Testability concept of PCB design professional testing organization

Date：2022-07-11 17:30:00Views：572

The testability of product design (de sign for testability. oft) is also the main content of product manufacturability. From the perspective of production, it is also one of the processibility of design. It refers to considering the degree of difficulty that the product performance can be tested when designing, that is, how to test the product performance and processing quality in the simplest way when designing the product, or the product design should make it easy for the product to test its performance and quality according to the specified method as far as possible. Especially the design of electronic products is essential to the performance test of products. DFT's good product design can simplify the preparation of inspection in the production process and final product inspection, improve the test efficiency, reduce the test cost, and easily find the defects and faults of products, so as to ensure the quality stability and reliability of products. Products with poor DFT design will not only increase the time and cost of testing, but also cannot guarantee the quality and reliability of products due to the difficulty of testing. Therefore, the design for testability, which is compatible with the methods and equipment of product design and testing, is one of the important contents that must be considered in the design of electronic products.

1、 Testability of printed board design testability and manufacturability of printed pole design belong to the technological design of printed board, and also include the manufacturing of printed board, the testability of finished printed board (optical board) and the testability of printed board assembly. The test methods and contents of these two parts are completely different, but they should be reflected in the design of the same printed board. For designers, they need to understand not only the performance and methods to be tested on the printed board, but also the installation and testing requirements and methods of printed board assemblies. There are unified standards for the test methods and performance requirements of printed boards and optical boards, which can be found by referring to the standards of relevant printed boards. For the test of printed board assembly, the designer should consider comprehensively according to the characteristics and requirements of circuit and structure, and take appropriate measures to reasonably set test points or decompose the test in the installation process during layout and wiring. Especially with the miniaturization of electronic products, the pitch of components is getting smaller and smaller, the installation density is getting larger and larger, and there are fewer and fewer circuit nodes for testing, so the online testing of printed board assemblies is becoming more and more difficult. Therefore, the electrical, physical and mechanical conditions of testability of printed boards should be fully considered in the design, as well as the use of appropriate mechanical and electronic equipment.

2、 The optical board test of printed boards is an important means to ensure the quality of printed boards of components to be installed, and it is also a powerful measure to ensure the quality of printed board assemblies and reduce the loss of repair, rework and scrap. The quality of optical board testing is guaranteed, which can improve the installation efficiency of printed boards and reduce costs. If the quality of the optical board cannot be guaranteed, the components and parts need to be removed when the quality problem of the board is found after the installation of the printed board, which is not only labor-consuming and time-consuming, but also may damage the components and parts, resulting in greater loss of time and economy. Therefore, the electronic industry at home and abroad attaches great importance to the performance testing of printed boards, and has formulated many testing standards and methods. The main inspection items include appearance inspection, mechanical performance test, electrical performance test, physical and chemical performance test and reliability (environmental adaptability) test. There are many test items for PCB and optical board, but there are few restrictions on the design.

Appearance inspection is generally carried out on the finished printed board through visual inspection or appropriate optical instruments, mainly to check the quality of manufacturing. Appearance inspection has few requirements on the testability of design. Among these performance tests, the most powerful one affected by the layout and wiring of printed board design is the electrical performance test, which includes voltage withstand, insulation resistance, characteristic impedance, circuit on-off and other test items, and the design of test points should meet the test requirements. The biggest relationship with the testability of the design is the circuit on-off test, which is the key to ensure the quality of printed boards. The performance requires 100% logic on-off test for each printed board. So when designing PCB.

The matching between the physical size of the test probe of the test equipment and the test circuit that may be used in the test should be considered. Otherwise, it will be due to the position and size error of the measured point on the printed board. Problems that cause errors in the test or poor repeatability of the test. The destructive mechanical, physical and chemical properties are generally tested and evaluated by sampling from the same batch of products or designing special test plates or attached test plates according to the standard. When designing, you should be familiar with the design and test requirements and methods of the test board and attached test board.

3、 Testability of printed board assemblies the testing of printed board assemblies refers to the electrical and physical testing of printed boards installed with components. There are many difficulties that affect the testing of printed board assemblies, and the testing method is much more complex than the testing of optical board of printed board. The testability design of the assembly shall include the testability of the system. The testability function requirements of the system shall be submitted to the concept of the overall design of the whole machine for review. The testability of printed board assemblies must be compatible with the integrity of design integration, testing and maintenance.

The testability of PCB assembly is before the design starts. The review shall be conducted with the technical personnel for the manufacture, installation and testing of printed boards. To ensure the effect of testability. The review shall cover the visibility of circuit graphics, installation density, test operation methods, division of test areas, special test requirements and test specifications.

There are mainly two types of PCB assembly level testing. One is to test the function of the assembly with independent electrical functions, apply a predetermined excitation signal on the input end of the assembly, and confirm whether the design and installation are correct by monitoring the results of the output end. Another kind of test is to carry out optical inspection and on-line test on the assembly. Optical inspection has no clear restriction on the design. It can be tested as long as the circuit has a certain visibility. This method mainly checks the quality of installation and welding. The limitation of on-line testing on printed boards is that the test points should be designed in the coordinate grid where they can match the test needle bed, and can be tested on the welding surface. If the flying pin test is used, it is necessary to ensure that the test points are located on the coordinate grid, and to maintain enough space in the layout so that the probe (flying pin) can touch the tested point. The flying pin test can be carried out on both sides of the board, mainly to detect the assembly and welding quality and whether the processing components are damaged.

The commonly used testing methods include manual testing and automatic instrument testing. Manual testing is carried out by multimeter, digital voltmeter, insulation resistance tester and other instruments, which has low efficiency, complex recording and data processing, and is limited by the assembly density. The miniaturized high-density assembled printed boards are difficult to be tested manually. Automatic instrument detection has the characteristics of high precision, good reliability, good repeatability and high efficiency, and some instruments also have the ability of automatic judgment, recording, display and automatic fault analysis.

The commonly used automatic testing technologies include automatic optical testing (AOD), automatic X-ray imaging (AX I), on-line testing (ICT) and functional testing. The test compatibility of printed board assemblies should be considered in the design of printed boards. If the test methods and necessary test mechanical rules are not considered, even if the circuit has good testability in electrical aspects, it is difficult to test on printed board assemblies, such as the location and size of test points and the matching of the pitch of test points with the test probe or needle bed. The problems of setting electrical conditions and preventing damage to components during online testing are all testability problems to be considered and solved.