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How to characterise mixed-signal IC for production

06 Mar 2013  | Robert Seitz

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Every device, meanwhile, has a specified drift over temperature, which may be a typical or a guaranteed minimum/ maximum specification. Temperature guard bands have tighter test limits than the IC's data-sheet specifications and need to be calculated based on the drift of the measurement over temperature. The advantage of using temperature guard bands is that you can skip test stages at other temperatures and instead calculate, based on the test results seen at room temperature, whether a device would fail at temperature extremes.

Figure 2 shows a temperature-characterisation report with guard bands included. The plots demonstrate that there is drift over temperature. From this data, you must be able to predict that when testing production parts at +25°C, the drift at the temperature extremes will be within specification limits. That is the point of temperature guard bands for production testing.

Figure 2: This sample temperature-characterisation report includes guard bands. The Y axis represents the number of parts; the X axis shows the measured value. Characterisation is shown for 100 parts tested at +25°C (a), 1072 parts tested at +25°C (b), 1073 parts tested at +125°C (c), and 462 parts tested at −40°C (d).

Silver samples are often used to show the stability of the test solution over temperature. They also can be stored for use as reference parts for later verification of the test solution. The testing procedure for these devices is to test three parts at three temperatures, hundreds of times each.

Using the gathered data, you can prove the stability of every device with the help of a statistical report tool. For example, a double distribution or instability can be seen immediately. You can keep the test results for later reference, comparing future measurements with the stored data.

Assume the device to be tested has the following operating conditions: a minimum operating temperature of −40°C, a typical operating temperature of +25°C, and a maximum operating temperature of +125°C. The black bar in the graph in figure 3 is the value at room temperature. The green bar shows the value at high temperature (+125°C); the blue bar shows the value at cold temperature (−40°C). The data clearly shows an increase and decrease of the value at high and low temperatures, respectively.

Figure 3: This silver-sample report shows the drift between temperatures. The data clearly shows an increase and decrease of the value at high and low temperatures, respectively.

GR&R for plastic parts
In addition to testing at the wafer level, engineers must test packaged parts to determine that no damage has occurred during the packaging process. To verify the repeatability and reproducibility on different testers, test a minimum of 50 plastic packaged parts on one tester twice in the same order, then repeat the procedure on another tester and compare the test results obtained using the different testers. An optimal result would be a 100% overlay of both sets of data. If you discover that the results are not closely matched, you must find the root cause for the discrepancy.

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