The Objective Of Environmental Stress Screening & Application Of Ess

5.2 The Objective of ESS:

Environmental testing is one of the ways to meet the reliability requirements. Products should not only be good out of the box, but that they also perform their intended functions throughout their expected lifetimes. ESS can be defined as a process or a series of processes in which environmental stimuli, such as rapid temperature cycling and random vibration are applied to electronic items in order to precipitate latent defects to early failures. In other words, ESS is a vehicle by which latent defects of the products are converted into detectable failures.

ESS is designed to precipitate defects introduced into the product by faulty components and the manufacturing process. These defects are often called latent defects, because they are not apparent unless some type of stress is applied to the product. Because ESS is inspection step, it is not a value added process, and should be eliminated as soon as possible. It must be noted that ESS is not a simulation of the product’s “mission environment” and has no relationship to the end use of the product. ESS is designed to apply appropriate stimulation (thermal, vibration, etc.) of such a magnitude which will cause defective parts and workmanship errors to precipitate. It is also important that the applied stimulation not approach the mechanical, electrical, or thermal stress limits of any component to avoid accelerating the fatigue and/or causing damage. Each screening profile must be tailored (modified) for each module, unit, or assembly undergoing ESS. ESS is capable of producing valuable data for product improvement.

The underlying principle of ESS is that there exist sub populations of latent defects, which, if not detected prior to shipment, would cause premature failures in service. These defects are responsible for the high initial failure rate of the bathtub curve, and if the ESS process is effective, they are precipitated to a large extent and the failure rate vs. time curve is flat when the product is placed in service. Therefore, ESS can be used effectively only when the failure rate Vs time curve has an initially decreasing slope. If this is not the case, there are no infant mortality defects, and any attempt at ESS will merely consume useful life without decreasing failure rate in the field. Some examples of latent defects that can potentially be converted to obvious defects are:

a) Parts
1. Partial damage through electrical Overstress or ESD
2. Partial physical damage during handling
3. Material or process defects
4. Damage inflicted during soldering (excessive heat)

b) Interconnections
1. Cold solder joint
2. Inadequate/ excessive solder
3. Broken wire strand
4. Loose screw terminations
5. improper crimp
6. Loose conductive debris

Once the product passes through the early life stage it is usually reliable. The critical factor to control product reliability is to ensure that products have successfully passed through infancy prior to delivery to the customer.

5.2.1 Burn_in:

A special case of ESS is burn-in, which is the screening of components and assemblies at elevated temperatures, under bias, to precipitate defects prior to shipment. Therefore, burn-in should not be confuse with more general ESS. However, sometimes, the term burn-in is used interchangeably with ESS.

5.3 Application of ESS in Production flow:

ESS may be conducted at any point in the manufacturing flow. It may be applied at more than one point, if justified. For electronic products, the three most common points are at the

1. Component level,

2. Sub-assembly level (circuit card level), and

3. Final assembly level.

Because the types of defects are different at each level, the stress conditions may be different at each level.

Thermal product cycling is by far, the largest precipitator of latent defects followed by random vibration. Therefore, majority of ESS facilities include both temperature and random vibration.