Planning and Execution of Environmental Test Sequences

Planning and Execution of Environmental Test Sequences

Electrical units are operated by electrical and electroniccomponents. An environmental test sequence is useful for testingelectrical units to reduce reliability issues.

Here we want to help out on testsequence guidelines based on relevant principles and considerationsfor electronic testing according to International Standard IEC-60068-X-XXand the United States Military Standard MIL-STD-810G.

1. INTRODUCTION

ELECTRICAL units are operated by electrical and electroniccomponents on battery power or AC/DC power. They include componentscontaining small printed circuit boards (PCBs) comprisingvarious devices, such as resistors and capacitors. Electrical unitsmay also be components in combination with mechanical partsand single- or double-sided PCBs, such as those found inmicrocomputers. Electrical units found in automobiles andrailway vehicles require a very high level of safety andreliability, yet reliability issues leading to recall are common.Test sequences are applied to electrical units to reducereliability issues, particularly tests related to environmentalconditions. A test sequence can reproduce various failure modes,and combination testing can be used to determine the properorder for individual environmental tests.

He we help you to understand the test sequence guidelines based on International Standards IEC-60068-X and the United States Military Standard MIL-STD-810G were developed. We have tried our best to interpret, and come up with test sequence which is accepted worldwide.

2. ENVIRONMENTAL TESTING USING STANDARDS

Most studies of environmental tests are performed in single or dual environment. That is, test sequences are notconsidered except for fields of automotive and aerospace electrical units.The proposed test sequence incorporates thevarious principles and considerations of IEC-60068 series of standards.

The following four philosophies were applied for arriving at the sequence:

• Information about failure trends should be found inthe early part of the test sequence. It would be appropriate that test sequenceshould begin with the most severe tests. However,destructive tests that result in the specimen being unsuitablefor further testing should be placed at the end of thesequence.
• All the test results should be accumulated as possible should be obtained priorto specimen damage. A sequence of tests that simulates the most appropriate environmental parameters most likely to occur in practiceshould be used.

The sequence of tests that provides the most significant information about damage caused by previous tests should be used.

According to the above principles, the most severe and leastsevere tests should be carried out during development testing.The tests that provide the most significant information and thetests most likely to occur in practice should be applied duringapproval testing.

Key test sequence considerations are as follows. A testthat includes a rapid change in temperature should come at thestart of the sequence, and tests for the robustness of terminationsand soldering should be placed early in the sequence of tests.

Next, all or a portion of the mechanical tests should beperformed to accentuate faults likely to have been caused byrapid temperature changes and to provoke new faults, such ascracks or leaks. Cold and dry-heat phases should be appliedearly in the climatic test sequence so that the short-term effectsof temperature can be revealed. A damp heat cycle phase willintroduce moisture into any cracks, the influence of which willbe accentuated by the cold phase and, potentially, by a low airpressure phase. In some cases, sealing tests may be used for therapid detection of cracks or leaks. A damp heat, steady-state testis often applied at the end of the entire sequence of tests, or,when not included in the sequence, on separate specimens todetermine the long-term behaviour of the component in a humidatmosphere. Tests such as corrosion drop and topple, and solar radiations are not normally included in the test sequence andshould, if they are required, be made on separate samples.

A description of each test and a test sequence based onMIL-STD-810G is also proposed. Tests associated withenvironmental testing of electrical units include low pressure(altitude), high temperature, low temperature, temperatureshock, solar radiation (sunshine), rain, humidity, fungus, saltfog, sand and dust, and immersion.

The low-pressure (altitude) test is performed early in the testsequence because of both its limited damage potential and itsgenerally early occurrence in the life cycle. However, other tests,such as the high- and low-temperature test, dynamic test, andnon-metallic parts test, may contribute significantly to theeffects of low pressure on the test item, and thus may need to be conducted prior to the low-pressure test. These tests provide information on failure mechanisms that can occur naturally during transport.

One testing approach to conserve the test item life is to apply those tests that are perceived to be the least damaging, such as the high- and low-temperature test, early in the test sequence.

The temperature shock test employs the test item response characteristics and performance information obtained from the high- and low-temperature tests.

The solar radiation (sunshine) test applies to all stages. However, high temperatures along with photochemical reactions effects could affect material strength or dimensions and could thus influence the results of subsequent tests, such as vibration testing.

The effectiveness of determining the integrity of an enclosure is maximized if the rain test is performed after the dynamic tests. Humidity testing may produce irreversible effects. Thus, one should consider performing dynamic tests prior to humidity tests. In addition, because of the potentially unrepresentative combination of environmental effects, it is generally inappropriate to conduct humidity testing on test samples that were previously subjected to salt fog, sand and dust, or fungus tests.

If using the same test item sample for more than one climatic test, it is usually recommend that the salt fog test be conducted after the other climatic tests, because salt deposits can influence the results of other tests.

When evaluating the cumulative effects of vibration and other environments, a single test item should be exposed to all environmental conditions. Vibration testing generally should generally be performed first. The placement of the shock test in the sequence will depend upon the general availability of test specimens and on the type of testing, i.e., on whether the goal of the testing is developmental, qualification, endurance, etc. Normally, shock tests should be scheduled early in the test sequence, but after any vibration tests. Shock usually occurs after vibration in practice, and shock testing without vibration is not meaningful.

3. SEQUENCE

Following will be recommended sequence for testing if possibly can be followed in sequence.

• Cold Test
• Dry Heat Test
• Rapid Change In Temperature Test
• Shock And Vibration Test
• Drop And Impact Test
• Low Air Pressure (Altitude) Test
• Damp Heat (Cyclic) Test
• Corrosion / Contamination / Fungus Test
• Rain / IP Protection Test
• Dust And Sand Test

CONCLUSION

Problems occurring in the field in electrical units have been difficult to predict and prevent, and new concepts for reliability testing are desired. Therefore, a composite reliability test sequence based on International Standard IED-60068-X and the United States Military Standard MIL-STD-810G was proposed to enable better prediction of field failures. Tests providing the most significant effects and tests mimicking model conditions most likely to occur in practice were identified from the test sequence guidelines on the basis of the principles applied to approval testing.