X-RAY TUBE SELECTION CRITERIA FOR BGA / CSP X-RAY INSPECTION

Transcription

1 X-RAY TUBE SELECTION CRITERIA FOR BGA / CSP X-RAY INSPECTION David Bernard Dage Precision Industries Inc. Fremont, California d.bernard@dage-group.com ABSTRACT The x-ray inspection of PCB assembly processes is becoming ever more important to undertake as the benefits of using area array packages / chip scale packages / flip chips are applied to more and more products. This is because automated optical inspection (AOI) cannot be used on these devices as their solder contacts are hidden from view. Achieving the best x-ray inspection for a particular application depends on selecting the appropriate x-ray inspection system. Whichever system is considered, from whatever manufacturer, at the heart of them all is the x-ray tube. The limitations of traditional closed x-ray tubes, in terms of achievable resolution and magnification, encourages the use of single-staged or double-staged, open (or demountable) x-ray tubes for today s x-ray analysis, particularly as component and board sizes continue to shrink. This paper will: Review the basics of x-ray tube type and operation Discuss the relative advantages and disadvantages of the various tube types including cost of ownership implications Suggest which tube type is most appropriate for current and future applications Key words: x-ray, inspection, BGA, CSP, resolution BACKGROUND X-ray inspection has traditionally been used for development and field-failure analysis applications within the printed circuit board (PCB) and semiconductor industries. More recently, the advantages of the technique have become ever more important for both production quality and production process control applications as well. In particular, the increasing use of area array packages / chip scale packages / flip chips within ever more products, further drives the need for x-ray examination because AOI cannot be used on these devices as their solder contacts are hidden from view. Therefore, x-ray examination becomes an important method of validating production processes at an earlier, rather than at a later and more expensive, stage in the manufacturing timeline. When an x-ray inspection system is being considered for acquisition, AOI attributes can be assumed to apply. However, x-ray inspection is a very different technique to AOI and needs to be considered in a completely separate light. To this end, this paper will concentrate on 2- dimensional, or 2-D, x-ray inspection and attempt to indicate the key factors that should be considered so as to provide the best analysis for a particular analytical application. 3-dimensional, or laminography, x-ray systems will not be considered. Figure 1: Basic 2-D x-ray system configuration A 2-D x-ray system captures x-rays that have passed through a sample and converts them into images that the operator views. Any object 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 shadow on the detector (see figure 1). In this way, solder and copper tracks appear dark compared with the laminated circuit board in a PCB, for example. Achieving the best x-ray inspection for a particular application, therefore, depends on selecting the appropriate x-ray inspection system. OPEN AND CLOSED X-RAY TUBES Whichever system is considered, from whatever manufacturer, at the heart of them all is an x-ray tube. An x- ray tube is the device that produces the x-ray radiation for the analysis 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 systems have used what are called closed x-ray tubes (figure 2) where the vacuum is produced

2 during manufacture and the tube sealed allowing no access to the components within. In recent years, open, or demountable, x-ray tubes (figure 3) have become more popular for PCB and semiconductor x-ray inspection because of their opportunity to provide higher magnification, better resolution and serviceability (by allowing access to the consumable items of target and filament). In contrast, evacuation of the open tubes is achieved through the use of vacuum pumps supplied with the x-ray system. Figure2: Closed x-ray tube with reflective target FEATURES OF X-RAY TUBES The key features that define the capabilities of any x-ray tube are (see figure 3): Figure 3: Open single-stage x-ray tube with transmissive target The filament, or other device, that produces electrons within the tube, sometimes called the electron gun. This is often from thermionic emission from a hot wire. The more electrons produced by the filament, as defined by the current passing through it, then the brighter the x- ray image can become. The focussing electronics electro-magnetic, or other, components within the tube that squeeze the accelerated electrons into as small a spot as possible on the target. This point on the target is called the focal spot. The smaller the focal spot that can be produced then the better the resolution that can be achieved for the final image. The target type transmissive or reflective. Transmissive targets require that the x-rays, once produced at the focal spot, must pass through the thickness of the target to exit the tube and irradiate the sample (figure 3). Reflective targets have the