Purpose of environmental stress screening
The purpose of environmental stress screening is to find and eliminate the early failure of products, make them enter the random failure stage when leaving the factory, and deliver them to users with inherent reliability level.
Principle of environmental stress screening
Environmental stress screening is to accelerate the internal potential defects into faults by applying reasonable environmental stress and electrical stress to electronic equipment, so that people can find and eliminate them.
Environmental stress screening is a technological means of equipment development and production. The screening effect depends on the applied environmental stress, electrical stress level and the ability of detection instruments. The magnitude of applied stress determines whether potential defects can be accelerated into faults within a predetermined time; The detection ability determines whether the potential defects that have been accelerated into faults by stress can be found out for troubleshooting. Therefore, environmental stress screening can be regarded as the extension of product quality control inspection and testing process. 2 defect classification
The loss of the specified function of the product is called failure. Repairable products are often referred to as faults. For the equipment, any quality feature that does not meet the specified technical standards constitutes a defect.
The failure of most electronic equipment is called failure. See Figure 2.11 for its decomposition based on the cause of failure. It can be seen from the figure that equipment faults are divided into accidental failure faults and defect faults. It is believed that accidental failure is random failure, which is caused by the inherent failure rate of components and parts; while defect type failure is caused by raw material defects, component defects, assembly process defects and design defects, component defects themselves are caused by defects such as structure, process and materials, and design defects include circuit design defects, structural design defects Process design defects, etc.
Classification of visual defects of electronic equipment
In terms of impact and consequences, defects are divided into fatal defects, heavy defects and light defects: from a visual point of view, the main process types of defects include: welding, non welding connection, excess of wires and cables, anti short circuit gap, contact, printed circuit board, parts manufacturing and installation, components, winding, marking, etc., most of which may produce fatal defects or heavy defects, Light defects are common.
Fatal defects refer to the defects that may cause harm or insecurity to personnel such as the use, maintenance, transportation and storage of equipment, or may hinder the tactical performance of some important equipment (such as ships, tanks, large artillery, aircraft, missiles, etc.).
Major defect refers to the defect that may cause failure or seriously reduce the service performance of the equipment, but does not constitute a fatal defect.
Light defects refer to defects that do not constitute heavy defects, but will reduce the service performance of the equipment or do not meet the specified technical standards, and have little impact on the use or operation of the equipment.
Visual defect refers to the defect that can be directly observed through human visual organs or judged by simple tools on the quality characteristics of equipment.
The quality inspector of the manufacturing unit can find most visible defects and submit them to relevant departments for elimination. Only invisible defects can be found by environmental stress screening or other methods, otherwise the product reliability will be affected.
Design principle of environmental stress screening scheme
The design principle of the environmental stress screening test scheme is to enable the screening stress to stimulate the faults caused by potential design defects, manufacturing defects and component defects: the applied stress does not have to simulate the life profile, task profile and environmental profile specified by the product; in the test, various working modes specified by the design shall be simulated.
Determine routine screening or quantitative screening according to conditions and necessity: formulate screening methods according to different stages and product characteristics case.
Screening in development stage
In the development stage, conventional screening is generally carried out according to the screening methods obtained from experience. Its main functions are: on the one hand, it is used to collect information such as the type and quantity of possible defects in products and the effect of screening methods; On the other hand, routine tests are carried out before reliability growth and engineering development tests, which can save test time and money. At the same time, it is conducive to the design of mature and fast development test methods.
The routine screening in the development stage shall collect data for quantitative screening in the production stage, prepare for quantitative screening, and design the outline of quantitative screening.
Screening in production stage
The screening in the production stage is mainly to implement the quantitative screening outline designed in the development stage; By comparing the recorded defect precipitation with the design estimated value, the measures to adjust the screening and manufacturing process are put forward; Improve or reformulate the quantitative screening outline by referring to the quantitative screening experience data of products with similar structure and maturity. These empirical data mainly include:
Models of components and components with high failure rate: suppliers of products with high failure rate; Receiving inspection, test and screening data of components: records of previous screening and testing; reliability growth test records: other test records.
Effect of environmental stress on defects
Field environmental stress is the main factor affecting the development of defects into faults. Any defect that develops into a fault needs to be subjected to a certain strength stress. After a certain period of time, only when the product is subjected to an environmental stress equal to or greater than the threshold can some defects become faults: in some mild environmental stresses, many defects will not develop into faults. Therefore, the purpose of screening can be achieved only by selecting the stress that can expose some defects as the screening condition.
Environmental stress screening is the technological process of the development and production of the whole machine. Each product must be carried out. In order to save resources and time, we should try to select the stress conditions with high efficiency and the assembly grade with low cost. Random vibration equipment and temperature cycling equipment are necessary, and the manufacturer should be equipped as soon as possible. Before being equipped, it can be implemented in nearby units with these conditions. At this time, it is more important to select and screen the assembly level.
The design of environmental stress screening scheme is a comprehensive work. It is necessary to master the history and current situation information of product design, process and components, and collect necessary data: when selecting environmental stress screening or quantitative environmental stress screening, and which screening assembly level to select, some risks shall be deducted. Therefore, the system engineering method must be applied to make a comprehensive trade-off around the product reliability requirements.
Environmental stress screening and reliability growth test belong to reliability engineering test. They have the same general objectives and their own characteristics. They can complement each other but can not replace each other. Environmental stress screening can be used to do some reliability growth things, so as to make full use of the results of environmental stress screening and improve the test efficiency.