Frequently Asked Questions - X-RAY

A two dimensional (2-D) x-ray inspection system is basically an x-ray microscope. X-rays are produced by an x-ray tube and pass through the sample into an x-ray image capture device, such as a digital image intensifier, which converts the x-rays into images that the operator views. Any object or material within the analysed sample that has material of higher density than the surroundings will absorb more of the x-ray beam and so cast a darker shadow on the detector (see figure). In this way, solder and copper tracks appear dark compared with the laminated circuit board in a PCB, for example, when viewed in an x-ray system. In-system sample manipulation is required to pan over the test object (x and y plane) as well as to alter the available magnification (z-direction). Further details and definitions of the key features of x-ray inspection systems can be found in the ‘glossary of terms’ section of this web site.

X-ray inspection systems are used by the printed circuit board and semiconductor industries to check and monitor the quality of their manufacturing processes and products. It offers a way to investigate, in a non-destructive way, the locations and joints within their products that cannot be inspected optically. For example, x-ray inspection allows investigation of the solder joint quality, in situ, for area array packages such as Ball Grid Arrays (BGAs), flip chip devices and other chip scale packages (CSPs). It can also be used for investigating the wire attachment quality within semiconductor device packages. Many other applications exist. Without x-ray inspection, the only alternative would be by the damage/removal of the part in question for subsequent optical investigation. However. This optical approach makes the product/device unusable in the future and destroys the environment that needs to be investigated in the process.

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• The right system will depend on your specific application. However, the questions you should be asking your potential x-ray system supplier (using the requirements for a system for general printed circuit board assembly needs, or PCBA, that includes BGA and flip chip inspection as an example) should include, amongst others:
• Does the system have an open x-ray tube to give me the highest resolution and magnification, especially as devices and joints are continuing to shrink in size?
• Does the system tilt the detector instead of tilting the sample to achieve oblique angle views without compromising the available magnification?
• What is the maximum oblique angle view?
• Can less skilled operators use the system simply and easily without compromising the analytical quality?
• Does the system have collision-free sample movement?
• Does the system have a digital detector to provide the best image quality in real time?
• What is the greyscale sensitivity of the system?
• Can I automate my inspection procedures?
• Can I measure BGA void percentages and BGA solder ball diameters manually and automatically?
• Does what I need come as standard or what additional system options will I need to ensure that I have the facilities that I need?
• What is the price – including all the necessary options for my needs if not provided as standard!
• Additional technical information to help you answer these questions can be found at the following locations:

Differences between tilting the sample and tilting the detector to achieve oblique angle views Selection Criteria For X-ray Inspection presented in the Proceedings of Nepcon, Shanghai, 2003.

Differences Between Open And Closed Tubes presented in the Proceedings of SMTA International, Chicago, September 2002.

Comparison Between Digital And Analogue X-ray Inspection For BGA, Flip Chip and CSP Analysis presented at the Proceedings of APEX, Anaheim, February 2004.

Using Digital X-ray Imaging As A Process Control Tool For Lead-free PCB Assembly presented at the Proceedings of SMTA International, Chicago, September, 2004.

• Your local Dage representative will be able to advise you as to the best x-ray system for your needs and can arrange to x-ray some of your samples so that you can see what x-ray inspection can offer your business. Please click here to locate your nearest Dage office or Dage representative.

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The fundamental difference is that digital x-ray systems use a digital detector to provide the x-ray image and analogue systems, an analogue detector. In performance terms, digital detectors provide substantially more pixels in the image than analogue detectors and, more importantly, provide a higher level of greyscale sensitivity. The greyscale sensitivity allows better separation in the image of similarly dense material within the sample allowing more subtle variations/faults to be detected. For example, only digital detectors have sufficient greyscale sensitivity to observe ‘champagne’ or ‘interfacial’ voiding within BGA solder joints in addition to the ‘process’ or ‘bulk’ voids that can be seen by analogue and digital systems.

Further information on the different types of voiding that can be seen using digital x-ray inspection systems can be found in the following technical paper presented by Dage at the Proceedings of SMTA International, Chicago, September, 2004 - Digital X-ray Process Control

Dage provides a digital detector, as standard, in all of its x-ray inspection systems. This is the XiDAT detector and is a digital image intensifier providing a 1.3 Mpixel image at 25 frames per second (real time imaging) with a 16-bit greyscale sensitivity. An alternative digital detector that could be used is a CMOS flat panel detector. Further information on the comparison between digital and analogue x-ray inspection for BGA, flip chip and CSP analysis can be found in the following technical paper presented by Dage at the Proceedings of APEX, Anaheim, February 2004 by David Bernard and Steve Ainsworth  – Comparison Between Digital and Analogue X-ray Inspection.

