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X-ray Inspection of Harsh-environment Electronics
December 31, 1969 |Estimated reading time: 12 minutes
By Jon Dupree, May Qadir, Chris Cherry, and Guy Cornett, YXLON International
In X-ray inspection of harsh-environment electronics, there usually are materials involved that are not easily X-rayed. Requirements for high penetrating power and high intensity dose make transmission tubes ineffective; directional tubes cannot achieve small enough focal spots. Engineers need a tool that can achieve the power needed to penetrate and image the parts, and the resolution to examine details.
There always has been concern about the quality of solder joints in electronic assemblies, and this concern is even greater when it comes to harsh-environment electronics. Here, a product will experience prolonged periods at high temperatures or short durations at extreme temperatures, severe temperature fluctuations, a high level of mechanical shock and vibration, corrosive environment, or a combination of these.1
Increases in worldwide demand for oil, evolving electronic needs in the automotive industry, and a resurgence of high-reliability military and aerospace sectors are driving demand. Applications include deep-well data-logging tools, automotive under-the-hood electronics, and fly-by-wire aircraft. Harsh-environment applications require electronic systems that can survive well beyond the traditional MIL-STD operating temperature.
Over the last few decades, electronics utilization has increased rapidly in automobiles. A growing number of devices have been designed to improve performance in engine, transmission, steering, and traction control. Because the majority of these sensing electronics and control devices are placed in under-hood and -body environments, they can cycle rapidly from 180° to -40°C operating temperature, depending on driving location and climate. Many of these electronic assemblies are designed with uncompromising materials and undergo extreme testing to ensure integrity.
The goal of fly-by-wire in aeronautics is to replace hydraulic control for weight reduction, ease of maintenance, and improved reliability. This requires the hydraulic actuator to be replaced by electronic control actuators throughout the aircraft. A challenge is how to cool these electronic devices operating in a high-temperature environment.
The lifecycle of harsh-environment electronic devices must be longer than normal. Due to the mission-critical nature or the inaccessibility where many of these electronics will be used, the cost of changing out or compensating for a failed component could far exceed the total cost of the product. Traditional organic PCB material, such as FR-4, will degrade under high temperature exposures and cause board-level failures such as trace and via cracking. Another main concern is the reliability of solder joint interconnections between component and board. At high operating temperatures, intermetallic formation at the connection junction can cause intermittent open joints; metal migration across conductor traces can cause bridging of signal paths. In addition, coefficient of thermal expansion (CTE) mismatches on packaging material can cause stress- and fatigue-related failure inside the components.2
Figure 1. A transmissive tube design.
Designing a piece of electronic hardware such that it works reliably from the get-go is a challenge, especially if it has to work in extreme environments where temperature, humidity, vibration, and radiation are the enemies of electronics. Most engineers assume that designing for a harsh environment is much like designing for a hospitable one, but with a greater emphasis on testing and improving the design empirically, addressing the observed failure modes.3 One common method is to test an electronic assembly nondestructively before and after highly accelerated stress and thermal tests. X-ray inspection, using real-time 2D or, increasingly, computed tomography (CT) 3D modeling, is the primary method for qualifying the integrity of a solder joint or the board’s traces and vias. High resolution and magnification X-ray inspection can detect changes to the assembly before catastrophic failure and assist in design improvement. Additionally, X-ray inspection is a key component of field failure analysis to document the state of the assembly before cross-sectioning or other destructive tests.
Challenge of X-ray Inspection
A unique challenge for X-ray inspection of harsh-environment electronic assemblies is that there are usually materials involved that are not easily X-rayed. Harsh-environment assemblies may have ceramic substrates, which generate more noise in an X-ray image than comprable FR-4. There may be heat shields or other heavy metal components around the electronic assembly, which hinder inspection. Components may have thermal transfer attachment materials, where air bubbles in the materials (voids) could cause hot spots to occur on the device. Components may have additional structural attachment materials, such as underfill, which may have voids causing structural deficiencies. More power or X-ray flux may be needed to inspect these assemblies than would traditionally be needed to inspect electronic assemblies.
Historically, an increase in power results in a loss of resolution, such that the lower-resolution and higher-powered X-ray source is unable to inspect the assembly sufficiently while fully intact. Partially disassembling the device causes delays in analysis, or may be impractical in the inspection of a highly accelerated tested part. Recent technological improvements in X-ray target materials have narrowed the gap between power requirements and resolution needs.
