Highly accelerated life test (HALT) and highly accelerated stress screen (HASS) are two processes that very quickly uncover problems in the design and production phases of your products. Both rely on the concept of time compression. That is, HALT and HASS use much higher stresses than exist in the field environment to force failures to occur in significantly less time than would be required in a normal environment.
In HALT, every stimulus of potential value is used during the design phase to find the weak links in the design and fabrication processes of a product. These stimuli may include all-axis impact vibration, broad-range thermal cycling, burn-in, over-voltage, voltage cycling, humidity and whatever else exposes relevant defects in the product.
The stresses are not meant to simulate the field environments but to find the weak links in the design and manufacturing processes using only a few units and in a very short period of time. They are stepped up to well beyond the expected field environments until the fundamental limit of the technology is reached. Reaching the fundamental limit generally requires fixing everything found, even if it is uncovered above the qualification levels.
HASS screens use the highest possible stresses, frequently well beyond the qualification level, to attain time compression in the screens. Many stimuli exhibit an exponential acceleration of flaw precipitation. Using these stimuli results in a much shorter duration of stress, if we use the correct stress, so a drastic reduction in screening equipment and manpower is realized.
The screens must be of acceptable fatigue-damage accumulation, or lifetime degradation, using proof-of-screen techniques where one or more products are screened repeatedly to show that enough product life is left after just a few screens. Generally, HASS is not possible unless a comprehensive HALT has been performed.
Without HALT, fundamental design limitations will tend to restrict the acceptable stress levels in production screens and prevent the large accelerations of flaw precipitation, or time compression, possible with a very robust product. A less-than-robust product probably cannot be effectively screened by the classical screens without substantial reduction in its useful field life since it will have none to spare.
The Phenomena Involved
Several phenomena, such as electromigration, chemical reactions, and mechanical fatigue damage, are involved when screening occurs. Each of these has a different mathematical description and responds to a different stimulus.
Chemical reactions and some migration effects proceed to completion according to the Arrhenius model or some derivative of it. Many misguided screening attempts assume that the Arrhenius Equation always applies; that is, that higher temperatures lead to higher failure rates. But this is just simply not an accurate assumption.1,2
The fatigue damage precipitated by temperature, rate of change of temperature, vibration, or some combination of them can be modeled in many ways, the least complex of which is Miner’s Criterion. This criterion states that fatigue damage is cumulative and nonreversible and accumulates on a simple linear basis: The damage accumulated under each stress condition, taken as a percentage of the total life expended, can be summed over all stress conditions. When the sum reaches unity, the end of fatigue life has arrived, and failure occurs.
The data for percentage of life expended is obtained from S-N (number of cycles to fail vs stress level) diagrams for the material in question. A general relationship based on the Miner’s Criteria is:3
D» ns b
where: D is the fatigue damage accumulated.
n is the number of cycles of stress.
s is the mechanical stress (in pounds per square inch, for example).
b is an exponent derived from the S-N diagram for the material. b ranges from eight to 12 for most materials.
The design or process flaws that will cause field failures usually, if not almost always, prompt a much higher than normal stress to exist at the flaw than at a position without the flaw. This is what causes the early failure.
For illustrative purposes, let’s assume there is a stress twice as high at a particular spot that is flawed due to an inclusion or void in a solder joint. According to the equation with beta assumed to be about 10, the fatigue damage would accumulate about 1,000 times as fast at the position with the flaw than it would at a non-flawed position. This means that we can fatigue and break the flawed area and still leave 99.9% of the life in the non-flawed areas.
Our goal in environmental stress screening is to make the flawed areas of the structure fail. With the proper application of HALT, the design will have several of the required lifetimes built into it and only an inconsequential portion of its life would be removed in a HASS. This would be verified in proof of screen.
The relevant question is “How much life is left after HASS?” not “How much did we remove in HASS?”. All screens remove life from the product. This is a fundamental fact frequently not understood. Properly executed HALT and HASS programs will leave more than enough life left in the product at delivery and will do so at a much reduced total program cost compared to the classical methods.
Flaws of other types have different equations describing the relationship
between stress and the damage accumulation, but all seem to have a very large time- compression factor resulting from the slight increase of the stress. This is precisely why the HALT and HASS techniques work.
Equipment Required
The application of HALT and HASS generally is very much enhanced by—if not impossible without—the use of environmental equipment of the latest design. This includes all-axis impact shakers and very high rate-of-change thermal chambers (60°C/min or more).