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X-ray Integrated Digital Acquisition Technology. This is result of Dage’s revolutionary and pioneering work into improving x-ray image quality and providing it as standard, yet without cost penalties, to all of its x-ray inspection systems. It has been achieved by improving the entire imaging chain within the x-ray system in an integrated fashion. As a result, Dage’s advanced digital x-ray inspection systems offer high magnification, increased resolution and more greyscale contrast at different angular views, as standard, compared with analogue image intensifiers. These features, so crucial for fault finding within the PCB and semiconductor industries, are coupled with Dage's ‘Image Wizard’ x-ray operating system, the most user-friendly on the market which includes the Dage Image Wizard, to provide deskilled, automated inspection without compromising analytical quality. This is achieved by using all-digital data transfer that allows real-time, digital x-ray inspection at a resolution of 1.3M pixels. All the data is displayed, as standard, on a 19" digital 1600 x 1200 LCD display.

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NO

By utilising the latest hardware and software technology, Dage can offer all these high specification features at a price that is not only dramatically less than competitors require for their nearest equivalent, but this price is also less than many competitors demand for their inferior specification machines. Why compromise having the best image quality for your x-ray analysis needs, when you can get the best for less?

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An x-ray tube is the device that produces the x-ray radiation for the inspection system. In essence, an x-ray tube is an evacuated cylinder within which electrons are produced, accelerated by an applied voltage and driven to strike a metal target. The effect of the electrons hitting the target is to produce the x-rays. The vacuum is required within the tube so that the electrons can travel down to the target without being absorbed by atmospheric contamination. Traditionally, x-ray inspection systems have used what are called closed x-ray tubes, where the vacuum is produced during manufacture and the tube sealed allowing no access to the components within. In recent years, open, or demountable, x-ray tubes have become more popular, if not the norm, for PCB and semiconductor x-ray inspection because of their ability to provide much higher magnification, much better resolution and serviceability (by allowing access to the consumable items of target and filament). In open x-ray tubes, evacuation of the tube is achieved through the use of vacuum pumps supplied with the x-ray system

Further information on the differences between open and closed x-ray tubes, and their effects on the capabilities of an x-ray inspection system, can be found in the following technical paper Differences Between Open And Closed Tubes presented by Dage in the Proceedings of SMTA International, Chicago, September 2002.

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Existing and future x-ray equipment will be able to see the faults generated by using lead-free materials, such as under BGA and CSPs, as well as within the rest of the SMT components. All that may be necessary is the adjustment of certain x-ray tube parameters, compared to those used for equivalent Sn/Pb products, to ensure the best image contrast is available to allow for easy analysis.

Further information on the implications for using x-ray inspection with lead-free materials can be found in the following technical papers presented by Dage at the Proceedings of SMTA International, Chicago, September 2003 - Implications Of Using Lead-Free Solders and at the Proceedings of SMTA International, Chicago, September 2004 - Does PCB Pad Finish Affect Voiding Levels In Lead-Free Assemblies

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The greater greyscale sensitivity that digital x-ray inspection systems provide allow a far more subtle investigation into joint quality, especially of BGAs. For example, they are able to interrogate the joint interfaces of a BGA far better and indicate the presence of interfacial voiding within the solder balls in addition to bulk, or process, voiding. Further information on the different types of voiding that can be seen using digital x-ray inspection systems can be found in the following technical paper presented by Dage at the Proceedings of SMTA International, Chicago, September, 2004 - Use Of Digital Imaging As A Process Control Tool

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AXiS is the Active X-ray image Stabilisation facility that is provided, as standard, on Dage’s XD7600 x-ray inspection systems. It is Dage’s unique solution to provide vibrational isolation of the system down to 4 Hz. It has been included to counteract vibrational interference that might affect image quality, especially when operating the x-ray system within the production environment to investigate samples at the highest magnification and smallest resolutions. Your local Dage representative will be able to tell you more. Please click here to locate your nearest Dage office or Dage representative.

