PCB Bare Board Test
In order to enable manufacturers to reduce production costs under the premise of fully guaranteeing the electrical performance of bare board interconnection, users will have to use 100% network data compatibility. It is also one of the problems encountered at present. People expect to develop industry standards in the foreseeable future. solve these problems. In the past, the work in this area was not perfect. For example, the industry provided circuit board manufacturers with Gerber machine code. This format was used to drive the light plotter and generate the mask tool that defines the light pattern of the production circuit board. It can be a single-sided, double-sided or multi-sided lightmap. Existing software tools can extract netlist information from Gerber graphs. This data does not include component information, it only defines the conductive connections that exist due to machine code commands. The early industry standard for data formats is IPC-D-356.
This data format extracts netlist information from the CAD system and replaces it with intelligent machine code. Many testers can use this code to determine the netlist characteristics corresponding to the physical condition of the circuit board. Since the bare board test is performed after the wiring process is completed, the bare board test information is provided in the IPC-D-356 format and is related to the pin information of a single component. Therefore, manufacturers tested according to the IPC-D-356 standard can provide information such as “the 16 pins of the U14 component are shorted to the 9 pins of the U20”.
Of course, the most needed electronic data for bare board testing is CAD netlist data. Many companies are reluctant to share this information with circuit board manufacturers, but this is still the most concise data to determine whether the bare board performance meets the CAD system design requirements. People expect the electronic description of the three formats of the circuit board to be consistent with each other, but in most cases, this is not the case. There are three reasons for this difference in description:
1. Hasty changes.
2. The problem of data conversion between machine code data and netlist data.
3. Problems in software implementation. In any case, data compatibility is very important.
Open circuit and short circuit tests using fixtures and needle beds also face challenges. The increasing complexity of circuit boards makes them unable to meet circuit test requirements. The complexity lies in the reduction in circuit size and the increase in component density. The test instruments used by most circuit board manufacturers include single-density, double-density, and four-density testbeds. The double-density testbed is suitable for spacing of 400 mm and above. When the board density exceeds 400 mm pitch, other technologies must be considered. This type of test mainly faces more array-shaped packages, which can be BGA or column grid arrays, or fine-pitch BGA packages with closer pins.
The topic of this section is bare board testing. However, in order to ensure the correct installation of various parts during assembly, the circuit board manufacturer must ensure that the signal inlet and outlet of each wiring network on each array pad are connected to the signal inlet and outlet of other wiring networks related to it and eliminate short-circuit faults. The four-density test fixture can have 62 test pins per square centimeter, but people are worried that the probe contact will cause potential damage to the circuit board. In addition, the cost of dual-density and four-density fixtures and the cost of the entire test instrument are relatively high, which makes it very difficult to adjust the entire test coverage within the expected cost range. This cost is expected based on the current understanding of electrical performance testing and linear expectations of existing test concepts. There are also circuit board manufacturers that use flying probe test (flyingprobetest). With this instrument, each wiring network can be stimulated and compared with neighboring wiring networks to establish open-circuit and short-circuit characteristics.
Flying probe testing eliminates the fixture, and its cost depends on the number of circuit boards produced. It is an alternative to bed of needle testing and an effective low-cost test program. However, the speed is relatively slow, and the price of the equipment is more expensive. When testing high-density circuits and other wiring networks, the test becomes more complicated.
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