The use of bypass capacitors on test fixtures often is overlooked as well as misunderstood. Time and again we find a wide variety of unusual symptoms appearing in test environments that are found to be attributible to Vdd fluctuations caused by unsupressed transients, rather than by some fault of the tester. Let's look at how these transients are caused and how they affect us even at slow test speeds.

1 MHz Scenario
In a real-world application, many events can take place in a single 1MHz cycle of 1000ns. Since different pins of the chip being tested require different applied timing, subsequent transients can occur at corresponding places on the 1000ns "time-line". In the list below of critical device pins, we show "where" in time these pins experience edge transitions, either as stimulus from the tester (as in the case of the Clock or RESET) or as output states from the chip (Q Data, for example).

Given this fairly typical timing, the following graphical representation can be drawn showing the timing relationships during a reset of the device.

Vdd Transients induced by device switching

It can be seen in the drawing that although the test rate of this device is 1MHz, frequencies as high as 100MHz might appear on the power pins due to the compounded effects of node switching. Of significant interest is the impact on Vdd of many pins switching state simultaneously, as is the case with the 32-pin address and data busses which typically goto logic zero at reset.

One user describes the effect and cure:
"Without a capacitor, there is a 'long' path back to the Power Supply and there is a voltage drop across the power line. Also sense lines can be 'slow' to react, compounding the problem. Caps create a local source of current that can be dumped at a high rate of speed directly to the problem. The cap speed depends on the ESR (Effective Series Resistance - effectively, a resistor in series with the capictor). ESR values depend on the size and type of cap. This is why there are often multiple sizes and types of capacitors used together as one."

How Many? Where?
What we're addressing is an issue of power distribution. As designers and test engineers can tell you, the number of ground pins needed on a chip has a direct correlation to the number of pins that might possibly change state simultaneously. If it is not possible to attach capacitors to all power pins, place them on (at least) the power pins sourcing those I/O pins that are most likely to change state/direction in unison. Such pins might be data or address busses that "zero out" during a reset or an address rollover. Making this determination may require intimate knowledge of the chip's substrate structure.

Bypass Capacitors
You will need .001, .01 and .1 uf bypass capacitors positioned right at the power pins of the DUT chip socket for a local reservoir of power. Keep the leads as short as possible, and No Electrolytics! The farther your DUT is from the capacitors, the less effective the capacitors are. Electrolytics can store power long after the test is completed, causing problems during subsequent continuity tests or when the test is a Production situation; the device may still be "powered" when the handler steps to the next chip. The different capacitor values and types (mylar, ceramic, etc.) serve to provide appropriate characteistics depending on the transient frequencies involved. As we've already seen, many frequencies may be present regardless of the test rate used.

'Scope it out
To view your Vdd on an oscilloscope follow these steps:

• Define a new group in the Pin Setup window as "Trig" for the scope trigger. Put an unused tester channel in this group and call it "trig", display in binary. Set direction to DUT Input, NRZ data format, Leading edge 0ns.
• Select a scope channel other than channel one as the trigger source, and connect the probe for that channel to the "trig" tester channel on the DUT board. Set the trigger for a positive (rising) edge trigger. Connect channel one of the scope to a Vdd+ pin on the chip, close to the body.
• Edit the data in the Vectors window. Fill the "trig" pin with logic 0 for the entire vector set by pulling down the "Vectors" drawer at the top of the screen. Locate the desired vector at which the greatest number of node state changes is taking place, such as the RESET in our example. Place a logic 1 on the "trig" pin at this vector.
• Prepare to execute by setting the Stop Condition in the Run Setup window to "NONE". All of the vectors will be executed up to the Last Vector defined on this window. If using the Programmed mode, you must put a JMP at the Last Vector returning to the start of the vectors.
• Power should be defined in the DUT Power window before running. When RUN button is clicked, the tester will cycle endlessly through the vectors until the stop sign is clicked. As it runs, the scope can be fine tuned to display the Vdd condition at the selected vector.

Conclusion
If you've done without capacitors in the past, reconsider your practices. Odd behavior you may recall from earlier test efforts may lead you to suspect poor power management that could have been avoided with better fixturing discipline.

Related info

Q'nApp #E9: Iddq Testing
Q'nApp #E20: Power and Ground
Q'nApp #E34: DUT board compatibility

Qe22.zip is a zipped Word file of this Q'nApp.

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