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If you are only able to inspect solder bumps from directly above the sample then the physical bulk (i.e. main mass) of the solder ball will mask any subtle variations that might occur, and be seen, in the interfacial areas between the solder ball and the device, on one side, and the pad on the other. It is these interfacial areas that will tell you if the joint quality is acceptable. By looking at the joint at an oblique angle view, and as important being able to select that angle view at any point around the joint, then the bulk of the solder ball is partially removed from interfering with the investigation of the interfaces, allowing solder ball shape and interfacial variations (as well as voids) to be more clearly seen in the x-ray. If you consider a good BGA solder ball joint to be like a soccer ball sandwiched (and partially squashed) between two flat pieces of wood and then consider the diagrams below, then by only being able to look form the top down will limit the information that can be obtained, compared to having access to oblique angle views. The larger the oblique angle that is possible then the better the separation of the bulk of the ball and the interface becomes.

Using digital x-ray inspection, which has much greater greyscale sensitivity compared with analogue x-ray inspection, then more information is able to be seen within the BGA solder balls. For example, the distinction and differences between interfacial voiding and bulk voiding within the solder joint, as well as being able to distinguish the actual interfaces from the bulk of the solder ball. An example is shown below where the interfaces have been highlighted to aid description. Further information can be found in the following paper presented at the Proceedings of SMTA International, Chicago, September, 2004 - Use Of Digital Imaging As A Process Control Tool.

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The geometric magnification of an x-ray system is the ratio of the (distance between the x-ray tube focal point and the system detector) to the (distance between the x-ray tube focal spot and the sample), as shown in the diagrams. So the closer the sample can get to the x-ray tube focal spot the greater the geometric magnification a system can achieve. Therefore, the thinnest samples will always be able to achieve the highest geometric magnifications in an x-ray system. With this in mind, being able to move the sample as close as possible to the x-ray tube without the danger of sample collision or damage becomes very important. The x-ray tube focal spot is the position within the tube where the x-rays are actually produced and this is not the same place as the outer cover of the tube from where the x-rays emerge.

With the above ratio in mind it could be assumed that by increasing the distance between the x-ray tube focal point and the system detector, or A in the diagram, then larger and larger magnification could be achieved. Whilst this is true in principle, the laws of physics unfortunately do not allow this luxury. This is because although the magnification will increase linearly with distance, the intensity of the x-rays coming from the x-ray tube decrease at the inverse square rate at the same time. In other words, doubling the distance A will double the geometric magnification but at the same time the intensity of the x-rays reaching this longer distance has decreased by a factor of 4. So it will take 4 times as long to acquire the same number of x-ray photons at this larger distance to produce the same quality image at the closer distance. Therefore, a compromise must be reached in real x-ray inspection systems to balance image quality, speed of use and realistic magnification.

Geometric magnification should not be confused with the system, or total, magnification. The system magnification is a product of the geometric magnification and the magnification of the imaging chain, as shown in the diagram. It is the ratio of (size of the object on the operator screen) to the (actual size of the object).

The system magnification provides a larger number than the geometric magnification. However, if the detector area used upon which the image is captured and/or an increasingly large operator display is provided, then the system magnification can be raised to very high numbers without actually providing any more meaningful magnification of the actual object under examination. Some image intensifiers, for example, can be operated in what is called double or triple-field mode and this will increase the system magnification values dramatically, but do not change the geometric magnification.

Further information on geometric magnification with open and closed x-ray tubes, and their effects on the capabilities of an x-ray inspection system, can be found in the following technical paper Differences Between Open And Closed Tubes presented by Dage in the Proceedings of SMTA International, Chicago, September 2002.

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The operator can quickly and simply obtain a ‘navigation map’ x-ray image of the entire inspection area, or of a single board or component, through the Dage "Image Wizard" operating software. The navigation map is built up from a sequence of x-ray images like a mosaic. The Dage "Image Wizard" software provides a location box on the navigation map that clearly identifies where in the sample the x-ray inspection system is looking. As the location box automatically adjusts in size over the navigation map as the magnification changes, relative to the size of the actual area being shown in the main x-ray image, the operator always knows where they are inspecting on the sample, whatever the magnification used. The navigation map and location box are automatically saved alongside the main x-ray image (as shown in the example below of an oblique angle view of a flip chip at medium magnification). In this way, the location and nature of the fault can be quickly identified once the sample has been removed from the inspection system to allow rework and repair to be quickly realised.

We do not require the use of laser pointers or separate optical cameras to show the inspection location.

