Automated test equipment saves cost and enhances efficiency for product manufacturers. Let us see out what drives the most popular automated test equipment used in the industry
Testing has always been a vital part of manufacturing industries, but rapid advancements at Silicon Valley have resulted in a highly competitive market. This has driven the need for reduction in the product development cycles, and at the same time ensuring product reliability and quality, which makes it even more impactful. The speed and effectiveness of product testing have a significant impact on quality assurance and time to market for the products.
Automated test equipment (ATE) contributes towards product success by providing swift and reliable testing and monitoring solutions. There are a variety of ATEs being used in the electronics industry today, each having a different approach for the testing and measurement.
Automated optical inspection
One of the efficient testing solutions for today’s small, compact and complex electronics is automated optical inspection (AOI). It is a non-contact test method for automated visual inspection, which can be used for the detection of a variety of surface defects in PCBs.
How does it work? AOI system scans the board with high-definition cameras and then processes the captured images for analysis and inspection of defects. Imaging systems are the heart of the AOI systems. Today we have more flexible and scalable imaging sensors—best suited for the high-end microelectronics. Strobed inspection modules for image acquisition have also contributed to better imaging sensor resolution, resulting in an increased speed and accuracy of the AOI systems. Introduction of 3D AOIs has simplified the inspection of SMT devices on finished PCB assemblies, yielding precise height measurements for the correct detection of lifted components and lifted lead defects along with the solder volume post reflow.
Rajeev Kaushal, general manager-PES, eInfochips, explains, “To improve performance in terms of speed and accuracy, algorithms are optimised and ported on DSP/FPGA platforms to carry out online AOI. To avoid computational complexities, the lighting and locational parameters are controlled, which results into minimal use of pre-processing algorithms, hence improvement in execution. Experience-based new logics are also designed and implemented on platforms for specific AOI applications. It has improved accuracy tremendously and the platforms are integrated with external systems for faster response and action. The new AOI platforms also support integration of multiple sensors that cater to the needs beyond optical inspection and include thermal inspection also.”
Most AOI systems’ lighting requirements depend upon the type of defects, product and distance from the camera. The positioning of lighting as well as type of light source is also very important, shares Rajeev. Ideally, full surface should be uniformly lit without shadow, shine or any artefacts because of lighting. Different types of lighting are available:
1. Fluorescent lighting. Thoug widely used for PCB AOI, the main problem with fluorescent lighting is that the lamps degrade with time. This means that the AOI system will be subject to constantly changing levels and quality of light.
2. LED lighting. LED lighting is very well received for AOI applications, as level of lighting can be controlled and it does not degrade.
3. Infrared or ultraviolet. Required for specific applications.
Trends in AOI. The component size and power consumption are extremely important factors in AOI as they help in mobility as well as maintenance. “Trend is towards miniaturisation of components and compact AOI machines. Application-based right type of sensors, platform with proper connectivity and integrated sensors with smaller size and ease of use are limited in market. The market is growing and near threshold. Once it crosses the threshold (say 2-3 years from now), availability of platforms, components, integration and services would not be a challenge,” explains Rajeev.
Automated X-ray inspection
Automated X-ray inspection (AXI) systems are almost similar to AOI systems and are also used for PCB inspection. High-density ICs and BGAs (ball grid arrays) are difficult to test with AOI systems as they have connections underneath the packages. AXI systems use X-rays instead of visible light and thus are able to inspect the elements that are not visible to AOI systems.
High-focus systems with sub-micron spot sizes are ideal for the MEMS, opto-electronics, mobile phones, telecom electronics, BGAs and loaded PCBs. 3D imaging has increased AXI performance manyfold, as AXIs are having angled 3D imaging with optional computed tomography (CT) system for reconstruction of 3D images to gain more insight into components and sub-assemblies. Higher magnification for zooming in on specific areas with user-friendly software adds to the AXI testing ranges.