x-rays reflect off the surface of the target before exiting the tube, see figure 2. The type of target used within a tube directly affects the magnification that is available within an x-ray system. Note how the minimum distance that a sample can be placed in relation to the focal spot location differs dramatically in the two target types mm or less for transmissive targets typically used in open tubes and ~ 15 mm for reflective targets most commonly used in closed tubes. The target material and the thickness of that material. This is particularly important for transmission targets as a trade off needs to be made to provide good x-ray flux for commercial applications (i.e. long lifetimes) whilst at the same time not self-absorbing too much of the x- rays as they pass through. Furthermore, as transmissive targets become thicker then there is more opportunity for the incoming electrons to broaden out the focal spot as primary, and secondary, excitation effects produce x- rays from within the thickness of the target. Tungsten is the most commonly used target material. The available accelerating voltage for the electrons discussed in terms of kilovolts, or kv. The greater the kv then the more penetrating are the produced x-rays. This means that higher kvs need to be used to image dense, or thick but relatively less dense, objects. At lower kvs, only thin and less dense samples can be inspected. Otherwise, the x-rays have insufficient penetrating power to travel through the sample and strike the detector, so creating the image. The tube power measured in watts. The higher the power then the greater the x-ray flux and so the brighter the final image. The vacuum window. In all x-ray tubes, the x-rays must have some way of escaping the evacuated tube without disturbing the vacuum. The most common method is to have a beryllium metal disk as the vacuum seal, and exit, for the x-ray beam. Beryllium is transparent to x-rays of the penetrating power used for x-ray inspection and is often called the beryllium window. However, thin light metals can also be used, such as aluminium in place of the beryllium, but some of the produced x-rays will be filtered by this material and so modify the energy spectrum of the produced beam.

3 COMPARING X-RAY TUBE FEATURES When comparing x-ray tubes, and systems, from different manufacturers, the features described above will vary and result in implications for the final x-ray image quality. As such, the following differences should be noted so that the best tube, and system, for a particular application can be selected. However, the selection should also consider that the best tube/system combination must also be reliable and fit for purpose. As will be seen, there are techniques to enable ultimate x-ray resolution but at the price of only being able to operate with special types of samples under special conditions. Such solutions may be appropriate for specific laboratory applications but may be entirely inappropriate for the vast majority of PCB applications, for example. When comparing x-ray tubes/system features, the first choice is between having an open or closed x-ray tube. Minimum feature recognition Most manufacturers use the term minimum detectable feature size instead of focal spot size when defining the specifications possible for their systems. This is used as it also takes into account the effects of the rest of the imaging chain in the system and relates it to objects that can be seen. Figure 4: effect on image quality as focal spot increases in size. However, for both open and closed tubes, the smaller the focal spot the electrons can make on the target then the better the resolution of the x-ray tube. Ideally, an x-ray tube would produce an infinitesimally small focal spot. This would then produce perfectly sharp images. In reality, the focal spot in an x-ray tube has a finite dimension. The larger the spot then the more that edge blurring occurs on the image, thus limiting final image resolution. This effect occurs, as shown in figure 4, by the edge in question being imaged from either side of the focal spot onto the detector. The result is unsharpness shown in the final image. Tube power For open and closed tubes, as the focal spot is reduced in size, so the energy density at the target rapidly increases. For example, if a tube produces 1W of power into a 1- micron spot then to achieve the same energy density with a 20-micron spot requires 400W. Although small sounding in value, such large energy densities at small focal spot sizes produces heat, which must be removed from the system. This energy deposition modifies / ablates the target surface, requiring target replenishment over time. If the target is thin, such as in a transmissive target, then the surface modification will remove the active layer and necessitate the ability to service the target. Even if the target is thick, such as in a reflective target, surface modification will occur but over a longer time period than with a transmissive target, and so will also need replacing. The commonly used closed tubes are unable to be serviced and require the whole tube to be replaced upon failure of the target or filament. The energy