2D or not 2D, that is the question
01 March 2019
The use of automated optical inspection (AOI) machines is a popular method of inspection – especially with today’s increasing use of microelectronics – and is fast becoming an integral solution for the surface-mount technology (SMT) electronics manufacturing industry.
This article was originally featured in the March 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
AOI is the automated visual inspection of PCBs during manufacture, using a camera to scan the assembly for defects that would lead to board failure. It can be used at many stages throughout the manufacturing process, including bare board inspection, solder paste inspection and populated board inspection (pre- and/or post-reflow). These systems have become more important for manufacturers, because they provide incomparable reliability to help maintain high quality of boards, with thousands of solder joints, and complex and intricate components.
However, does this mean AOI can be the ultimate gatekeeper to identify defects early in the process? The answer may be both yes and no. Due to increasing component miniaturisation, optical inspection systems face many physical limitations, which true 3D-based systems are now trying to overcome.
The path of AOI inspection
To understand 3D AOI, let’s first consider its predecessor. 2D AOI was introduced in the early 80s, and continues to be one of the most popular forms of inspection today. However, with the increasing ‘micro’ trend, and the use of multiple stacked dies in components, due to the small footprints being used, the 2D AOI inspection method does not always live up to expectations. As technology advances, and the world’s insatiable demand for the smallest, smartest and most powerful electronic devices increases, so too must production techniques and inspection processes.
Many customers felt that 2D AOI was ‘good enough’ for their commercial application. However, with increasing interest in 3D AOI technology, and solutions that intelligently combine 2D and 3D technologies to overcome any limitations and provide superior results, the tide is changing.
When looking at the path of the optical inspection market, there has clearly been advancement, and we can see how unsolved difficulties help drive innovation. Solder paste inspection (SPI) has undergone a shift from 2D to 3D, because 2D technologies that were used to collect solder deposit images could not solve shadow problems.
We now see the same need for component inspection with AOI systems. As today’s board complexity is increasing, with more components, more joints, higher density, and new package technologies, 2D AOI technology using greyscale image analysis, or angled camera view of colour images, is no longer a viable option. Most decisions made based on ‘good-bad’ comparison of reference images can be affected by variables like component surface finish, board condition, component proximity, and more.
Although 2D AOI is still a major technology in the market, more and more manufacturers are adopting 3D AOI to increase board quality. 3D AOI improves measurement accuracy by providing true measurement of features, and detecting aspects like ‘lifted leads’ or slight changes in ‘coplanarity’. This enables detection of process drift, and allows correction of potential problems before they result in expensive value-added boards being scrapped.
Why choose 3D AOI?
Measurement accuracy is increasingly important within the electronic assembly industry, with the decreasing geometry of components, and the need to offer 100% reliable and traceable processes for critical applications. This is especially apparent in the automotive, aerospace, medical and telecoms industries, but is becoming increasingly important to most manufacturers as they strive to improve the reliability of electronic assemblies.
Not only does 3D AOI ensure inspection tolerances are precise, it provides meaningful insights about the process, helping eliminate the root causes of defects through measurement data generated from 3D AOIs. The 3D SPI, together with 3D AOI, enables manufacturers to accurately control and monitor the solder printing and component placement processes.
It is important when using 3D AOI equipment to ensure it covers the 3 ‘Rs’: reliable, repeatable and relatable. Another feature to consider is systems which intelligently combine 2D and 3D technologies to offer the best inspection and generate meaningful data (also satisfying the three ‘Rs’).
The development of 3D AOI has led to a step change in product quality. Greater visibility of problem areas is possible, with the ability to measure features, monitor process changes and take corrective action before the line starts producing failures. Without this, process changes, such as screen print deposition, go unnoticed – and boards are assembled, possibly passing 2D inspection, but with insufficient solder paste in some areas, the joints may be prone to early life failure. In the worst case, they may pass a functional test and be used in an assembly which ends up in a critical part.
For example, the world’s leading car manufacturers do not want ABS modules causing brake failures in the field, where recalls and litigation cost could run to millions, so will insist on sub-contractors making such parts using tried and tested AOI systems that they have qualified, after exhaustive testing.
What’s out there?
When looking at 3D AOI equipment, check its limitations. Some may offer restricted levels of 3D coverage for components and solder joints. Look for innovative systems that provide full 3D coverage to identify boards with defects and monitor performance. Think of the 3 ‘Rs’: ensure it inspects common defects like missing or wrong components, but also accurately identifies coplanarity and lead bridging issues, which are prevalent in small packages and BGAs.
There are systems on the market that measure the z-axis profilometry of the entire board. By measuring components and solder joints, and then offering critical height information to the inspection algorithms, manufacturers can identify errors like pad overhang and insufficient solder. This results in process optimisation, something that is difficult to achieve for 2D AOI systems, as they do not offer height information, and it is impossible to accurately measure and quantify shape, coplanarity and solder amount.
Unlike 3D AOI systems using a brightness threshold, those which use the height threshold to extract critical body and the lead tip information result in data which provides reliable body and joint measurements with CAD dimensions.
Look for systems that combine innovative new technology. Does the 3D AOI system include superior hardware and industry-leading optical technology that generates reliable and repeatable data? Can this data be stored in a central database, which can be used in deploying an optimal program for multiple operations and reducing programming and setting condition times? There is a checklist of features that need to be ticked before committing to the installation of 3D AOI system. To ensure your list contains the correct items, it’s always best to consult with an expert in the field.
As boards become more complex, with increasing component miniaturisation, the inspection process, and how it can be achieved accurately, becomes increasingly important to electronic components’ longevity. The challenges and limitations of optical inspection systems can no longer be overlooked, because they can be the cause for false calls and escapes. 3D AOI systems are the answer, but be sure to think twice when suppliers claim 3D inspection – is it modified 2D or full 3D measurement capability? Look for systems that combine technologies with superior hardware to provide true 3D optical inspection solutions – and help guarantee that your inspection process is faultless.
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