The choice between an open tube and a sealed tube is driven by the application, and understanding it starts with what the two have in common.
How any X-ray tube works
Every X-ray tube contains a filament and a target inside a vacuum. Heating the filament releases free electrons, a high voltage accelerates them toward the target, and X-ray photons are produced where they strike it.
The vacuum is not optional. Electrons cannot travel from filament to target through air, because oxygen and nitrogen molecules in the path would scatter and absorb them before they arrive.
Everything else is refinement: focusing the electron beam more tightly, achieving a smaller spot size, improving resolution. The underlying arrangement has not changed since the earliest tubes.
The sealed tube
A sealed or closed tube encloses the filament and target inside a permanently evacuated glass envelope. Structurally it resembles a light bulb, which is roughly what the earliest ones were.
The failure mode is the filament. It heats and cools through every duty cycle, and thermal cycling eventually ruptures it. In a sealed tube there is no way to reach the filament, so the entire tube is replaced when it fails.
The open tube
An open tube uses the same filament and target arrangement, but the vacuum enclosure can be opened. When the filament fails, it can be replaced rather than discarding the tube.
The cost is that the vacuum has to be re-established afterward. That requires a vacuum pump and the associated procedure, which means an open tube carries ongoing maintenance a sealed tube does not.
The actual tradeoff
Sealed tubes are simpler to own. Nothing to pump down, no filament replacement procedure, no vacuum system to maintain. When the tube dies, a new tube goes in.
Open tubes cost less per failure and more per year. The consumable is a filament rather than an entire tube, but the system requires vacuum equipment and someone competent to use it.
Neither is better in the abstract. The right answer depends on duty cycle, on what the application demands from the source, and on whether the maintenance capability exists in-house.
Reflection targets and the magnification problem
There is a second distinction inside the sealed tube category, and it matters more than the open versus closed question for many electronics applications.
In a conventional tube the target is a bulk piece of material set at an angle, and X-rays leave it at roughly a right angle to the incoming electron beam. This is a reflection target.
The consequence is geometric. The target sits back from the exit window, and the window itself has thickness. Together they establish a minimum distance between the target and anything placed outside the tube.
Magnification depends on how close the sample can get to the source. A gap between the target and the sample sets a floor on that distance, and therefore a ceiling on achievable magnification. For a BGA, a silicon wafer, or any small feature requiring high magnification, that ceiling is a real constraint.
Transmission targets
A transmission tube solves the geometry. Instead of a bulk angled target, the target is a thin layer deposited on the exit window itself. Electrons strike it and X-rays continue in the same direction, emerging from the window rather than off to the side.
Because the target is effectively at the surface of the tube, a sample can be brought extremely close to the point where X-rays are produced. That removes the gap and allows much higher magnification.
It is still a sealed tube, so it carries none of the open tube’s maintenance requirements. For small, high-magnification electronics work, this combination is very attractive.
Why transmission tubes are power limited
The limitation is thermal, and it follows directly from the geometry.
A thin target film on a window has very little material to conduct heat away from the point where the electron beam strikes it. A bulk reflection target has far more mass and a far better thermal path. Since electron beam power becomes heat at the target, the thin target reaches its limit at much lower power.
Commercially available transmission sources are generally limited to around 110 kV. That covers most electronics inspection comfortably. It does not cover dense or thick samples requiring greater penetration, and those applications need a reflection target or an open tube capable of higher power.
Choosing
The decision breaks into two questions.
Open versus sealed is about maintenance economics and power requirements. Open tubes support higher power and lower cost per failure, at the price of vacuum maintenance.
Within sealed tubes, reflection versus transmission is about magnification against penetration. Transmission targets allow the sample closer and achieve higher magnification, capped at lower power. Reflection targets handle denser samples and give up some magnification to the geometry.
Small features in thin samples point toward transmission. Dense or thick samples point toward reflection or open tubes. Most application requirements resolve fairly clearly once framed that way.
