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Fundamentals of HALT/HASS Testing

Highly accelerated stress screening and highly accelerated life testing (HASS and HALT) systems require sources, instruments, switching systems, signal routers, plug-in cards, and data-acquisition elements. This equipment must be reliable, accurate, and repeatable. Here's an overview of what's required, and why.

by Jon Semancik,
Digital Multimeter Product Line Manager,
Keithley Instruments, Inc.,
Cleveland, Ohio

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Shortening Time-to-Market HALT and HASS Are Related A Fundamental Assumption Test Equipment Considerations A Switching Power Supply Example The Description Parallel Cycles Synchronization Sources of Error Relay Life Putting It Together Glossary of HALT/HASS Terms A Freebie

Today's switching power supplies, DC-to-DC converters, telecommunication devices, and related assemblies generally require highly accelerated stress screening (HASS) or burn-in testing to make sure they perform reliably.

Since thousands of products may be aged and monitored simultaneously during HASS on the factory floor, the challenge for designers implementing measurement equipment is the large number of test channels needed, and the immense amount of electrical noise that can be produced by switching power supplies.

Shortening Time-to-Market
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But even before HASS testing can take place, similar highly accelerated life testing (HALT) can be performed during product development. The aim is to shorten time-to-market.

Meeting the HALT and HASS challenges requires careful design and operation of test system hardware and software in order to achieve high throughput and accuracy. As always, the size and cost of test equipment are issues that cut across a range of disciplines involving product development and production engineers, QC/reliability engineers and managers, production test engineers, and even technicians.

HALT and HASS Are Related
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When properly implemented, HALT and HASS can quickly uncover problems associated with product design and production. Both rely on techniques that shorten the time required to identify potential causes of failure. This is done by applying much higher stresses than exist in actual product use, which forces failures to occur in significantly less time than under normal conditions.

In HALT, temperature and vibration stress conditions are used during product development to find weak spots, both in the design and in the product's planned fabrication processes. Test stimuli may include humidity, thermal cycling, burn-in, over-voltage, voltage cycling, and anything else that could logically expose defects.

This phase of testing requires only a few units and a short testing period to identify the fundamental limits of the technology being used. Generally, every weak point must be identified and fixed (re-designed) if it doesn't meet the product's specified limits.

In production, HASS uses the highest possible stresses—frequently well beyond the qualification level—to greatly reduce the amount of equipment, time, and manpower required for this type of testing. Appropriate proof-of-screen techniques must be used to protect good product (a proof-of-screen is used to verify that HASS test limits, derived from HALT, will catch production defects without damaging good product). Keep in mind that HASS usually isn't possible unless comprehensive HALT is done earlier, because fundamental design limitations will tend to restrict HASS stress levels.

A Fundamental Assumption
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The fundamental proposition behind HALT/HASS is that if a product and its manufacturing processes are properly designed and verified with HASS, then production operations should turn out reliable units. It follows that units failing prematurely are a result of improper manufacturing practices.

Typically, a population of products will exhibit reliability characterized by a bathtub-shaped curve (see Figure 1) with three distinct failure-rate regions.

The first region is the infant mortality section of the curve, which has a decreasing failure rate and is associated with built-in (i.e., not designed-in) defects. These are the types of defects that can often be identified by HALT testing.

The amount of time required for a given HASS test is determined by the width of the infant mortality region of the reliability curve. In general, the higher the stress, the sooner the failures, the narrower the infant mortality region, and the shorter the HASS test period.

As mentioned, HALT and HASS test profiles include temperature and vibration as well as other stimuli that can reveal a wide range of problems. Let's discuss some common examples and test objectives, starting with temperature profiling.

Temperature profiling can:

• Determine the minimum and maximum product operating temperature limits,
• Accelerate the aging process of the device under test (DUT) by applying extreme temperatures,
• Test the DUT under various climatic and pressure (i.e., altitude) conditions, and
• Burn-in the DUT under elevated temperatures at a high-duration rate.

Next we can discuss humidity profiling and test objectives. Humidity profiling can:

• Determine the effects of high/low humidity on the DUT (high humidity can cause corrosion; low humidity can lead to electrostatic discharge, or ESD, damage),
• Find latent shorts on printed circuit boards (PCBs) that are related to humidity (typically due to ion migration),
• Accelerate the aging process of the DUT by applying extreme humidity conditions, and
• Test the DUT under global humidity and pressure (i.e., altitude) conditions.

Vibration profiling can also be done. Vibration profiling can:

• Determine the maximum product operating vibration levels,
• Find mechanical defects early in the work-in-process cycle, and
• Simulate global transportation conditions on the DUT.

The parameters measured with any of these HALT/HASS profiles can be any number of signals that indicate whether or not the product is functioning properly. The measured signals may range from DC values to high-frequency RF variables. In some cases, a stimulus must be applied to the DUT and its response measured.

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