Several factors affect the reliability of a product or the reasons for its failures searched. In this post we go through some of these factors.
1. Poor design
Sometimes the cause for failure may be due to poor design. The error may have occurred for example due to too small margins of safety or due to too optimistic design. Such mistakes may occur if the use environments and requirements are not adequately known. Too little knowledge of materials and components used or pressure to cut down the costs may also lead to selection of improper materials/components for the needed use conditions or use life. Sometimes reliability considerations are not properly analysed in design, and they are found only in later stages of production or in use.
2. Manufacturing defects and mistakes
For complex products manufacturing may require lots of different steps and processes. These can significantly stress the products and some failures may be caused already during manufacturing of the product. For example, processing may cause internal stresses or thermal damage to the product, which results in a failure immediately or later in the hands of customer. Human errors may also happen in the manufacturing line, especially, if the process control is poor, which may cause poor reliability to the product.
One typical cause of failure is the change of some material/components used in the product. This can happen for example when material is no longer available or somewhere in the supply chain it is changed to a cheaper one. On paper the materials may look the same but in real life the new one may have much poorer properties than were originally designed for the product. For example, plastics with same name may have very different properties.
3. Quality problems
Sometimes certain batches of products have large number of failures even though most of the products manufactured are functioning fine. This may be due to a quality problem. Quality problems may be caused due to manufacturing errors, but also due to differences between supply batches. For example, the properties of materials may vary markedly, and this may cause problems for some batches. Therefore, properties of materials should be occasionally measured for quality control.
In addition to materials, quality problems also apply to components. Quality of components may be poor for certain batch or for certain supplier. Components may also be fake. In this case the insides of a component are not what they should be, and the manufacturer is different from the one stated in the papers.
4. Environmental conditions
The environment in which the product is used or stored has naturally a considerable effect on the reliability. For example, high temperature, temperature changes, high humidity or corrosive environments tend to cause failures. These are important factors to consider, when reliability testing method is chosen or when reason for failure is investigated. When environmental factors are considered, often use conditions are thought. However, the determination of use conditions can be tricky especially, if the product is used internationally and can see any conditions from tropical heat and humidity to cold seashore or highly polluted big cities.
It is important to notice that the use environment is not the only environment which should be considered. Products are also exposed to environments which are present during their transportation and storage. This might mean hot temperatures due to being stuck in a shipping container in direct sunlight, resulting in much higher thermal exposure that was designed for the use environment. Or in the worst case the products are stored outside in rain and polluted areas for prolonged time while waiting for installation.
Overstress often causes rapid and surprising failures. In overstress a critical limit of material, structure or component is exceeded which leads to failure. Overstress can be any kind of stress, such as too high temperature, too quick temperature change, too humid or dry environment, exposure to chemicals which can break for example polymer chains in plastics, exposure to highly corrosive environments etc.
Overstress may occur in manufacturing if for example chosen processing temperature exceed the thermal limits of some material present in the product. Another cause for overstress is misuse. The product may be stored in wrong conditions or the customer may be handling the product incorrectly, for example using it in conditions exceeding the specifications of the product.
Wear means that a failure occurs because products, their materials, and components age and therefore lose their original properties. All products wear in time. The process is slow, and therefore failures due to wear develop slowly over time. Wear causes lots of different failure mechanisms depending on the materials, structures, and conditions. For example, solder joints tend to crack due to fatigue or plastic casings become fragile and crack due to prolonged exposure to increased temperatures or UV. When a failure due to wear occurs in the end of useful lifetime of a product, it is not reliability issue but a part of the normal lifecycle of a product.