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 is generally sealed in a glass tube containing a vacuum (analogous to a light bulb). An open-tube source is generally all-metal and maintains a vacuum with a two-stage pumping process.

To determine which type of system is required, several fundamentals must be considered: detail detectability, tube voltage, geometric/total magnification, cost of ownership, and system investment.

Detail Detectability - Also known as feature recognition, detail detectability is the ability to resolve the smallest feature.  It can be estimated as approximately half focal-spot size of the X-ray source. Thus, the smaller the focal-spot size, the sharper the image at high geometric magnification. In general, most open tube systems will have a smaller focal spot size than sealed tubes and therefore a sharper image.

Tube Voltage - The voltage requirement depends on the sample.  The greater the sample thickness, density, and mass, the higher the voltage needed to penetrate.  The tube current controls the quantity of x-ray photons (flux) that is generated.  The higher the current, the lower the noise levels allowing a higher contrast view.  Since focal-spot size is directly proportional to power (voltage × current), the tube current is typically smaller at higher tube voltages to maintain image sharpness. Low-density samples typically require higher current and lower voltage. Maximum voltages for sealed tube X-ray sources generally range from 80 kV to 150 kV while open-tube systems vary from 100 kV to 225 kV.

Geometric and Total Magnification - Understanding the distinction between geometric and total magnification capabilities is essential. Geometric magnification is the actual magnification with no enhancement. Total magnification is the geometric magnification plus magnification enhancement tools such as digital zooming and software. 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 feature size that can be observed. 

This is defined as:
Mgeo= FID / FOD
where: FID is the Focus-to-Intensifier Distance,
and FOD is the 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 FOD resulting in a lower achievable geometric magnification.  Because of this, open-tube systems provide a more complete package, yielding a higher level of electronic analysis capabilities including BGAs, flip-chip bumps, package inspection, and a wider range of failure analysis.

Cost of Ownership - The up-front costs of open-tube systems are generally higher due to added support systems such as the vacuum pumps and a more complex x-ray control. However, both technologies have their associated costs.  Since all major components of open-tube systems may be exchanged, virtually unlimited tube life can be achieved.  This is at the expense of a required scheduled maintenance program and consumables such as filaments, targets, and seals. Sealed-tube systems require minimal maintenance but eventually need a tube replacement.

System Investment - Ultimately, the system selected must meet the current needs as well as future applications. It also must allow flexibility when new technologies enter the market.  Since image quality is essential, actual samples should be submitted to each of the qualified vendors for imaging comparisons.  After determining which systems provide the best quality, go step by step through the features, further narrowing the list. Then, request a hands-on demonstration of each vendor’s recommended configurations. Testing a system yourself will tell how well a system is going to fit the application and facility needs.