X-ray Fundamentals
X-ray systems for inspecting electronic assemblies traditionally use an electron gun, or X-ray tube. In all X-ray tubes, an electron beam is emitted from a cathode by a high voltage and focused on an anode, otherwise called a target. When the electron beam hits the target, electrons collide with the target material and are slowed down and deflected, transferring their kinetic energy to the target material. About 98% is released as heat. For this reason, the target material must have a high melting point. Typically, less than 2% of the energy appears as X-rays.
X-ray power is a combination of the X-ray wavelength and the quantity of X-rays. The amount of voltage applied determines wavelength. Higher voltages (kV) accelerate the electrons faster and produce shorter, more penetrating wavelengths. The tube current (mA or µA) defines the intensity of the X-rays generated.
As the power of the X-ray is increased, so is the heat that is seen by the target. This heat must be dissipated, or the target will burn up. Heat dissipation can be accomplished either by cooling the target or by increasing the surface area of the target where the electron beam interacts. This surface area is called the focal spot, and it is a prime element that affects the final resolution of a magnified image. The electrons are focused on as small a focal spot as possible. The focal spot is a key factor in determining image resolution and, therefore, the quality of the X-ray image. As the focal spot size decreases, resolution and the ability to detect detail are improved, enabling geometric or projection magnification without peripheral shadowing, an effect called geometric unsharpness, or penumbra.
Ideally, the focal spot would have a diameter close to zero. The spatial resolution of an X-ray tube is approximately one half the focal spot size. Detail detectability for the tube is approximately one half of the spatial resolution.
The other key factor in geometric unsharpness is geometric magnification. Image magnification is determined by the ratio of the focal spot to detector distance (FDD) to the focal spot to object distance (FOD). For a given focal spot size, the closer the object is to the focal spot, the higher the magnification (m) and the greater the geometric unsharpness. However, the smaller the focal spot, the closer the object can be to the focal spot to obtain higher magnifications with minimum geometric unsharpness.
Figure 2. The heat conductivity of a high-power target versus standard backing material. The aluminum target requires de-focusing.
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Microfocus Tubes
Microfocus tubes are essential for applications where the required resolution needs a focal spot size down to 1 µm. Many microfocus X-ray systems are used in the inspection of electronic components, packages, circuits, and substrates, BGA solder bumps after reflow, and bond wires in a package after encapsulation or other hidden interconnects.
A microfocus tube is defined as a tube having a focal spot size from 100 to 1 µm in diameter. Almost all conventional and sealed tubes are a directional design.
Directional X-ray Microfocus Tube
In directional tubes, the target and window are separated. The target and focal spot are located at a small distance before the X-ray window from which the X-ray beam is issuing at a 30°, 60°, or 90° angle.
The target is a tungsten cylinder. This type of target design allows for higher power output, which allows higher intensity for better contrast in more dense samples.
The target power can range up to 280 W depending on the tube model. Target power is the maximum power the target can dissipate before melting at 100% duty cycle.
The minimum FOD for an open directional tubes is about 6.75 mm. Sealed tubes have a greater distance between the focal spot and the tube end window. This distance limits the magnification achievable with the tube compared to transmission tubes.
Transmission X-ray Microfocus Tube
In transmission tubes, the target and window are one piece, and the window is directly in line with the target and electron beam. X-rays emit from the end of the tube.Transmission targets consist of a thin layer of target material that is sputtered onto a thicker backing material. The backing material provides mechanical strength for the target in which the X-rays are generated. The target layer is usually a 3?5 µm-thin layer of a metal with high density and a high atomic number and weight, such as tungsten, molybdenum, or copper. The backing material has a low density, low atomic number, and low atomic weight, such as beryllium or aluminum. Due to the low atomic number, this material does not produce hard X-rays; it converts beams that penetrate the target to soft X-ray energy and filters them out. The backing material also is called the X-ray window.
Because the focal spot and X-ray window are at the same level and the X-ray window is typically 0.25?0.50 mm, the focal spot can be placed close to the sample and extremely high direct magnifications can be achieved. The minimum FOD for transmission tubes is about 250 µm. This distance is much shorter as compared to directional tubes and allows much higher magnifications.
At high energy, the focal spot is determined only by the area of electron impact on the sputtered target material.
