 20 15 10 5 0 0 2 4 6 I [A] 8 GWN 0111 Figure 9 Temperature rise as a function of drive current* The difference between all curves is small since contact temperature between pulses drops to almost ambient between pulses. Cause for this is the relatively short contact length and low thermal mass together with heat sinking of the contact at either end. It results in a 120 msec thermal time constant, which is short compared to the 3 second cycle time for the 10% case and also short compared to the pulse length of 300 ms. Thus all curves follow essentially the same trajectory. The resulting maximum currents are as follows: GateWave Northern, Inc. 10