Keep the pressure on
An exclusive interview with a test engineer
By Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/1/2010 12:00:00 AM
Doug Kmiec is the director of test and package engineering at Silicon Microstructures, a manufacturer of MEMS pressure sensors. The sensors measure absolute, differential, and gauge pressure from 0.15 psi to 100 psi, producing analog and digital outputs that are proportional to pressure. Kmiec is currently developing automated test stations that will eliminate some manual test procedures, increase throughput, and provide easier upgrade paths than the current test stations. Senior technical editor Martin Rowe met with Kmiec at his office in Milpitas, CA.
Q: How do you test a pressure sensor?
A: A sensor’s circuitry is essentially a piezo-resistor Wheatstone bridge built on a flexible membrane. We calibrate the devices by applying known pressures and temperatures to the part and trimming external resistors. Some of our parts use ASICs to get the desired output instead of trim resistors. We calibrate these parts to establish the sensor’s coefficients. The ASIC stores and compensates the sensor’s output.
Q: Do you test your parts outside published specifications to provide design margins?
A: We typically characterize parts to meet customer requirements, but we always look for ways to enhance production test. Our new test platform will let us test a device at pressures that exceed published specifications. We will test a 100-psi part at 225 psi to ensure the die and the package integrity.
Q: Why did you decide to develop a new test platform?
A: Cost, quality, and data reporting were the reasons. Stabilizing the pressure and temperature of the parts takes time. The new system will stabilize parts more quickly, and it will have shorter load and unload times. The new platform also will improve consistency of test data. It will consolidate and store data in a data warehouse. We will displace several hardware and software platforms giving Silicon Microstructures one system for hardware upgrades, maintenance, software language, and algorithms. We’ll use the new system for production, R&D, engineering, and failure analysis. With a single test platform, performing tester measurement subsystem calibrations will require fewer spare subsystems when the subsystems go out for calibration validation.
Q: How will the new tester work?
A: The new tester tests up to 32 sensors at once. It has four VTI Instruments chassis, each containing a DMM (digital multimeter) that measures sensor output voltages. Multiplexers route the signals from the DUTs (devices under test) to the DMMs. Using multiplexers eases tester conversion. Power supplies excite the sensors with either constant voltage or constant current.
There are four gas flowmeters that measure gas flow during test. If flow is detected, it is due to an unseated or leaking part. Leaks cause a pressure gradient that diminishes the results of nearby parts. This flow measurement allows valves to be closed on bad sockets but continues the test on other parts. Ethernet provides communication to temperature, pressure, electronics, and transport equipment.
Q: Why did you choose Ethernet?
A: Ethernet is a standard that’s available on all PCs. It’s independent of vendor and hardware generation. Each PC has two Ethernet connections: one without a firewall for tester component communications and another with a firewall for external connections. With our short measurement time and long stabilization time, Ethernet overhead isn’t an issue. Utilizing Ethernet may eliminate hardware dependences, latent software bugs, and obsolescence issues in the future.
Every other month, we publish an interview with an electronics engineer who has test, measurement, or inspection responsibilities. If you would like to participate in a future column, contact Martin Rowe at mrowe@tmworld.com. To read past "Test Voices" columns, go to www.tmworld.com/testvoices.
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