Screening method and scheme design principle of electronic components

Date：2021-08-02 18:29:07Views：1852

If the electronic components are not screened and tested before installation, once welded into the printed circuit board, it is found that the circuit can not work normally, and then check it, it will waste time and energy, and damage the components and printed circuit board. It is best to list the required components according to the circuit schematic diagram before hands-on preparation. In order to ensure that there is no waste of time and reduce errors in the trial production process, and ensure that the finished device can work stably for a long time, it must also be screened and tested after all components are prepared.

1. Appearance quality inspection

After you get an electronic component, you should check its appearance for obvious damage. For example, check whether all leads of the transformer are broken, whether the appearance is rusted, and whether the wire package and framework are damaged. For example, check whether the appearance is damaged, whether the pin is broken or rusted, and check whether the model on the device is clear and identifiable. For adjustable components such as potentiometers and variators, it is also necessary to check whether their activities are smooth and flexible within the adjustment range, whether the tightness is appropriate, whether there should be no mechanical noise, good hand feel, and ensure good contact of all contacts.

Various electronic components have their own characteristics and requirements. We should know more about the performance, parameters and characteristics of various components and accumulate experience.

2. Performance filtering

The screening of electronic components is an essential process to ensure that the trial electronic devices can work stably for a long time and stand the test of application environment and other possible factors. The so-called screening is to apply one or more stress tests to electronic components to expose the inherent defects of components without damaging their integrity. The screening theory is that if the test and stress level are properly selected, the inferior products will fail and the excellent products will pass. In the long-term production practice, people found that the newly manufactured electronic components generally have a high failure rate when they are just put into use, which is called early failure. After early failure, the electronic components enter the normal service life stage. Generally speaking, in this stage, the failure rate of electronic components will be greatly reduced. After the normal use stage, electronic components will enter the wear and aging stage, which will mean the end of life. This law is just like a bathtub curve, which is called the efficiency curve of electronic components,

The failure of electronic components is caused by improper raw materials or process measures in design and production. The early failure of components is very harmful, but it is inevitable. Therefore, we can only artificially create early working conditions, so as to eliminate inferior products before making products, so that the components used for product production can enter the normal use stage from the beginning, reduce failure and increase their reliability.

In a formal electronic factory, the aging screening items generally include: high temperature storage aging; High and low temperature cyclic aging; High and low temperature impact aging and high temperature power aging, etc. High temperature power aging refers to energizing the tested electronic components, simulating the actual working conditions, and experiencing a high temperature of + 80 ℃ - + 180 ℃ for several hours. It is an effective measure to test a variety of potential faults of components, and it is also the most widely used method at present. For amateurs, it is unlikely to adopt these methods for aging detection in the process of single piece electronic production. In most cases, natural aging is adopted.

The design principles of component screening scheme are defined as follows

Screening efficiency w = rejected defective products / actual defective products

Screening loss rate L = damaged number of good products / actual number of good products

Screening elimination rate q = number of defective products / total number of products screened

The ideal reliability screening should be w = 1 and L = 0, so as to achieve the purpose of reliability screening. The Q value reflects the problem of these products in the production process. The larger the Q value, the worse the reliability of this batch of products before screening, that is, the greater the problems in the production process, the lower the yield of products.

The more screening items are selected, the stricter the stress conditions are, and the more thoroughly the inferior products are eliminated, the higher the screening efficiency is, and the reliability level of the screened components is closer to the inherent reliability level of the product. However, it will cost more and take longer time to do so, which will eventually reduce the screening efficiency.

Therefore, if the selection of screening conditions is too high, it will cause unnecessary waste. If the screening conditions are too low, the inferior products will not be eliminated completely, and the service reliability of products can not be guaranteed. It can be seen that insufficient screening strength or too strict screening conditions are unfavorable to the reliability of the whole batch of products.

In order to carry out reliability screening effectively and correctly, the screening items and screening stress must be determined reasonably. Therefore, the failure mechanism of the product must be understood. Different types of products, different production units and different raw materials and technological processes may not have the same failure mechanism, so the conditions for reliability screening should also be different.

Therefore, a large number of reliability tests and screening tests must be carried out for various specific products, so as to master the relationship between product failure mechanism and screening items.

The following principles should be mastered in the formulation of component screening scheme:

1) Screening should be able to effectively eliminate early failure products, but should not improve the failure rate of normal products.

2) In order to improve the screening efficiency, strong stress screening can be carried out, but the product shall not produce new failure modes.

3) Reasonably select the best stress sequence that can expose failure.

4) Master the failure modes of all products.

5) In order to formulate a reasonable and effective screening scheme, it is necessary to understand the characteristics, materials, packaging and manufacturing technology of relevant components.

Conclusion: while following the above five principles, the screening time should be reasonably formulated in combination with the production cycle. Due to the current situation of the design, manufacturing and process of electronic components in China, as well as many uncontrollable factors in the procurement of imported components, the secondary screening of electronic components has become an indispensable part to stimulate the potential design and production defects of electronic components, effectively eliminate early failure products and improve the reliability of the whole system.