Classification of Environmental Tests
Environmental tests are roughly divided into three categories:
- Mechanical environmental test
- Climate environmental test
- Comprehensive environmental test
As shown in Figure 1.
Figure 1 Classification of Environmental Tests
- The mechanical environmental stresses include: vibration, shock, collision, constant acceleration, noise;
- The comprehensive environmental stresses include: environmental factors that combine mechanical and climatic environments;
- The climate and environmental stresses include: temperature, humidity, low pressure, salt spray, rain, mold, solar radiation, dust;
It is well known that environmental stress can cause product failure. The technical information of a well-known foreign electronic equipment company clearly shows the relationship between environmental stress and failure and its related proportion. As shown in Figure 2, among the effects of various stresses, failures caused by temperature, humidity, and vibration environmental stresses account for 88% of all environmental stresses.
Figure 2 Proportion of Various Environmental Stresses in Failure
Relationship Between Temperature, Humidity, Vibration Stress & Failure
Relationship Between Temperature Stress & Failure
The data show that temperature stress is the most important factor causing product failure, as shown in Table 1.
Table 1 Main Types of Failure Caused by Temperature Stress
| Lapse | Environmental stress conditions | Sensitive components & materials |
| classification | Subdivision | Reason | Failure mode |
| High Temperature | High Temperature Aging | Aging | Tensile strength aging Insulation aging | Temperature + Time | Plastic, resin |
| Chemical Reaction | Thermal decomposition | Temperature | Plastic, resin |
| Softening, melting, vaporizing, sublimation | Distortion | Temperature | Metal, plastic, thermal fuse |
| High temperature oxidation | Oxide layer formation | Temperature + Time | Connection point material |
| Thermal diffusion | Lead break | Temperature + Time | Heterometallic connection |
| Heat build-up | Remaining heat burning | Combustion | Heating + Drying + Time | Wood material with vinyl and polyurethane paint |
| Migrate | Electromigration | Break lead | Temperature + Current | Tungsten, copper, aluminum (especially aluminum leads in integrated circuits) |
| Spread | Metal | Fatigue, damage | Temperature + Stress +Time | Springs, structural elements |
| Plastic | Fatigue, damage | Temperature + Stress + Time | Springs, structural elements |
| Low Temperature | Brittle at low temperatures | Metal | Damage | Low Temperature | Zinc, titanium, magnesium and their alloys |
| Plastic | damage | Low Temperature | Low elasticity amorphous |
| Temperature Cycling or Thermal Shock | Expansion & contraction | Different materials have different expansion coefficients | Peeling, cracking | Temperature + Times | Paint coating |
| Mechanical displacement | Electrical performance changes | Temperature + Times | Adjustable resistance, potentiometer |
| Different materials have different expansion coefficients | Deformation of seals | Temperature + Times | Hermetically sealed container |
Relationship Between Humidity Stress & Failure
Moisture is also one of the main environmental stresses that cause product failure. The product is in a humid environment, and the moisture absorbed by the material will cause expansion, decrease in strength, and change in performance, and the insulation material will cause a reduction in electrical performance. The details are shown in Table 2.
Table 2 Main Types of Failure Caused by Humidity Stress
| Lapse | Environmental stress conditions | Sensitive components & materials |
| Classification | Reason | Failure mode |
| Water vapor absorption or diffusion | Diffusion | Swell | Humidity | Components encapsulated, covered, or constructed using low crystallinity resin materials |
| Microburst | Poor insulation performance and deliquescence |
| Corrosion | Electrolytic corrosion | Increased impedance | Humidity + DC electric field | Resistance, integrated circuit |
| Crack corrosion | Solder joint cracking | Humidity | Solder joint |
| Stress corrosion | Color changes | Humidity | Alloy |
| Migrate | Ion migration | Short circuit | Humidity + DC electric field | Copper, lead, tin, zinc |
| Poor insulation performance | Humidity + DC electric field + halogen ion | Metals such as gold and platinum that migrate when coexisting with halogens |
| Mold | | Deterioration of insulation performance | Temperature +Humidity | Plastic material rubber material |
Relationship Between Vibration Stress & Failure
The impact of vibration stress (vibration, shock, collision, constant acceleration, etc.) on the product is also one of the main factors causing product failure, as shown in Table 3.
Table 3 Main Types of Failure Caused by Vibration Stress
| Lapse | Environmental stress conditions | Sensitive components and materials |
Classification | Reason | Failure mode |
Resonance | Produce displacement | Loose, separated | Vibration + shock | Material connection |
| Poor contact | Vibration + shock | Solder joint |
| Abrasion | Vibration | Material connection |
Resonance, durable vibration | Strength | Fatigue, deformation, bending | Vibration + shock | Metal structure |
| Cracking | Vibration + shock | Metal structures, plastic structures, cables and wires |
Relationship Between Comprehensive Application of Three Kinds of Stress & Product Failure
Applying separate environmental stresses to the product can trigger product failure. Then applying 3 different environmental stresses to the product can easily achieve 3 to 5 times the acceleration effect. At the same time, by combining different environmental stresses, failures that cannot occur when individual stresses are applied can be stimulated.
Assume that X, Y, and Z are failure modes that occur when stress factors A, B, and C are applied. X, Y, Z, XY, YZ, and XZ modes appear under the combination of two stress factors. Then under the combined situation of the three stress factors, a new XYZ mode will appear. At the same time, X, Y, Z modes are also accelerated. In addition, the environment where the stress factors A, B, and C are comprehensively applied is closer to the product use environment than the environment where the stress factors are applied alone. Therefore, better results can be achieved through comprehensive environmental tests.
In the comprehensive experiment of simultaneous application of three kinds of stress: temperature, humidity, and vibration, from the mechanism of failure occurrence, the product undergoing temperature cycling expands and contracts due to the difference in the expansion coefficient of the material, and loosens at the joint. At this time, if humidity and moisture are applied, it will invade from the gap and reduce the friction coefficient of the joint and the joint. When vibration stress is applied, the resonance phenomenon of the product will occur with respect to a specific frequency. Through the repetitive processes of motion, moisture absorption, freezing, and resonance, the emergence of new failure modes (caused by the greatly accelerated single-factor failure mode and the combined effect of the three factors) become possible.
Figure 3 Mechanism of Failure Caused by Comprehensive Stress Application
Summary
Because the temperature, humidity, vibration three comprehensive test is very close to the product use environment test. And this test can very effectively provoke product failure problems. Therefore, in recent years, this test technology has been widely used in engineering.
Our company has this Temperature Humidity Vibration Combination Test Chamber, which meets the following standards:
- IEC68-2-1 (GB2423.1-2008)
- IEC68-2-2 (GB2423.2-2008)
- IEC68-2-3 (GB2423.3-2006)
- IEC68-2-30 (GB2423.4-2008)
- IEC68-2-14 (GB2423.22-2008)
- MIL-STD-810D (GJB150.3A-2009)
