by Jeff Stong
The EMPF utilizes x-ray systems to inspect every PCB assembly, whether it is a military or a commercial product. In doing this, the EMPF insures the quality and reliability of the solder joints of CSP, Flip Chip, BGA and Micro BGA packages.

By design, x-ray systems provide the capability to look inside opaque/solid substances. Many of today's microfocus x-ray system manufacturers offer either open-tube or sealed-tube technologies. A sealed-tube x-ray source generally is a glass tube analogous to a light bulb where the contents of the tube are in a vacuum. An open-tube source, which generally consists of an all-metal construction, creates a vacuum by performing a two-stage pumping process every time the system is switched on. The vacuum is maintained throughout the system via a secondary pumping system. Although open tubes may use either directional or transmission target optics, for the purposes of this writing, any reference to an open tube is indicative of a transmission target.

To determine which type of system fits your needs, several fundamentals must be considered: detail detectability, tube voltage, geometric/total magnification, application requirements, cost of ownership, the image chain, software and system investment.

Detail Detectability, also known as feature recognition, can be estimated as approximately half of the x-ray source focal-spot size. In general, most sealed tubes will have larger focal spots than open tubes.The focal-spot size of microfocus x-ray is a fundamental parameter. One technique used for quantification is lateral resolution, which is the smallest feature that can be differentiated by the system in a top-down view. The smaller the focal-spot size, the sharper the image at high geometric magnification.

Tube-voltage Requirements depend on the application. Penetration of the sample or photon attenuation is based on three factors: material thickness, density, and mass. The higher the voltage, the greater the penetration capabilities of an x-ray source.

Voltages for sealed tubes generally range from 80 kV to 150 kV; open-tube systems vary from 100 kV to 225 kV. Both technologies usually have variable voltage ranges starting around 10 kV with increases in increments as low as 1 kV up to the maximum tube voltage.

Tube current controls the quantity of x-ray photons that the tube generates (flux). The end effect of a higher current is a higher contrast due to less noise. Since the focal-spot size is directly proportional to the power (voltage × current), the tube current typically is smaller at a higher tube voltage to maintain image sharpness. Low-density applications typically require higher current and lower voltage.
Geometric and Total Magnification
Beware of randomly spoken magnification values. The distinction between geometric vs. total magnification capabilities is essential. Geometric magnification is the true or actual magnification without enhancing capabilities, including optical and digital zoom, pixel, and software. Total magnification is the geometric magnification plus magnification enhancement tools such as zooming. Total magnification takes into account the optical characteristics of the detector and size of the display monitor.

The larger the geometric magnification, the smaller the observed feature can be defined as:
Mgeo = FID/FOD
where: FID = focus-to-intensifier distance
FOD = focus-to-object distance

Open tubes have a minimal focus-to-object distance and are best suited for applications demanding high magnification. Sealed tubes have a larger minimum FOD resulting in lower achievable geometric magnifications.

Application Requirements
A quality sealed-tube system is of great value for many applications. These systems, many capable of 70× geometric magnification, can inspect various components. For example, sealed-tube systems with penetration requirements up to 150 kV and a tube output power up to 10 W yielding current up to 250 µ are not only popular but are also excellent tools for BGA inspection (see Table 1).

Open-tube systems provide a more complete package, yielding a higher level of BGA analysis capabilities. For example, in addition to the higher level of BGA analysis, open-tube systems permit analysis of flip-chip bumps, package inspection, and a wider range of failure analysis in general. These systems accommodate applications requiring higher geometric magnifications, voltage, and tube current.

Open-tube systems are more powerful than sealed tubes and can be even easier to use with a properly designed control system. With a sealed tube, you have the ability to see the hair on a cricket's leg. With an open-tube system, you can see if the hair on the cricket's leg has split ends.

Cost of Ownership and System Price
The up-front costs of open-tube systems generally are higher due to the added support systems such as the vacuum pumps and a more complex x-ray control. However, both technologies have their associated costs.

All major components of open tubes may be exchanged, resulting in a virtually unlimited tube life. Open-tube systems require a scheduled maintenance program and have consumables such as filaments, targets, and seals. Sealed-tube systems require minimal maintenance with the tube itself as the only consumable. Typically, sealed-tube systems have a life span of three to five years depending on usage and design.

The Image Chain
The detector is the next important piece of the x-ray system. Like the x-ray tube, detectors come in different flavors and are broken down into two categories: analog or digital.

The most popular and economical analog image acquisition method is the x-ray image-intensifier device coupled with a high-resolution CCD camera. The image intensifier is an analog device contained in a large evacuated glass envelope. It comprises a scintillation screen and a photo cathode by which the x-rays are converted back to electrons, then accelerated to produce a smaller, brighter image on a second, phosphorescent screen similar in operation to a CRT.

The output from the image intensifier enters a chain consisting of lenses, a CCD camera, the image processing system, and the final output onto a monitor. Many manufacturers use general-purpose cameras that, although relatively inexpensive, do not offer the best contrast.

A small handful of manufacturers uses cameras that are sensitive to the spectrum of light emitted by the intensifier. These optimized cameras usually are identifiable by their pancake or disk-like design.

Image-intensifier packages are offered with 9", 6", and 4" input areas. Image intensifiers also are available with combination modes such as dual-field (6"/4" or 4"/2") and tri-field units (9"/6"/4").

The magnification factor and image resolution will vary with intensifier selection. The x-ray-to-light conversion efficiency increases with the diameter of the input field. A smaller intensifier will provide higher magnification but requires more flux.

The digital or flat-panel detectors come in various configurations, such as a solid-state amorphous silicon sensor operating as a two-dimensional photodiode array. X-rays are converted to light using a vertical structured scintillator. The flat-panel detector consists of a photo-diode matrix and a scintillator screen for x-ray conversion.

The amorphous silicon-based digital detectors provide up to 16-b, 64k gray-scale images as compared to 8-b, 256 shades of gray provided by the camera of an image-intensifier system. The digital detector, with its higher cost, is useful for capturing small changes in material density and examining low absorbing materials such as nonconductive epoxy. Some cameras can be upgraded from 8-b, 256 gray scales to 10 b or 12 b, providing some improvement.

Software
Programming capabilities and a user-friendly graphical user interface (GUI) are key. Having the tools is great, but being able to use them is essential. Image processing and filtering functions that provide comprehensive capabilities for custom and standard applications plus a user-friendly GUI are necessary.

System Investment
Ultimately, the system you select must be appropriate for your current as well as future applications. It also must accommodate the changes in production to smaller, denser materials to reflect today's technology trend. The maximum value for the dollar is not the least expensive system out there, but the best system for the money.

Contact X-ray vendors that can provide you with a solution within your budgetary constraints. Since image quality is essential, submit samples of your product to each of the qualified vendors for imaging.

After determining which systems provide the best quality, go step by step through the features, further narrowing down the list. Then, request a hands-on demonstration of each potential supplier's recommended configurations. Driving a system yourself will tell how well a system is going to fit your application and facility needs.