Man Versus Machine
March 04, 2015
By Thorsten Rother, YXLON International GmbH and Keith Bryant, SMT Solutions
Who would you rather entrust the inspection of the ABS circuit board in your car? In our latest article, published in the U.S. Tech in February 2015, we discuss this everlasting question in the field of X-ray inspection.
This discussion has gone on in bars for many years, ever since men raced against the early automobiles, now it carries on when they face each other over a chessboard. When someone has a car with Parking Assist it can provoke many adverse comments from friends and soon our cars will have auto drive systems. If you believe Professor Stephen Hawking, this will end in the nightmare scenario when AI (Artificial Intelligence) makes Human obsolete: “It (AI) will take off on its own and redesign itself at an ever increasing rate. Humans are limited by slow biological evolution, so couldn't compete and would be superseded”.
Within the X-ray world a similar debate has rumbled on, also for a long time, basically the merits of fully automated inline 2D and 3D X-ray versus offline 2D systems. It falls quite neatly into those who believe a human interface causes issues with results and those who believe that the human interface is needed to make accurate judgement calls on viewed images. The crux which we will investigate here is can a fully automated system be consistent and accurate enough and work with almost no false fails or passes, which cause issues within the factory or with customers. Or does the flexibility of an offline system allow the human at the GUI or human interface to have the ability to make the correct judgement call on a defect.
Inline Computed Laminography has been in use for the inspection of printed circuit boards for many years and 2D Offline has been around a long time in electronics too, FeinFocus being the original system in the 1970’s. Technology has moved on dramatically, but the basic split remains. Inline is about throughput and repeatability with no human intervention, it needs programming and also fine-tuning to get the best from it and the production line stops if it breaks.
2D offline requires an operator to make the key decisions in many cases, unlike inline it can zoom in and out and show angled views to allow the operator to fully inspect potential fails. However it is not really fast enough to do 100% inspection in anything but small batch shops.
Principle of Computed Laminography
Laminography was originally developed for medical applications for imaging with X-rays. By a coordinated movement of X-ray source and sensitized film in opposite directions above and below the patient, a certain plane within the patient, the so called focus plane, is being projected onto the same position during the entire scan and thus gives a sharp image. For all other planes the position of the projection moves during the scan and they are therefore blurred. In Medical X-ray this technique is called tomography. However, with this technique only one layer can be imaged per scan. In modern digital laminography the film is replaced by a digital flat panel detector, which acquires many images during the movement. By means of a computer these images are superimposed to generate a blurring tomogram as with the classical technique. However, by applying an appropriate shift before superimposing the images, it is possible to obtain sharp images from many other layers and such to receive a full 3D image of the object. This reconstruction technique is called tomosynthesis.
Fig.: Basic principle of laminography. Features in the focus plane are always projected on the same position of the detector.
Features outside this plane are blurred.
Principles of 2D offline X-ray Inspection
Again the design of these systems varies, but the basics are really the same, an X-ray source fires a beam through the sample and some radiation is absorbed and some gets to the collector or detector. To allow a more detailed inspection of the assembly, the sample itself, the tube and/or detector can be rotated and the sample moved closer to the tube for higher magnification, this allows the operator to view the potential fault in much more detail than the inline options.
So now we have the technology and ethos comparisons of these two ways to X-Ray inspect PCB Assemblies, very different ways of solving the problem of how to inspect the things that cannot be seen with the naked eye, or even AOI systems. In Line relies on the skill of the programmer to make and debug a program to inspect assemblies and make deductions based on the results of algorithms as to the acceptability of the products. These will be based on measurements taken at reasonably low magnifications with information from only some slices through the assembly.
Offline relies on the ability of the system to display the faults and an operator to decide what is acceptable and what is a failure, he does this using zoom, angled view, measurement algorithms, filters, contrast stretch etc. All tools under his control to allow him to make an informed decision on the fault, there is flexibility in this approach that allows more checking before making a decision. So dear reader the answer is now for you, whose brain would you prefer checking the ABS circuit board on your car?
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