The renaissance of functional test technology is an inevitable consequence of the miniaturization of surface mount devices and circuit boards. Once any system is so small that it is difficult to probe the internals, all that is left are some input and output channels that communicate with the outside world of the system, and this is where functional testing comes in.
This situation is exactly the same as it was in the early days of functional testing development thirty or forty years ago. However, what is different from the past is that today's international standards for functional test instruments (such as PXI, VXI, etc.) have gradually matured, and standard instrument modules and virtual instrument software technologies have been widely used, which greatly increases the versatility and flexibility and help reduce costs. At the same time, the results of design-for-testability of circuit boards, and even the results of design-for-testability of very large-scale hybrid integrated circuits may be transplanted into functional test technology. Utilize the standard interface of the boundary scan technology and the corresponding testability design, the functional tester and the online test equipment can be used to program the system online. There is no doubt that future functional testers will tell us much more than a "pass or fail" verdict.
The continuous miniaturization of surface mount devices and circuits has relentlessly driven the obsolescence and evolution of some related test techniques. Under the evolutionary pressure of electronic miniaturization, technology, like species, follows the simple law of "survival of the fittest". Keeping an eye on the path of testing technology can help us predict the future.
Since surface mount technology (SMT) began to gradually replace socket mounting technology, the components mounted on the circuit board have become smaller and smaller, while 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 overwhelmingly used ten years ago, account for only about 10% of the total number of similar devices today; and the consumption of 0603 devices has also begun to decline four years ago. , replaced by 0402 devices. At present, the smaller 0201 devices are gaining popularity. It took about ten years to switch from 0805 to 0603. Undoubtedly, we are in an era of accelerated miniaturization. Let's look at surface mount integrated circuits. From the dominant quad flat package (QFP) ten years ago to today's flip chip (FC) technology, various packaging forms have emerged, such as thin small pin package (TSOP), ball array package (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 have decreased significantly, while the pin density of the device has increased rapidly. For a chip with the same logic function complexity, the area occupied by the flip-chip device is only one-ninth of the area occupied by the original quad flat package device, and the height is only about one-fifth of the original one.
Micro-packaged components and high-density PCBs bring new challenges to testing
The shrinking size of surface-mount devices and the consequent high-density circuit mounting have brought great challenges to testing. Traditional manual visual inspection is not suitable even for medium-complexity circuit boards (such as a single panel with 300 devices and 3500 nodes). Someone once conducted such a test, allowing four experienced inspectors to inspect the solder joint quality of the same board four times. The result was that the first inspector found 44 percent of the defects, the second inspector had 28 percent of the same results as the first, and the third inspector had the same results as the former. Two had 12 percent agreement, while the fourth inspector had only 6 percent agreement with the first three. This test exposed the subjectivity of manual visual inspection, which is neither reliable nor economical for highly complex surface mount circuit boards. Manual visual inspection is practically impossible for surface-mount circuit boards using microball-array no-package, chip-scale packages, and flip-chips.
Not only that, due to the reduction of the pin pitch and the increase of the pin density of surface mount devices, the bed of needles in-circuit test is also facing the dilemma of "no vertical cone". According to the forecast of the North American Electronics Manufacturing Planning Organization, after 2003, the use of online testing to detect high-density packaged surface-mounted circuit boards will not be able to achieve satisfactory test coverage. Based on the 100% test coverage rate in 1998, it is estimated that after 2003, the test coverage rate will be less than 50%, and after 2009, the test coverage rate will be less than 10%. As for the troubles of backside current drive, test fixture cost and reliability in the online test technology, there is no need to think more about it. Just because the test coverage rate is less than 10% in the future, the fate of this technology is already doomed in the future. .
So, can we submit a circuit board to final functional testing when human eyesight is beyond the reach and machine probes have nowhere to reach? Can we bear to test for several minutes and only know that the circuit board is faulty, but not what is going on in this "black box"?
Optical inspection technology brings new testing experience The development of technology will never stand still because of the above difficulties, and test inspection equipment manufacturers 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 the semiconductor chip manufacturing and packaging process before being widely used in the circuit board manufacturing industry. However, they will require further innovations to truly address the testing difficulties posed by the miniaturization of surface-mount devices and high-density boards.
At the same time, the major in-circuit test and functional test equipment manufacturers in the industry have been unable to meet the future development trend. The countermeasures they take are to acquire relatively small manufacturers of automatic optical inspection equipment and X-ray inspection equipment, so that they can quickly master related technologies and quickly enter the market.
Whether it is automatic optical inspection technology or automatic X-ray inspection technology, although they can help complete the work that manual visual inspection is difficult to do, 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 optical and X-ray technology, so in the next few years, it is expected that the technology in the generation of high-resolution optical images of circuit boards and true 3D X-ray images will continue to grow. progress.
In addition, today's relatively cheap storage and computing technologies make it possible to process large-capacity image information. What needs to be innovated in this field is the algorithm of image processing, and the combination of the most basic image enhancement and pattern recognition technology with expert system. These expert systems are based on the computer-aided design and manufacturing data (CAD-CAM) of the circuit board, combined with the empirical data on the production line, can carry out self-learning and self-improvement of the algorithm of inspection and discrimination. Another possible development direction in this field is to expand the scope of the use of light spectrum. At present, the industry has begun 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 "too hot" or "too cold" points, which reflect the manufacturing defects of the board.
Online testing is the end of the line. For online testing technology, manufacturers and the industry are working hard to find such a goal: to pass as much information about circuit board electrical performance defects as possible. There are three main areas of work being carried out around this goal:
The first is to strengthen the research and implementation of circuit board testability design, including the use of boundary scan technology (digital devices: IEEE1149.1; hybrid devices: IEEE1149.4) and other built-in test technologies that have become industry standards.
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 state of other nodes by testing some nodes.
The third is to balance the resources of online testing and other testing equipment, and optimize the overall testing and inspection architecture.
But despite these efforts, the importance and dominance of online testing has faltered. On the contrary, the functional testing technology, which was relatively slow due to the rise of online testing, will regain its development momentum.
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