density into the target is also one of the main reasons why closed tube focal spots are, at best, 5 microns in size (typically 8 20 microns) compared to 1 2 microns, or less, for open tubes. Closed tube manufacturers must consider the trade off between the focal spot size that is achievable and the tube lifetime because of the major replacement cost on tube failure. Therefore, a closed tube with a reflective target continues to degrade from its factory specification from the day of purchase to the time of failure. The factory specification may only apply for a limited period at the beginning, even though the closed tube can continue to be used for an extended period. In contrast, as open tubes permit easy access to the consumable items (filament and target), they can continue to be returned to their factory specification indefinitely. When comparing the specifications for open and closed tubes, the power that a tube offers must be related to the focal spot that it can achieve. In this way, closed tubes often state dramatically higher power values than open tubes, but such power values are only available at the largest spot sizes the tubes can produce. For most semiconductor and PCB applications the smaller spot sizes are always desired because of the size of the components involved. Therefore, the correct tube to use will also depend on the dimensions of the object to be inspected. For example, if a tube produces a 20-micron focal spot and the item to be inspected is 25 microns in diameter, such as gold wire within a package, then this tube would be unacceptable for this purpose. With devices becoming ever smaller, the functionality of x-ray analysis will of a necessity migrate towards smaller spot sizes. Air-cooling is preferred by tube manufacturers for removing the heat generated, to the complexities and additional service requirements, of water-cooling. The operational advantages of air-cooling, however, will limit the spot size and power achievable in all tubes using it, as it is much more limited in the heat dissipation that can be handled compared to water-cooling.

4 Magnification X-ray systems used for inspection are basically shadow microscopes see figure1. The geometric magnification of an x-ray system is the ratio of the distance between the tube focal point and the image capture device and the distance between the tube focal point and the sample. See figures 5 and 6. An image intensifier is the most commonly used image capture device and has replaced the use of film in PCB and semiconductor applications. Figure 5: Geometric magnification Focussing electronics and low kvs Closed tubes traditionally have a single set of focussing electronics to produce the focal spot. Open tubes can have a single (figure 3) or double-stage focussing arrangement. Open tubes with double-stage focussing arrangements, possibly together with added proprietary apertures, or skimmers, within the beam, are sometimes called nanofocus tubes. Nano-focus tubes are able to provide even higher levels of resolution than standard open tubes because the electrons are further constricted and focussed whilst travelling to the target. This provides a narrower focal spot but at the price of more limited operational capability (see operational considerations section below). Whichever focussing arrangement is selected, achieving the smallest focal spot size requires that low accelerating kvs are used. Low kvs are necessary as although the electrons can be focussed to a very small point on the target, once they have arrived they spread out, produce x-rays from over a wider area and so broaden the focal spot (see ref 1). This broadening is reduced in transmissive targets because the depth of the thin tungsten layer itself limits the electron volume spread. Ideally, an infinitely thin target layer would be best for transmissive targets. In reality, a compromise thickness must be used; otherwise target ablation will happen too quickly, so requiring frequent target replenishment. This would make such a tube inconvenient for the majority of applications. It has already been mentioned that the target layer cannot be too thick either for transmissive targets, in case some of the desirable low kv x- rays are self-absorbed by the target before they exit the window. The best compromise for the majority of PCB applications permits reasonable x-ray flux at low kvs together with reasonable target lifetimes, so providing the best operational parameters. Figure 6: Geometric magnification with open and closed tubes The design of closed x-ray tubes requires that the focal point is at a distance from the x-ray exit point from the tube because of the insulation required to handle 10 s of kv potential between the anode and cathode. This causes the closest distance that a sample can approach the focal point with a closed tube x-ray system to be large (typically 15 mm or more see figures 2 & 6) compared to an open tube. In the open, transmissive tube, the sample can be placed on top of the exit window and be mm away from the focal spot (figures 3 & 6). These differences directly affect the available magnification and can be shown though an example. If you were to take a closed