In-circuit testing (ICT) is a comprehensive PCB testing technique, which measures circuit parameters such as resistance and capacitance, along with the operation of analogue components such as operational amplifiers and some functionality of digital circuits.
ICT systems gain direct electrical access to the components on a PCB via an electromechanical ‘bed of nails’ fixture and test for manufacturing faults. Advancements in ICT systems have contributed towards lowering overall manufacturing test costs by improving fault coverage, reliability and throughput of in-circuit production tests.
Fully automated ICT systems having compact size and integration with high-volume production lines minimise the need for operator handling. This saves labour costs and reduces the risk of product damage due to electrostatic discharge (ESD). Capabilities of ICT systems have advanced far beyond when they were first introduced. Now, with all the additional electrical test capabilities that do not require actual physical test access, we can categorise today’s versatile test systems as ‘electrical test controllers’ rather than ‘in-circuit testers.’
“Although ICT is appreciated for component-level testing, testing of components that are connected in parallel is almost impossible,” adds T. Anand, managing director, Knewron.
ICT to ETC transition. ICTs have been popular since long due to their usage in high-volume production environments, mostly for simple tests. “However, ICT is the most tedious, cumbersome and expensive type of testing. Creating an ICT fixture is costly and time-consuming activity. It is typical example of White Box (Glass Box) testing. Now, electrical test controllers (ETCs) aka automated test controllers ATCs do much better job than ICTs. Their testing method can be categorised as Black Box (full function) testing too,” says T. Anand.
ETCs do excellent job in simulating end-user, which gives more valuable information than ICTs. In short, ICTs have grown up to be ETCs.
Typical digital ICTs deploy boundary scan technique and perform basic digital verification, unlike ETCs where test case is generated, voltage/data is applied to respective pins and output is read and verified.
Boundary scan is the unique solution to many test requirements and provides information about the board without accessing the complete board. It is ideal for testing those complex boards that could not be tested otherwise due to lack of test access.
Boundary scan systems have little hardware and their significant part is software. The well-established test technique boundary scan JTAG IEEE Std. 1149.x requires test program to be generated before it can be used. Being an integral part of a boundary scan system, test program generator uses the net list of the unit under test and the BSDL files of the boundary scan components contained within the circuit to create test patterns for the rest. The test program generator also creates test vectors that enable the system to detect faults on the nodes and non-boundary scan components that are surrounded by boundary scan devices.
Functional automated test
Digital boards are well suited for testing with functional automated test (FATE) systems where much of the program can be generated automatically by entering the circuit data into the tester, which builds up its own picture of the circuit and then, with knowledge of the pin connections, it can build up a test program for the board. The simulations can reveal design problems such as race states or even the non-required circuitry. Unfortunately, the program generation usually needs a lot of finishing, which is a time-consuming process. Also, the analogue areas often require analogue-measuring instruments to be used and need to be programmed manually. This manual programming can be very expensive to implement.
Although, FATE systems can be very fast at finding functional faults with a board, they are not always so fast in finding the problem area. In most cases they are unable to locate a problem because of lack of view of the internal areas of the board. A guided probe can be connected to the tester that can be manually applied to different points on the circuit under program control to check the points on the board that are not accessible via the bed of nails. Again, in the case of analogue areas, the routines required for fault finding using guided probe need to be programmed manually, which can be particularly time consuming.
Design for testability
Boundary scan—and any other test technology—requires design rules that must be considered. If such design rules are disregarded, the achievable test depth might be considerably affected, or in extreme cases, completely lost. “Nothing is ‘sadder’ than a board that cannot be tested because of one missing interconnection. But there’s no need to worry about possibly ‘many’ design rules. Convenient software provides support for rule compliance,” adds Harish. Moreover, it once again demonstrates that it makes sense to start with test generation at a very early stage of product design. Once the layout is finalised, things are relatively hard to change.
Pankaj V. is a technical journalist at EFY, Gurgaon, while Dilin Anand is senior technical correspondent at EFY Bengaluru. You can read the full story on www.electronicsforu.com in the Test and Measurement section