The development of PCB test technology

The revival of functional testing technology is the inevitable result of the miniaturization of surface mount devices and circuit boards. Once any system is small enough to be difficult to detect inside, the only thing left is some input and output channels that interact with the outside world of the system, and this is where functional testing comes in.

This situation is exactly the same as the early stage of functional testing development 30 or 40 years ago. However, unlike the past, today the international standards for functional test instruments (such as PXI, VXI, etc.) have gradually matured, and standard instrument modules and virtual instrument software technology have been widely used, which greatly increases the versatility and flexibility of future functional test instruments and helps to reduce costs. At the same time, the testability design results of circuit boards and even the testability design results of ultra-large-scale hybrid integrated circuits may be transplanted into functional testing technology. Using the standard interface of boundary scan technology and the corresponding testability design, functional testers can be used to program the system online just like online test equipment. Undoubtedly, future functional testers will tell us much more information than judgments such as "pass or fail".

Surface mount devices and circuits have been in an endless process of miniaturization, which has relentlessly driven the elimination and evolution of some related test technologies. Under the evolutionary pressure of miniaturization of electronic products, technology, like species, follows the simple law of "survival of the fittest". Paying attention to the development path of test technology can help us predict the future.

Since surface mount technology (SMT) began to gradually replace socket mounting technology, the components mounted on circuit boards have become smaller and smaller, while the functions contained in the unit area of ​​the board have become more and more powerful.

就无源表面贴装器件来说，十年前铺天盖地被大量使用的0805器件，今天的使用量只占同类器件总数的大约10%;而0603器件的用量也已在四年前就开始走下坡路，取而代之的是0402器件。目前，更加细小的0201器件则显得风头日盛。从0805转向0603大约经历了十年时间。无疑，我们正处在一个加速小型化的年代。再来看表面贴装的集成电路。从十年前占主导地位的四方扁平封装(QFP)到今天的芯片倒装(FC)技术，其间涌现出五花八门的封装形式，诸如薄型小引脚封装(TSOP)、球型阵列封装(BGA)、微小球型阵列封装(μBGA)、芯片尺度封装(CSP)等。纵观芯片封装技术的演变，其主要特征是器件的表面积和高度显著减小，而器件的引脚密度则急聚增加。以同等逻辑功能复杂性的芯片来讲，倒装器件所占面积只有原来四方扁平封装器件所占面积的九分之一，而高度大约只有原来的五分之一。

Micro-packaged components and high-density PCBs bring new testing challenges

The continuous reduction in the size of surface mount devices and the subsequent high-density circuit installation have brought great challenges to testing. Traditional manual visual inspection is not suitable even for circuit boards with medium complexity (such as single-sided boards with 300 devices and 3,500 nodes). Someone once conducted such an experiment in which four experienced inspectors conducted four inspections on the quality of solder joints on the same board. As a result, the first inspector found 44% of the defects, the second inspector had a 28% consistency with the first inspector, the third inspector had a 12% consistency with the first two, and the fourth inspector had only a 6% consistency with the first three. This experiment exposed the subjectivity of manual visual inspection. For highly complex surface mount circuit boards, manual visual inspection is neither reliable nor economical. For surface mount circuit boards using micro-ball arrays without packaging, chip-scale packaging and flip chips, manual visual inspection is actually impossible.

Moreover, due to the decrease in pin spacing and increase in pin density of surface mount devices, the bed-of-nails online test is also facing the dilemma of "no place to stand". According to the North American Electronic Manufacturing Planning Organization, after 2003, the use of online testing for high-density packaged surface mount circuit boards will not be able to achieve satisfactory test coverage. Based on the 100% test coverage 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 testing technology, there is no need to consider them anymore, because the future test coverage of less than 10% has doomed the fate of this technology.

So, can we submit the circuit board to the final functional test when human vision is not up to the task and the machine probes are out of reach? Can we endure several minutes of testing and only know whether the circuit board is broken or not, but have no idea what is going on in this "black box"?

Optical inspection technology brings new testing experience. The development of technology will never stagnate because of the above difficulties. Test and inspection equipment manufacturers have launched 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 they were widely used in the circuit board manufacturing industry. However, they still need further innovation to truly cope with the testing difficulties brought about by the miniaturization of surface mount devices and high-density circuit boards.

At the same time, the industry's major online test and functional test equipment manufacturers are unable to meet the future development trend. Their countermeasures are to acquire relatively small automatic optical inspection equipment and X-ray inspection equipment manufacturers to quickly master the relevant 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 cannot do, their reliability is not completely satisfactory. These technologies are highly dependent on computer image processing technology. If the original optical image or X-ray image does not provide enough information, 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 for generating high-resolution optical images of circuit boards and true three-dimensional X-ray images will also make progress.

In addition, today's relatively cheap storage and computing technologies make it possible to process large amounts of image information. What is urgently needed in this field is the image processing algorithm, as well as 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, and can self-learn and self-perfect the inspection and discrimination algorithms. Another possible development direction in this field is to expand the range of spectrum used. The industry has begun to try to capture and analyze infrared images of circuit boards when the boards are powered on. By comparing the infrared image with the standard image, the "overheated" or "overcooled" points can be found, thus reflecting the manufacturing defects of the board.

Online testing is a spent force

For online testing technology, manufacturers and the industry are striving to achieve the goal of obtaining as much circuit board electrical performance defect information 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 that has become an industry standard (digital devices: IEEE1149.1; hybrid devices: IEEE1149.4) and other built-in test technologies.

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 allows the electrical status of other nodes to be inferred by testing some nodes.

The third is to balance the use of resources of online testing and other testing equipment to optimize the overall testing and inspection architecture.

However, despite these efforts, the importance and dominance of online testing have been shaken. On the contrary, functional testing technology, which once developed relatively slowly due to the rise of online testing, will regain its momentum.