Standard transmission target materials can only accommodate a limited heat per square area of target. An open tube will defocus the electron beam to accommodate the heat generated over 4 W of power. The maximum defocused target power is 10 W, with a focal spot size greater than 10 µm. Target power is the maximum power the target can dissipate before melting at 100% duty cycle. Harsh-environment electronics may require the small focal spots achievable with a transmission tube, but demand greater power than the small focus spot can support.
Limitations of Standard Transmission X-ray
Using standard target materials, the focal spot typically must be increased by approximately 1 µm in size for each power watt. This increases the geometric unsharpness and limits the usable magnification.
The primary reason for increasing focal spot size is to avoid melting the target material. The maximum thermal target loading depends on the specific loading factor of the target material and the focal spot size. P = A × D, where P is the maximum loading in watts, A is the specific loading factor [watts µm-1], and D is the focal spot size in µm. If, for example, the target is tungsten with a loading factor of 1.2 w µm-1, and the focal spot size is 3 µm, then P = 1.2 w µm-1 × 3 µm = 3.6 W. The maximum target load for that focal spot size is 3.6 W.
Focal spot size is a critical factor in determining geometric magnification and image resolution, while tube power influences the X-ray intensity output. The essential limitations of the transmission type tubes are determined by these two factors. The tube cannot run at high power without making the focal spot larger. This limits the amount of magnification before the geometric unsharpness becomes unacceptable. At low power, the X-ray intensity is low, losing contrast and the ability to penetrate more dense materials.
High-power Target
This shortfall is addressed with a high-power target with 10× increase in thermal conductivity compared to conventional transmission targets. High-energy electron beams can be kept in focus maintaining small focal spot size for high image resolution.
A high-power target is a transmission target consisting of the interaction layer made of tungsten and a special backing layer that provides improved heat transfer and lower focal spot temperature. Its thermal conductivity reaches over 2,000 W/mK, whereas copper hits 400 W/mK, aluminum 200 W/mK, and beryllium less than 200.
The use of target backing materials with a higher thermal conductivity allows a transmission tube to run up to full power with little or no increase in the focal spot size. The most significant contribution is for high-kV applications where high-resolution imaging is nearly impossible using a standard target due to the necessary increase in focal spot size at higher wattages. This also is significant for the imaging of low-kV applications that benefit from the available increase in current, increasing contrast within the X-ray image without sacrificing resolution.
Figures 3A and B. The same component imaged with a conventional target (A) and a high-power target
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Harsh-environment Electronics
Due to the requirements of harsh-environment electronics for high penetrating power and high intensity dose, transmission tubes traditionally have been ineffective in resolving the level of details required. Directional tubes also have not been able to achieve small enough focal spots to resolve subtle details that are useful to diagnose structural changes from highly accelerated tests. With the introduction of high-powered transmission targets, engineers have a tool that can achieve the power need to penetrate and image the parts, and the resolution to examine the details clearly.
Figures 3A and B show an electronic component mounted to a board located in a heavy-cast assembly, which requires significant X-ray power to penetrate the casting. Conventional targets defocus to a large focal spot, creating high geometric unsharpness and lowering the resolution such that the bond wires and die-attach material are difficult to resolve (Figure 3A). In contrast, the image produced by a high-power target shows that the focal spot is not defocused and greater detail is visible (Figure 3B). The wire bonds can be identified and examined easily. A crack in the die is apparent, as well as insufficient die-attach material.
Figures 4A and B. Wire bonds and die-attach material are visible but too blurry with a conventional target (A). Proper inspection data are provided by the high-power target (B).
An image produced with a conventional target has a defocused focal spot, so wire bonds can be identified, but not inspected. The die-attach material can be seen extruding from the die edge, but no decision can be made regarding voiding in the material (Figure 4A). Using a high-powered target with a small focal spot, voiding in the die-attach material is clearly evident, and the non-uniform bends of the wire bonds can be seen (Figure 4B). SMT
REFERENCES:Contact the authors for a list of references.
Jon Dupree, May Qadir, Chris Cherry, and Guy Cornett, YXLON International Inc., may be contacted at 3400 Gilchrist Road, Akron, Ohio 44260; jon.dupree@yxlon.com, may.qadir@yxlon.com, chris.cherry@yxlon.com, and gcor@ameritech.net.