tube and open tube and place them in the same x-ray system, the distance between the focal spot and the image capture device would be the same for both tubes, e.g. 350 mm. The maximum geometric magnification for the closed tube system would then be 350/15 or ~23 X. (This assumes the closed tube has a minimum distance from focal spot to sample of 15 mm.) In contrast, the open tube system, with a 0.25 mm minimum focal spot to sample distance, would have a maximum geometric magnification of 350/0.25 or ~ 1400 X. The difference in the available magnification between open and closed tubes can be seen in figures 7 and 8. Figure 7 shows an optical image of a thin PCB with a flip chip of 6.35mm diameter in the middle. Underneath is an x-ray image of the same board. Note how the previously hidden

5 connections become visible on the flip chip and other components. examined with both a closed tube system and an open tube system. Figure 8: Maximum available magnification for the sample shown in figure 7 if a closed tube (left) and open tube (right) were used in the same x-ray system. Both images are Dage x-ray images using an open x-ray tube. The focal spot to sample distance has been set in the left image to emulate the closed tube condition. Figure 7: Optical image above a Dage x-ray image of a PCB with a flip chip shown in the middle. In figure 8, an open tube has been used to produce two images of the flip chip connection seen in figure 7. The sample has been placed on a carrier plate within the x-ray system such that the sample is as close to the focal spot as would be possible with a closed and open tube, i.e. 0.5 mm plus the sample plate thickness for the open tube and ~15 mm plus the sample plate thickness that would occur in a closed tube. The difference in the available magnification is clear. The ball shown has a diameter of 189 µm. Figure 8, does not show the difference in image quality that may be expected with a closed tube. This can be indicated with figures 9 and 10, where the same sample has been Figure 9: Dage x-ray image using open x-ray tube. The closed tube image (figure 10) makes it difficult to see the failure of the gold bond wires when compared to the open tube image (figure 9). The open tube also allows more magnification so the bond wires can be inspected in greater detail to help with the failure analysis (see figure 11). As the geometric magnification relies on the focal spot to image capture device distance; it would appear that increasing this distance would improve a system s specification. Whilst the geometric magnification would increase in value, the compromise that must be made is a rapid decrease in the x-ray flux hitting the image capture device. This is caused by the x-ray flux decreasing as the inverse square with distance. So doubling the focal spot to image capture device distance decreases the amount of x- rays hitting the detector by a factor of 4. Therefore

6 manufacturers have to make a balance between practical equipment sizes that will fit through doors, etc. and the ability to provide good images quickly by not having to wait for sustained signal averaging to overcome the decrease in x-ray flux. Figure 10: Closed x-ray tube image of sample shown in figure 9. Figure 11:Higher magnification Dage x-ray image of sample seen in figure 9. Sometimes, manufacturers state the maximum magnification for their systems as a combination of the geometric magnification and the magnification of the image chain that takes the captured x-ray image and displays it on an operator view screen. This is called the system, or total, magnification. Geometric magnification will give a better comparison between x-ray systems than the total magnification, as displaying the final image onto a larger monitor will increase the value of the total magnification but not provide any better geometric magnification, or, indeed, any additional resolution or image data. Brightness of filament The x-ray tube s design and function is to focus the image of the electron cloud produced at the filament onto the target. The amount of current passing through the filament in an open tube defines the number of electrons being emitted. This, in turn, affects the number of electrons striking the target and so affects the brightness of the final x-ray image. It would be assumed, therefore, that a thick filament could handle high filament currents, provide many electrons and have a very long lifetime. In reality, to maximise the thermionic emission, and to provide a concentrated cloud of electrons that can be accelerated and focussed into the smallest focal spots, the standard design for an open tube filament is of a hairpin shape made of thin wire, usually tungsten. Such an arrangement automatically limits the current that the filament can handle without rapidly being destroyed. Using a thicker wire, and a less sharp hairpin point, would increase the filament current that could be used but at the expense of the focal spot created. OPERATIONAL CONSIDERATIONS Whilst manufacturers strive to reduce focal spot sizes using the techniques described above so as to keep with the pace of component shrinkage, the downside of these approaches is that the