We intentionally do fatigue damage as rapidly as possible to reduce the costs. It is not unusual to reduce equipment costs by orders of magnitude by using the correct stresses and accelerated techniques. An example in Reference 4 shows a decrease in cost from $22 million to $50,000 on thermal chambers alone—not counting power requirements, associated vibration equipment, monitoring equipment, and personnel—by simply increasing the rate of change of temperature from 5°C/min to 40°C/min.
The basic data for this comparison is given in Reference 5. Another example in Reference 4 shows that increasing the rms vibration level by a factor of two decreases the vibration system cost from $100 million to $100,000 for the same production output.
The use of an all-axis impact shaker would further reduce the cost ratio. With these examples, HALT and HASS, when combined with modern screening equipment designed specifically to do HALT and HASS, provide quantum leaps in cost effectiveness.
Results
Perhaps the best way to illustrate the acceptance and success of the techniques is to list a few examples. Ron Horrell of the Hamilton Standard Division of United Technologies reported that the facility costs dropped by a factor of 88%, and the time to complete the design dropped by a factor of 98% using HALT.6
Larry Edson of the Cadillac Luxury Car Division of GM reported that high levels of reliability are possible with a few short HALTs. He said that the measurement of this level of reliability is darn near impossible.6 If a component or subassembly lasts many car lifetimes, who cares what the mean time between failure (MTBF) is?
Ed Minor of Boeing found that everything identified in HALT was 100% correlated with documented field failures.6 The Boeing 777 was the first commercial aircraft certified for extended over-water flight in its initial certification.
When considering the level of quality obtained, Kevin Granlund of EMC Corp. stated the cost of such an investment over the life of the product is easily justified. What the design and manufacturing communities learn will be incorporated into the next project, yielding even higher reliability. Certainly, this type of investment provides better results since it is a proactive step to finding problems and fixing them rather than waiting for them to happen and simply counting them.6
Otis Elevator reported significant cost reductions in design and excellent correlation between what was found in HALT and what was found in the field. 6 HALT and HASS are used by Nortel and considered to be “the cornerstones of any successful reliability growth program.”7
At Hewlett-Packard, a problem which took about 10 weeks to uncover using conventional methods was discovered in seven minutes using HALT. Another problem could not be found at all using classical vibration equipment. Using an all-axis impact shaker and the HALT approach, it was unearthed in two minutes. Still another problem took one year to uncover in the field, but it took only one week to unveil using HALT. 8
Summary
HALT has provided substantial (five to 1,000 times) MTBF gains even when used without production screening and has reduced product time to market, warranty expenses, design and sustaining engineering, and total development costs. HALT still is an emerging technology and continues to be improved at an amazing rate.
Today, HALT and HASS are required on an ever-increasing number of commercial and military programs. Many of the leading commercial companies use HALT and HASS techniques with all-axis impact vibration and moderate to ultra-rate thermal systems successfully; however, most are being relatively quiet about it because of the phenomenal improvements in quality and reliability and vast cost savings.
The aerospace industry and the military have been slow to accept the advanced techniques because stressing the product above the expected field environment is quite foreign to them. And even if they wanted to use HALT and HASS, it is very difficult to specify or interpret in contractual language. You must want to obtain top quality for your products to adopt HALT and HASS.
The basic philosophy is: Find the problems however we can and then fix them. This constitutes a paradigm shift.
References
1. Quality and Reliability Engineering International, Vol. 6, No. 4, September-October 1990.
2. Hakim, E.B., “Microelectronic Reliability/Temperature Independence,” Quality and Reliability Engineering, U.S. Army LABCOM, Vol. 7, pp. 215-220.
3. Lambert, R., “Case Histories of Selected Criteria for Random Vibration Screening,” Journal of the Environmental Sciences, IES, Jan./Feb. 1985.
4. HALT and HASS Seminar Notes, Hobbs Engineering.
5. Smithson, S.A., “Effectiveness and Economics-Yardsticks for ESS Decisions,” Proceedings of the IES, 1990.
6. Proceedings of the Accelerated Reliability Technology Symposium, Hobbs Engineering, Sept. 16-20, 1996.
7. Stone, K., Presentation at the QualMark ARTC, Dec. 7, 1993.
8. Anderson, E., Presentation at QualMark, Sept. 1993.
About the Author
Gregg K. Hobbs, Ph.D., P.E., is the president Hobbs Engineering. He has been a consulting engineering since 1978 and the person responsible for coining the terms HALT and HASS. Dr. Hobbs has taught related courses in North America, Europe, and the Middle East and has several patents pending on HALT and HASS equipment. Hobbs Engineering, 10218 Osceola Court, Westminster, CO 80030, (303) 465-5988.
Copyright 1997 Nelson Publishing Inc.
November 1997