A The spatial resolution is defined as the ability to spatially differentiate the structure of objects. In case if the user is checking an object for any spatial defect, there will be a change in the adjacent position within the object. This is considered as a defect between two lines. Since each line has a geometric shadow (vignette) while viewing under the X-ray beam, you can find the defects where the geometric shadows (vignettes) of two adjacent lines overlap exactly.
 
For example, consider that there is a BGA crack between the tin ball and the pad. The pad is geometrically shaded in X-rays, whereas the BGA ball is geometrically shaded in X-rays. Hence when the geometric shadows of the BGA ball overlap exactly with the geometric shadows of the pad, the crack cannot be seen.

A Below is the list of attributes to be checked on PCB:
● Any break in the solder joint
● Any short circuit
In the case of LED, BGA, SMT, and IC chip, we check:
● Internal deformation
● if the gold wire is normal
● Off-welding
● Empty welding
● Bubble and other defects
● Soldering Void Rate
● Solder bridge

A ● Parameters of X-ray tube such as spatial resolution, target current, detail detectability, and focus spot size
● Image Detector
● Geometric magnification
● Manipulator system

A One should pay attention to the following specifications while selecting a fluorescent tube for the X-ray machine:
 
1. X-ray tube type:
Opt for the open tube or closed tube type, which is correlated with the resolution and life expectancy of the inspection devices. The more the resolution, the more will be the view of intricate and delicate details. If you inspect the target at a large scale, then it’s not an issue to choose the device with a relatively low resolution. As far as CSPs and BGAs are concerned, the resolution of 2μm or smaller is needed.
 
2. Target type:
The target type plays a vital role in influencing the distance between the sample and the X-ray tube focus. This type will eventually influence the magnifying level of inspection devices. Hence, the target type should be reflective or penetrating.
 
3. X-ray voltage and power.
The penetrating ability of the X-ray tube is always proportional to the voltage. Hence when the voltage is large, it is easier to inspect the objects with higher density and thickness. If the inspected target is single-sided boards, select the devices with low voltage. Similarly, if the inspected target is multi-layer boards, select the high voltage. At a certain voltage level, the image definition is proportional to the X-ray tube power.

A The image detector is the major part of the PCBA X-ray inspection system. It converts invisible X-rays into visible light. Hence, it is possible to view the images easily through the naked eye. By sensing the intensity of the X-ray beam passing through the object, the detector produces varied black and white values (gray levels)
on the image. That is why if the detector gets damaged, its replacement costs will be tremendous.
So, let us have a fair idea of how to select the right X-ray detector:
● Before choosing the detector, check for the best brand by searching via the internet.
● Check the flat detector of modern technology that can intensify the image.
● Try to select the amorphous silicon flat-panel detector.
● Do not opt for CMOS flat-panel digital detector.

A The following are the parameters considered for the X-ray tube:
● Voltage in KV
● Tube current and tube power
● Target current and target power
● Relationship between tube power and target power
● Target current measurement
● Focus spot size
● Spatial resolution
● Focal size VS geometric shading
● Feature resolution
● Detail detectability of the system

A There are three types of X-ray tubes listed below:
 
1. Open X-ray tube (open tube)
● Expensive
● Need regular maintenance
● Higher X-ray image magnifications
● Can self-calibrate
● Filaments and targets are the consumables
● X-rays will be transmitted through the target
2. Conventional enclosed radiographic tube (closed tube)
● Cost effective
● Maintenance-free
● The life span of the tube life will be about 8,000-10,000 hours which can be last for over 8-10years if the X-ray only be operated by sampling check even with 24 hours production.
● As the tube ages, its focus will increase followed by the blurring of the image
● Here, tubes are the consumables
● X-rays are emitted by refraction
3. New type closed tube (marketed as “NT” ray tube)
● Not so expensive
● The life span of the tube life will be about 8000 hours. After that, the tube should be replaced with the new one.
● As the tube ages, its focus will increase followed by the blurring of the image
● Here, tubes are the consumables for users. Filaments should be changed for manufacturers.
● X-rays get emitted by the transmission

A X-ray inspection systems for PCBs, BGAs, and CSPs are divided into two categories. The inspection systems may be 2D or 3D, which may be operated either online or offline. Let us go through them in detail:
 
● 2D X-ray inspection system:
 
It shows the two-dimensional images from both sides of the SMT PCBA at the same time, thereby delivering a clear picture of the board’s components. This mechanism is similar to the traditional medical X-ray, which was used to view the bone fractures. These 2D systems can be operated either online or offline.
 
● 3D X-ray inspection system:
 
It creates the three-dimensional image of a PCBA by forming a series of 2D cross-sections. This mechanism is similar to the CT scan used in the medical industry. This system also works using the Computer tomography method, where it combines the cross-sections and eliminates the images from other sections to build up a specific area precisely. The CT method of the 3D system can only work offline due to its complex algorithms. But the Computer tomography method can be used both online and offline.
 
● Online X-ray inspection system:
 
Several X-ray devices are used online(AXI) for data collection and comparison. Most of these devices are put after the reflow oven. Hence, it will be easier to process enormous quantities of complex PCBs, based on the additional cost and safety elements. However, the online operations will slow down the efficiency of the X-ray machine, thereby consuming more time and expenses. It is the slowest part of the assembly line, where the ability of the fabrication line becomes low.
 
● Offline X-ray inspection system:
 
All types of X-ray devices can be operated offline to achieve panel inspection and sampling inspection. This operation type is comparatively quick to run. It is possible to inspect PCBs conveniently at any stage in the assembly line. Hence, it is less cost-effective with higher productivity.
 
Selecting the perfect system for your operation must be done carefully. Opting for a 2D or 3D system with online or offline abilities usually depends on the measure of the target product inspected, the expected quality of the inspection, as well as the quantity of time required to complete the inspection process.
 
The X-ray inspection system with CT function is usually operated offline due to the requirement of more 2D images and complicated algorithms. Hence, this CT X-ray inspection system is used only in the less significant professional research areas. Other 2D and 3D systems were instructed with the best image at the least time in order to cut the cost of the inspection.