improvement always comes at a price. Specifically, the available x-ray flux is dramatically reduced as the focal spots are reduced, making the x-ray image dim and requiring very long exposure times to generate sufficient image quality for analytical purposes. Therefore, if image acquisition takes a long time when using the ultimate available resolution, this may be inappropriate to use in all but highly specialised laboratory applications, where a great deal of care can be taken for each specific measurement. In addition, if images take many minutes, or longer, to acquire then the effects of vibration on the x-ray system may negate the enhanced resolution offered by the tube without specialist arrangements / conditions being applied to the x-ray system. Therefore, the ultimate resolution approach may not be appropriate for production and more general usage within the test and inspection environment. By only using the lowest kvs to achieve the ultimate resolution, the type of sample that can be inspected to this level becomes more limited. Typically, the best resolution for a tube will be available at less than 50kV. As long as the sample is not very dense and/or very thin, then this accelerating voltage will be acceptable for inspection at these conditions. However, typical PCB applications do not fit in to these requirements, as the board itself is likely have sufficient density to absorb most, if not all, of these low energy x-rays. Standard packaging around a device from semiconductor manufacturers may also absorb the x-rays making realistic analysis at these resolutions impossible without specially modifying the sample. Such modifications could include removing the component from the board, removing the packaging, thinning the sample, etc. When this

7 is considered, the use of the highest resolution tubes lends themselves more directly to failure-analysis laboratories, where special care and attention can be applied to individual examinations. The opportunity for production process control and manufacturing quality analysis does not usually lend itself to detailed inspection because of the time pressures involved. Therefore, the correct compromise for tube performance against application must be decided prior to x-ray system acquisition. Some manufacturers consider thinning the target to further reduce the broadening of the focal spot, as well as increase the maximum available geometric magnification The compromise that must be made is that a thinner target will be modified much more quickly by the electron beam and require more frequent replenishment. This may be acceptable within a laboratory environment where special conditions can be maintained but may not be acceptable for manufacturing situations. CONCLUSION The x-ray tubes supplied with commercially available x-ray systems can be of different configurations and the appropriate tube must be selected for a specific application (see table 1). Closed tubes, offer an integrated package that does not allow servicing of the tube components but offers relatively long lifetimes compared to open tubes, but the achievable factory specification may only be available for the initial portion of the tubes life. Closed tubes also offer poorer resolution and less magnification than open tubes in x-ray systems. The cost of eventual replacement of the whole tube when the unit fails is very high compared to the serviceable (consumable) components, filaments and targets, in an open tube. The cost of ownership over the lifetime of an x-ray system would therefore be much higher with a closed tube system. With open tubes, the resolution that is possible can be enhanced by reducing the focal spot on the target using a combination of some, or all, of the techniques mentioned above. The ultimate resolution of an open tube can be more than 5 times that of the highest specified closed tubes (1 micron or less). The price that must be paid is that this resolution is only possible under very specific conditions and, very often, with specially prepared samples. This may not be appropriate for the vast majority of applications. Instead, most open tube manufacturers compromise the performance of their tubes so as to still have 3 4 times the resolution of the closed tube but provide a more robust system that can be used reliably within realistic working environments. Although focal spot size has been the main thrust of this paper, the final image resolution produced by an x-ray system will also be influenced by the resolution of the image capture device and any image processing that may be achieved if the image is captured digitally. References [1] Kerridge, B, Sharpen x-ray images, Test & Measurement Europe, January This article was originally published in The Proceedings of SMTA International Conference, September Property Closed Tube Open Tube Single Focus Open Tube Double Focus Min. feature recognition 5 microns or more 2 microns < 1 micron Max. tube kv 90, , 160 Serviceable No Yes Yes Available magnification Low High High Cost of Medium-High Low Low ownership * Target thickness Large ~ 5 microns 2 3 microns Table1: Features of open and closed tubes * - based on 10,000 hour closed tube lifetime