Keep the ink flowing
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/1/2006
Richard Clark is a product engineer at Hewlett-Packard’s Corvallis, OR, facility. He evaluates and characterizes ICs used in the print heads of inkjet printers. He performs most of his testing at the wafer level, controlled by drivers. In a recent interview, Clark described the measurements he performs using numerous test instruments. He also explained how he automates his tests and how he writes instrument drivers.
Q: What functions do the ICs perform?
A: The ICs are the electrical interface between the printer and the nozzles that eject the ink. They receive digital commands and control the firing of the ink nozzles.
Q: What measurements do you perform on an IC?
A: Starting with schematics and circuit descriptions as test guides, I evaluate the performance of analog components within the IC, including DACs, ADCs, current sources, amplifiers, voltage references, and resistances. The rack includes an LCR meter, a source-measure unit, an oscilloscope, and a VXI chassis that contains a DMM, a digital pattern generator, and a switch matrix. I use the pattern generator to simulate digital commands from the printer. The oscilloscope lets me check output waveforms that drive the print nozzles. I perform the analog and parametric tests with the LCR meter, DMM, and source-measure unit. The wafer under test sits atop a thermal wafer chuck.
Q: How do you automate your tests?
A: Because we use HP Unix workstations to control the instruments, I often have to write custom drivers in C for each instrument. Rather than use the drivers that come with instruments, I write drivers that perform specific measurements. A driver contains only the code needed to set up an instrument and make the measurements I need. Then, I use Vee version 4 to produce test sequences.
Q: How do you make use of the drivers?
A: The drivers let me change parameters such as voltage, frequency, or temperature before a series of measurements so I can produce plots of device performance. Once I import the driver into Vee, I can see the function and the input variable names. Often, that’s enough information for me to understand what the function does. Source code comments provide additional information.
Q: How has automation improved the way you perform evaluations?
A: It doesn’t pay to perform a manual test on a device. Because I have a library of test functions already written, I can run tests faster and easier using automation, particularly because each test may require repeated measurements using different parameters.
Q: What else have you done to streamline the tests?
A: I’ve developed interface boards that let anyone quickly connect instrument probes and cables to the wafer prober. The board becomes a “mini test head” that contains connectors, switches, and relays that minimize setup time.
Q: What do you do with the test results?
A: I share test results with designers so we can compare measured results against simulated results. Often, we find differences in how a device will work when integrated into a printer. A test is, in effect, an experiment.
Q: Why does a designer need to run tests?
A: Designers often place circuits on test wafers and then test the circuit using the automated test station. A designer can often find the driver’s measurement functions he or she needs to run a test. If I can write a driver that someone else can use, I’ve improved productivity.
