The development of circuit board test technology
Abstract: The renaissance of functional testing technology is an inevitable result of miniaturization of surface-mount devices and circuit boards. Once any system is too small to detect the inside of the base, there are only a few input and output channels that deal with the outside of the system, and this is where functional testing comes in.
This situation is exactly the same as the early days of functional test development thirty or forty years ago. However, it is different from the past. Today, the international standards of functional test instruments (such as PXI, VXI, etc.) have gradually matured, and standard instrument modules and virtual instrument software technologies have been commonly used, which greatly increases the versatility and future of functional test instruments Flexibility and help reduce costs. At the same time, the testability design results of circuit boards, and even the testability design results of very large-scale hybrid integrated circuits, may be transplanted into functional test technology. Using the standard interface of boundary scan technology and the corresponding testability design, the functional tester can be used to program the system online as well as the online test equipment. Undoubtedly, future functional testers will tell us much more than predicates such as "pass or fail".
Surface mount devices and circuits have been in an endless miniaturization process, and have ruthlessly driven the obsolescence and evolution of some related test technologies. Under the evolutionary pressure of miniaturization of electronic products, technology also follows the simple rule of "survival of the fittest" just like species. Watching the road of testing technology helps us predict the future.
Since surface mount technology (SMT) has gradually replaced jack-type mounting technology, devices mounted on circuit boards have become smaller and smaller, and the functions contained in the unit area of the board have become more and more powerful.
As far as passive surface mount devices are concerned, 0805 devices, which were used extensively ten years ago, today account for only about 10% of the total number of similar devices; and 0603 devices have started to decline four years ago. Replaced by 0402 devices. At present, even smaller 0201 devices are gaining popularity. It took about ten years to go from 0805 to 0603. There is no doubt that we are in an era of accelerated miniaturization. Look at surface-mount integrated circuits. From the predominant quadrangular flat package (QFP) ten years ago to today's flip chip (FC) technology, a variety of packaging formats have emerged, such as thin small pin packages (TSOP), ball array packages (BGA) , Micro ball array package (μBGA), chip scale package (CSP), etc. Throughout the evolution of chip packaging technology, its main feature is that the surface area and height of the device are significantly reduced, while the pin density of the device is increasing rapidly. In terms of chips with the same logic and functional complexity, the area occupied by flip-chip devices is only one-ninth of the area of the original square flat packaged devices, and the height is about one-fifth of the original.
Micro-packaged components and high-density PCBs bring new testing challenges
The continuous reduction in the size of surface-mount devices and the consequent high-density circuit mounting have brought great challenges to testing. The traditional manual visual inspection is not suitable even for the medium complexity circuit board (such as a single panel with 300 devices and 3500 nodes). Someone once conducted such a test, in which four experienced inspectors performed four inspections on the quality of the solder joints of the same board. As a result, the first inspector found 44 percent of the defects, the second inspector and the first inspector had 28 percent consistency, and the third inspector and the former inspector Two had 12% agreement, while the fourth inspector had only 6% agreement with the top three. This test exposed the subjectivity of manual visual inspection, which is neither reliable nor economical for highly complex surface-mount circuit boards. For surface-mount circuit boards that use micro-ball arrays without packaging, chip-scale packaging, and flip-chips, manual visual inspection is virtually impossible.
Not only that, due to the reduction of the pin pitch and the increase of the pin density of surface-mount devices, the needle-bed type online test is also facing the dilemma of “no cone”. According to the North American Electronics Manufacturing Planning Organization, after 2003, the use of online testing for high-density packaged surface-mount circuit board inspection will not achieve satisfactory test coverage. Based on the test coverage of 100% in 1998, it is estimated that after 2003, the test coverage will be less than 50%, and after 2009, the test coverage will be less than 10%. As for the problems of back-current drive, test fixture cost, and reliability that still exist in online test technology, there is no need to think any more. Just because the test coverage of less than 10% in the future, it is doomed to the future of this technology .
So, can we hand over the circuit board to the final functional test when human eyesight is incompetent and the machine probe is nowhere to be reached? Can we tolerate the test for several minutes and only know whether the circuit board is broken or not, but we don't know what is happening in this "black box"?
The development of new testing technology brought by optical inspection technology will never stagnate because of the above difficulties. Manufacturers of test and inspection equipment have introduced products such as automatic optical inspection equipment and X-ray inspection equipment to meet the challenges.
In fact, these two devices have been widely used in semiconductor chip manufacturing and packaging processes before being widely used in the circuit board manufacturing industry. However, they need further innovation to truly cope with the test difficulties caused by the miniaturization of surface-mount devices and high-density circuit boards.
At the same time, the major online test and functional test equipment manufacturers in the industry have been unable to meet future development trends. The countermeasures they adopted were to acquire relatively small automatic optical inspection equipment and X-ray inspection equipment manufacturers, so that they could quickly grasp the relevant technology and enter the market quickly.
Whether it is automatic optical inspection technology or automatic X-ray inspection technology, although they can help complete the difficult tasks of manual visual inspection, their reliability is not completely satisfactory. These technologies are highly dependent on computer image processing technology. If the information provided by the original optical image or X-ray image is insufficient, or the image processing algorithm is not effective enough, it may lead to misjudgment. Fortunately, engineers have accumulated considerable experience in the application of optics and X-ray technology, so in the next few years, it is expected that the technology of high-resolution circuit board optical images and true 3D X-ray image generation will continue Some progress.
In addition, today's relatively inexpensive storage and computing technologies make it possible to process large-capacity image information. What needs to be innovated in this field is image processing algorithms and the combination of the most basic image enhancement and pattern recognition technology with expert systems. These expert systems are based on computer-aided design and manufacturing data (CAD-CAM) of circuit boards, combined with empirical data on the production line, can perform self-learning, and self-improve the algorithm of inspection and discrimination. Another possible development direction in this field is to expand the range of the used spectrum. At present, the industry has started to try to capture and analyze the infrared image of the circuit board when the board is powered on. By comparing the infrared image with the standard image, find the "overheated" or "overcooled" points, which reflect the manufacturing defects of the board.
Online testing is the end of the crossbow
For in-circuit test technology, manufacturers and the industry are striving to seek such a goal: through as much information as possible about the electrical performance defects of circuit boards.
There are three main areas of work being carried out around this goal:
The first is to strengthen the research and implementation of testability design of circuit boards, including the use of boundary scan technology (digital device: IEEE1149.1; hybrid device: IEEE1149.4) and other built-in test technologies that have become the industry standard.
The second is to make full use of circuit theory and computer-aided design data of circuit boards to develop more advanced test algorithms. This algorithm makes it possible to estimate the electrical states of other nodes by testing some nodes.
The third is to balance the use of resources of online testing and other test equipment to optimize the overall test and inspection architecture.
However, despite these efforts, the importance and dominance of online testing has wavered. On the contrary, functional test technology, which was relatively slow due to the rise of online testing, will regain its momentum.