Minimizing Power Supply Disturbances During Semiconductor Burn-In
To ensure the integrity of the burn-in process for semiconductor devices, test parameters must be well-regulated and devoid of transients. The DC voltage supplied to the DUT is one essential parameter that deserves careful consideration.
Fine geometries require lower supply voltages, and the devices are more susceptible to transient deviations. Often the results of burn-in tests are used to assess the quality of an entire lot of devices. Erroneous results, due to transient voltage deviations, may cause increased costs, loss of time and effort, and lot rejections.
Three power supply-related parameters are significant to successful burn-in: AC line transient insertion loss, power supply noise, and control-loop interactions with the DUT. Each condition plays an important role in verifying product yield and reliability.
AC Line Transient Insertion Loss
A burn-in power supply shares the AC lines with many noise- producing devices, such as switching supplies, copiers, fluorescent lights, motors, relays and thyristor-controlled ovens. Large capacitive loads can produce line-voltage dips when energized, while large inductive loads can produce voltage spikes when interrupted. Also, there is the unpredictable phenomenon of lightning, which will produce transients in the kilovolt range at the input to a facility.
Line transients will appear at the input to the power supply both in normal (transverse) and common-mode, depending on the nature of the source. Voltage dips due to switched capacitive loads or motor start-ups are usually dominated by normal-mode effects.
Line transient overshoots due to switched inductive loads and lightning strikes produce both normal and common-mode effects. Simulation of these line conditions is possible in a laboratory environment through the use of an appropriate transient generator.
prEN 50093 (an EC standard) specifies that an instrument shall perform transparently under the influence of a 30% line decrease lasting 10 ms. IEC 801-5 calls for the absence of a state change of an instrument following the application of a 1.2- us x 50-us x 1-kV transient applied normal-mode to the power input and 1.2 us x 50 us x 2 kV applied common-mode.
Fast transients are defined by IEC 801-4 to be applied to the power input of an instrument in common-mode fashion only, with the pulse geometry 5 ns x 50 ns x 1 kV. For the designer of the burn-in system, the question might extend beyond one of regulatory compliance to one of power-supply response (overshoot/undershoot) to line surges, sags or spikes so that semiconductor exposure to voltage stress is minimized.
If the power supply allows a portion of these transients to pass through to its output, the DUTs may be damaged (Figure 1c). Phase-controlled power supplies have the tendency to magnify the energy of an applied pulse due to false triggering of the control thyristor. On the other hand, properly designed switching power supplies attenuate this energy due to their large energy storage capacitors which provide high-frequency isolation of the switching devices from the power line.
Some power-supply manufacturers can provide data demonstrating the performance of their products under the influence of these various line sags and voltage overshoots in terms of output disturbance. This data can be useful to determine whether an isolation transformer or power conditioner is required to properly decouple the semiconductors from the effects of power-line noise or to determine whether a higher-performance power supply should be used.
Power Supply Noise Parameters
The sum of noise voltages on the output of a power supply is described as a normal-mode output parameter and is often specified both in terms of its rms and peak-to-peak values. Since many frequencies will be present, typically the bandwidth specification extends to 20 MHz.
Power supplies can produce noise voltages containing these components:
(o) Line frequency and related harmonics.
(o) Switching frequency noise and related harmonics (Figure 1d).
(o) Common-mode switching transistor dv/dt feed-through.
(o) Line dip or surge feed-through (Figure 1b).
The industry de-facto standard for measurement bandwidth is 20 Hz to 20 MHz. Some vendors choose not to specify this parameter, which renders their noise specification ambiguous.
The lower-corner frequency is particularly important regarding the characterization of phase control supplies, since the supplies have the tendency to feed through line dips. The upper-corner frequency is critical in the characterization of switchers, since the switchers tend to use relatively high dv/dt in their power conversion process.
Another noise parameter which is rarely specified is common- mode output noise current. This is the current produced between either output bus bar with respect to the power-supply chassis. This current inevitably appears in the system ground conductor, generating noise voltages and degrading system performance (Figure 1d). Some switching supplies produce upward of 500 mA peak-to-peak, causing considerable voltage conversion at the DUT.
Control Loop Interactions With the DUT
The feedback loops in power supplies have a limited bandwidth. Consequently, abrupt changes in load current produce transient changes in the output voltage. The magnitude of this change as well as the time duration are essential elements of a transient response specification.
Generally, only constant voltage load effect transient recovery time is specified in an attempt to describe loop behavior. A complete specification should include both the value of load current change and a voltage settling band. Otherwise, the specification is almost meaningless.
In a burn-in system, the most obvious dynamic condition is the load-current change. This occurs due to shifts in operating conditions on the semiconductor being burned in, for example, system reset Figure 1e or an operator removing a tray of semiconductors.
The instantaneous transient voltage is merely the product of the load current change times the equivalent series resistance of the system power supply’s output capacitor (Figure 1f and 1g). The recovery time is related to the power supply’s control-loop bandwidth.
The voltage overshoot is the cause of damage to the DUTs. Phase control-regulated supplies typically require around 50 ms to recover to within several hundred millivolts of the nominal output voltage, whereas switching regulated supplies typically require around 1 ms for the same recovery when subjected to a 50% load change.
Last, poorly designed control loops can produce voltage transients during up-programming, turn-on, turn-off, overvoltage reset, and mode crossover from constant current to constant voltage following a bus fault as shown in Figure 1a.
Summary
Although there can be many sources of transient voltage in a burn-in system, the DC power supply is the key isolation link among the various sources and the semiconductors being burned in. Some of the most vital specifications required to determine system performance will not be found in vendors’ data sheets. Consequently, it is up to you to get more information or perform some characterization of the system to determine the suitability of a given power supply for a particular voltage.
March 1995