5.4 ESS Implementation:

ESS is product-specific, since each product has its own set of potential defects, and since the applied ESS stresses affect each product differently. Eventhough the ESS process must be set up separately for each product, there are many common features of both products and stresses which cause many ESS processes to be similar.

The stresses applied in ESS are those which are expected to precipitate manufacturing defects. They are not necessarily those which the product will see in service. The two most common ESS stresses for electronic products are temperature cycling and vibration. They may be applied sequentially or simultaneously. ESS may be conducted anywhere in the manufacturing process flow.

5.4.1 Types of Failures detectable by Thermal and Vibration environments:

Parameter	Thermal	Vibration
Wrong component	Yes	-
Parameter drift	Yes	-
Defective Solder Joint	Yes	Yes
Loose hardware	-	Yes
Broken(partially) component/ lead	Yes	Yes
Improper Crimp	Yes	Yes
Chafed or pinched wires	-	Yes

Normally, 100% of a group of product is subjected to environmental stimulus for a pre determined time for the purpose of forcing failure to occur in-house. ESS on a sampling basis is worthwhile if the design and production methods are mature and stable. In practice it was observed that about 80% of the failures are sensitive to atmospheric tests and about 20% to vibration. Temperature and Humidity tests are effective for defect types like wire insulation, improper crimp or mating, parametric drift, corrosion, and hermetic sealed failures. Vibration is more effective for the defects like loose contacts, debris, Chafed & pinched wire and adjacent boards/parts shorting.

5.4.2 Differences in the screening of components and equipment:

	Components	Equipment
1. Operating environment	In general, precise operating environment is not known. It is likely to be used in different conditions and variety of equipment in which it is used.	In general, the operating environment of equipment is known at the time of manufacture.
2. Test Procedure	Highly standardized test procedures are necessary so that results on different samples of the same type taken either from different suppliers or at different times from the same supplier are comparable.	It is less commonly necessary to compare the results between similar equipment from different sources of supply.
3. Severity of environment	Components are usually tested to more severe levels than equipment in order to ascertain their performance limits.	For equipment, the severity level is determined by the weakest link ( by design) and is generally less severe.
4. Sample size	Components are frequently available in sufficient quantities to permit simultaneous tests to be applied to several lots.	Few equipment are available for test and each specimen undergoes the entire test programme. For complex equipment, the tests can be involved and costly.

5.4.3 Design of screens, important points:

1. Screen parameters are independent of field environment
2. Screen parameters are set subject to equipment design capability.
3. Applied stress must reach flaw location
4. Overstress must be avoided. Overstress can be detected by unnatural failure modes.
5. Factors which determine level of assembly to perform screening,
   1. Cost
   2. Effectiveness of the screen to expose failures
6. Screening programmes must be reviewed dynamically.

1. Thermal screen design:

* Determine the initial ESS test conditions which is characterized by the following parameters:

-Cycle characteristics
-Low temperature
-High temperature
-Rate of change of temperature
-Dwell times at temperature extreme

* No of cycles

* Equipment Condition

-Powered ON or Powered OFF
-Monitored or NOT monitored

* Analyze failures that occur

** If no failures, increase initial test conditions
** For all failures that occur, perform failure analysis and corrective action. If no failures occur, initial test conditions can be increased in steps such that the screening is effective. If failure was caused by overstress, decrease the severity of test conditions. Failures occurring due to Overstress is a subject of Engineering judgement and experience.

* Continue modifying thermal stress until only ‘latent defects’ are disclosed.

2. Vibration screen design:

* Begin with initial g rms level (e.g. 6 grms)

-Vibration step-stress is increased in one g rms increments

-Functional testing of unit during vibration is required

* Failure modes are noted at each vibration level

* Rigorous failure analysis and corrective action is essential

* Vibration level in g rms is increased until non-defect failure modes occur. This represents the design capability

* Further review of screen is essential on accumulated data.