As the Dage navigation map is an x-ray image, both sides of the board can be seen at the same time, unlike in an optical image where only one side can be seen. Therefore, bottom-side fault locations are clearly identified and do not have to be inferred from their relative position to the side that can be seen, which could make subsequent fault location for rework and repair more confusing.

New inspection locations to be investigated within the sample can be quickly selected by simple mouse click on to the navigation map. The operating software quickly drives the manipulator to the necessary position. The oblique angle view currently being displayed will also be used in the new location. The software and system design ensures that the sample CANNOT collide with any other part of the system as the new view is achieved.

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If 2D x-ray inspection for PCB and semiconductor applications can be seen like taking an x-ray of a broken arm or leg in a hospital (although providing large magnification) then CT, or Computerised Tomography, can be seen like taking a CAT, or CT, scan of the body in a hospital. In a CT scan, a whole series of 2D x-ray images are taken at every angle all around an object and this is repeated along the object’s length. These images are then taken into a PC and computationally re-constructed so as to provide the ability to look at a complete x-ray image of any slice through the object. The accuracy of the reconstruction is dependent on maintaining the positional accuracy of the sample throughout taking the x-ray images. That is why the person in the hospital has to remain as still as possible when undergoing a CAT scan. Unfortunately, the magnification and resolution that are needed for PCB/Semiconductor CT examinations technically means that the final results of such a CT investigation is much more limited compared to what would be perceived as being the case based on a hospital examination. For example it is typically limited to samples of around 10mm x 10mm and requires a stand-alone system to provide the positional accuracy to enable the best CT re-constructions possible.

As it is, this process for PCB and semiconductor applications still requires that many x-ray images are taken for each sample and then the computing time and power needed to re-construct the final slices is also very high. This is why a CT scan will typically take many tens of minutes, if not hours, to complete. If more resolution is needed and the sample is larger then this will increase the time to produce the CT image substantially.

So to consider if a CT system is appropriate for your needs the following questions might be asked:

Does the substantial cost of a CT system, suitable for electronics applications, actually provide much more information on the sample for your needs than can be obtained, simply and quickly, from a single 2D image in a 2D x-ray system?

What is the maximum size of the object that can be inspected?

Must I destroy my sample to make it into an object of sufficiently small size to fit within the CT system?

How long will it take to produce a CT image for an object of that size and at what resolution?

If I sacrifice resolution to speed up the time of CT acquisition then will it provide sufficient information for my needs?

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The mechanical layout, with the x-ray tube below the sample and the digital detector above, together with the Dage "Image Wizard" software control, ensures that the sample CANNOT collide with anything in the system. This allows Dage to offer the advantage of having a simple ‘point and click’ user interface for all sample movement and control, including achieving oblique angle views at the highest magnifications, and there is no need for any complicated joystick controls.

To achieve the highest magnifications within an x-ray inspection system, the sample must be placed as close as possible to the x-ray tube. Because all Dage x-ray inspection systems have a system configuration where the x-ray tube sits below the sample, it means that our system design and interlocks can always ensure that the sample can be placed as close as possible to the x-ray tube simply and easily. Therefore, the Dage systems can always provide the highest magnifications through simple mouse click without any danger of sample collision.

In contrast, a system configuration that has the x-ray tube located above the sample, demands that any safety interlock against sample collision is limited to the tallest part of that sample. For any object on the sample below this tallest height then the magnification that can be achieved for that object will be severely reduced, as it cannot be placed any closer to the x-ray tube. This may limit the analytical information that can be obtained. Therefore, the only alternative for this configuration is to deliberately allow disabling of the anti-collision features. This would then allow a highly trained operator to very carefully move the sample, manually, closer to the x-ray tube than the safe limit permits so as to improve the magnification of the smaller objects. However, the danger of sample collision will always remain, whatever the skill of the operator.

Our sample manipulator keeps the sample horizontal at all times and allows x, y and z (magnification) movements. Our XiDAT digital detector, provided as standard in all Dage x-ray inspection systems, moves in the x and y directions. This allows the viewing of up to 70-degree oblique angles at any position around an item of interest, anywhere in the entire system inspection area, as standard. For the Dage XD7600 and XD7500 x-ray inspection systems there is a maximum inspection area of 18" x 16" (458 x 407 mm) and for the XD7000 and XD7100 systems it is 24" x 20" (601 x 508 mm). The maximum board size that the systems can hold is larger than the maximum inspection area.

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NO

There is sufficient movement within the manipulator to allow its entire inspection area to be investigated at any available magnification.

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