E NVIRONMENTAL S TRESS S CREENING
6.2. Environmental Stress Screening: an overview overview
The process for detecting flaws (i.e. imperfections that could result in failures) by applying environmental and/or operational stresses to precipitate them as detectable failures is usually called Environmental Stress Screening (ESS) [191]
or Reliability Stress Screening (RSS) [192].
ESS is the tailored application of electrical and environmental stresses to electronic parts, module, units and systems to identify and eliminate defective, abnormal or marginal parts and manufacturing defects. ESS is composed by a process or a series of process in which environmental stimuli, such as rapid thermal cycling and random vibration, are applied to the device under test.
The processes of ESS or RSS are used to detect flaws in a population of items, usually components, leading to the subsequent removal of these flawed items from the population. The removal of such components facilitates rapid achievement of the reliability level expected for the population over the useful life.
ESS is a common and widely-conducted practice used to eliminating latent defects due to infant mortalities in electronic equipment (see Fig. 6.1) [148], [191], [193]–[196].
Fig. 6.1 illustrates the classical trend of a component failure rate (usually called the bathtub curve) which is divided into three sections [131], [183]:
• Early failure or infant mortality: in this phase failure are caused by intrinsic material defects, design or assembly mistakes and so on.
• Random failure: this section is also called useful life, and the failure rate trend is approximately constant.
• Wear-out failure: in this phase, the failure rate increases due to fatigue and material deterioration.
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Fig. 6.1. Effects of environmental stress screening on the bathtub curve.
Electronic components are adequately described using a constant failure rate model (i.e., exponential failure distribution). This is due to the fact that such type of items is characterized by a long-time useful life, which is predominant respect to the other two sections. This is a basic assumption for many reliability approaches existing in literature, like reliability prediction (see for instance [146], [147], [156]), Reliability Block Diagram (RBD) [197], reliability allocation (as detailled described in section 5), reliability importance, and so on.
As a matter of fact, fatigue is not a real failure mode for electronic components because wear-out will occur when the item will be already obsolete, so the last zone of the bathtub curve could be easily neglected for this type of device.
Quite the opposite, the infant mortality section could not absolutely be neglected because intrinsic defects due to the manufacturing process are very common, especially in low-cost commercial components. Therefore, environmental stress screening must be used to evaluate design and materials problems and consequently decrease the failure rate in the first zone of the bathtub curve (see the green trend in Fig. 6.1).
ESS is typically conducted on 100% of manufactured products to accelerate early failures in the most cost-effective solution. It can identify failure modes that usually are not discovered through simple inspection or testing, such as:
• Parameter drifts;
• Shorts and open on the electronic board;
• Incorrect installation;
• Wrong part installation;
• Contaminated part;
Time [h]
Failure rate [failure/h]
Bathtub curve Effects of screening
RANDOM FAILURE WEAR OUT
FAILURE ESS
EARLY FAILURE ("INFANT MORTALITY")
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• Hermetic seal failure;
• Foreign material contamination;
• Cold solder joints;
• Defective parts.
The screening level should not exceed the design limits, but they must be of sufficient strength to precipitate failures due to weak parts and manufacturing defects at the earliest time such that corrections are most cost-effective.
Considering the entire population of generic manufactured products just came out of the production process. The robustness of these population is usually distributed according to a bimodal normal probability density function. The highest peak stands for the strong population (i.e. subset of the total population of items made up of non-weak items), the items that belongs to this group generally fail because of random failures or wear out failures. The lowest peak represents the intrinsically weak population (i.e. subset of the total population of items made up of only weak items), that covers the first zone of the bathtub curve (early failures). Effective screening requires stresses of sufficient magnitude and time duration to precipitate failures from latent defects without accumulating significant damage to the remaining non-defective structural elements (see Fig. 6.2).
Fig. 6.2. Comparison between population robustness and ESS strength.
There are a number of common types of screening procedures which could be used with some degree of success. These common screening procedures (stresses) are shown in
TABLE VI.I in compliance with [192].
X
f(X)
Population Stress Screening Intrinsically weak
population
Strong population
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TABLE VI.I
COMMON SCREENING TYPES AND TYPICAL DEFECT TYPES PRECIPITATED BY ESS.
STRESS DEFECT TYPES PRECIPITATED
Thermal cycling
Component parameter drift Hermetic seal failure
Poor thermal coefficient matches Stress relaxation
Loosening of connections or parts Cracks
Vibration
Particle contamination Defective oscillator crystals Poorly bonded internal parts Poorly secured high-mass parts Mechanical flaw
Loosening of connections or parts Part mounting issues
Combined thermal cycling and vibration
All mechanisms under vibration and thermal cycling Interaction between mechanisms
High voltage Shorted connections
Humidity
Sealing properties
Hygroscopic contamination Circuit stability
Corrosion
High temperature Performance degradation Chemical reaction Acceleration Cracks
Mechanical defects
Gas pressure test Leaks and hermetic failure Power cycling In-rush current response
Circuit transients
147 There are a number of types of ESS: constant stress screening, step stress screening, and Highly Accelerated Stress Screening (HASS).
More in detail, a constant stress screening is a screening procedure where a constant environmental and/or operational stress is used for the duration of the process.
A step stress screening is a screening procedure where environmental and/or operational stresses are changed at planned intervals, usually increasing in strength for the duration of the process. Step stress screening is often used to shorten process times, and to give some idea of likely failures rates at different stress levels.
Highly Accelerated Stress Screening (HASS), is intended to be an on-going process either performed on the whole production (100 % screening) or on a sample from the production or from a batch. The HASS process is typically set up as a rapid temperature change between the upper operating limit reduced by some amount and the lower temperature limit plus the same amount.
If no operating limits have been identified, a level as high as appropriate for the item’s technology is chosen. Normally the screening strength of the HASS screening is adjusted by increasing or decreasing the number of temperature cycles. HASS normally stays within the items' operational limits to allow continuous monitoring of the function of the item, but operational limits can be exceeded where the items under HASS are not monitored during the screening. However, it is important that the stress levels remain below the destruction limit for good items. The items should then be tested for function after the HASS.
Generally, the purpose of all of these screening types is to cause relevant failures to occur in the item. Such relevant failures are those that would have prevented the item from achieving its reliability requirements in service.