From design to production
An exclusive interview with a test engineer
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 7/1/2008
Stewart Wilde is a test-development engineer at International Rectifier. Working in the company’s Durham, NC, design center, Wilde is responsible for test programming and design verification for packaged ICs used for power-monitoring and pulse-width-modulation (PWM) applications. Senior technical editor Martin Rowe spoke with Wilde by phone.
Q: When do you get involved in a new product design?
A: I start just after a part has a product objective data sheet, which is a list of specifications that a designer wants to achieve. I begin to formulate a test plan based on specifications for each IC pin. I also take part in design reviews, where I can suggest design-for-test improvements.
Q: What’s involved in developing a test?
A: The test plan spreadsheet contains the parameters for each test. We develop tests for Credence ASL series ATE systems, using the Credence test executive to write test sequences that call out test code. The test code makes calls to test functions provided by the ATE manufacturer. They include force, measure, arm, and trigger.
Our code also maps these functions to the appropriate device pins. We use Microsoft Visual C++ to write the code needed to test each IC pin and debug on one of two ASL systems in the lab. A test plan and code takes several months to develop.
Q: What tests do you perform in design verification?
A: Tests include measuring current consumption, input bias current, reference voltages, and PWM duty cycle. We also measure RDS(on) for field-effect transistor (FET) devices. We measure these parameters over a specified temperature range, looking for unacceptable drift. To perform voltage and current measurements, we use the ATE system’s voltage and current sources. Test parameters range from 20 V at 30 mA to 100 V at 100 A.
The tests are designed to help us find inconsistencies that a design engineer can’t predict during design simulation. Sometimes, we run tests that the designer didn’t request but we know are necessary based on prior experience.
Q: How many devices do you test in a design validation?
A: We typically test three lots, each with at least 30 devices. The lots are produced at the same factory, but at different times. We look for variations that could be related to the manufacturing process. We also look for measurement variations from devices tested several times with the same test fixture. Then, we change the load board and analyze measurement variations because component tolerances and calibration between the test systems can affect measurements. If we find inconsistent test results, then we may have to write new code. We must make the test results repeatable in production. Furthermore, we’ve found timing variations between the ASL 1000 and ASL 3000 ATE systems, so we have to compensate for that.
Q: Do you reuse code written for design verification in production?
A: Yes, we reuse most of the test code for production because the production line uses the same ATE systems that we use in the design center.
