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A guide to environmental testing

Addressing some of the common misconceptions and differences between simulation and stimulation testing

 

Simulation and stimulation are two fundamental forms of environmental testing. The differences, advantages, and applications between these types of environmental tests frequently cause confusion and debate.

Simulation mimics the conditions a product would see in its normal use environment. It is often referred to as ‘test to pass’ because the environmental stresses applied to the product – such as temperature, humidity, and vibration – do not exceed its intended use. Once the product passes the simulation test it’s ready for market or consumer use.

Stimulation uses stresses to uncover a product’s weaknesses and limits, often stressing a product to the point of failure and beyond. For example, the temperature would gradually increase in an attempt to find the highest temperature at which the product will operate and where damage will occur. Once the product fails, it is determined whether the failure is acceptable or if it needs to be fixed prior to market introduction. Stimulation is also referred to as accelerated testing.

The following are some examples of different types of simulation testing:

Burn-in
This is a traditional form of testing when large batches of products are tested together at an elevated temperature to aid in the precipitation of premature failures. Elevated temperature conditions are achieved using the heat from products under test or from a heating element.

Burn-in tests are commonly done with the products under test powered, and temperatures may be cycled occasionally, which causes some degree of thermal cycling as the devices heat and cool. The ease with which burn-in is implemented must be weighed against the relatively low screening effectiveness and long test times, which is why many companies are turning to temperature cycling.

Temperature cycling
As demands increase to reduce testing time and as understanding stimulation grows, companies are transitioning from burn-in to temperature cycling for testing electronics.

Temperature cycling is the rapid change between pre-determined temperature extremes for multiple cycles. While temperature cycling is considered more effective and efficient than burn-in, there are a few complications to overcome. Good airflow, for example, is important because it maximizes heat transfer and ensures the actual product temperature follows the chamber air temperature. It also prevents the formation of hot/cold spots within the workspace of the chamber.

Ramp rate, or the speed in which the chamber can change the air temperature, is also an important consideration. Faster ramp rates are more effective stresses for a given number of cycles; however, if the ramps are too fast they might cause undesired damage. A typical temperature profile transitions between -40°C and 125°C, at 5°C to 15°C per minute. At the temperature extremes, dwell periods (a period of time when the temperature is stable) are used to allow the product temperature to catch up to the air temperature.

Thermal shock
This is a form of temperature cycling, which exposes a small number of products to severe and extreme temperature changes. This is normally accomplished by moving the products between hot and cold zones of preconditioned air or fluid. Thermal shock can be used for design validation and pre-production validation testing.

ESS
Environmental stress screening identifies weaknesses in manufacturing materials and processes while in production. In an ESS process, every product is subjected to stimulus. The objective of stress screening is to eliminate products that would fail early in use to reduce product infantile mortality rates.

The following is an example of stimulation testing:

HALT and HASS
Highly accelerated life testing (HALT) uses aggressive temperature change rates and multi-axis repetitive shock vibration to reduce required testing time. HALT tests a product to the point of failure to uncover product weaknesses, operating limits, and destruct limits prior to production. HALT is used to detect design defects in order to make design enhancements.

Highly accelerated stress screening (HASS) is a production process used to screen out defective products prior to shipment. HASS uses similar, but less aggressive, stresses to those used in HALT.

Overall, stimulation testing applies stresses to products, bringing them to their breaking points, to improve and ruggedize them. Simulation testing ensures products are reliable and will function as intended. Environmental testing is an important aspect of product design and development. Each product may need different types of testing depending on their intended use environment, design, and materials.

Environmental chamber selection checklist:

» Desired temperature range and profile:
» Temperature change rate
» Humidity range requirements
» Duration and frequency of tests
» Number of devices to be tested
» Size of the device(s) under test (DUT)
» Mass and material of the DUT
» Power dissipation of the DUT
» Fixture requirements of the DUT
» Available lab floor space and utilities
» Budget and lead time.

General testing rules
Don’t test the tester. Minimize or eliminate any part of the test equipment in the environmental chamber. This can increase reliability and prevent false failures caused by test equipment continually subjected to environmental stresses.

Don’t put all of your eggs in one basket. It’s a good idea to break up the equipment required to separate self-contained systems. This has the advantage of minimizing batch sizes in production and allows for at least partial operation should the test system fail.

Continuous monitoring is preferred. Intermittent failures are possible, if not likely, during changing environmental conditions. Ensure ‘soft’ failures are not missed by using fast multi-plexing or dedicated test equipment channels.

September 27, 2016

 

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