The Importance Of X-Ray Inspection For Main PCB Assembly

 
X-Ray inspection is carried out to analyze the out-of-sight features of objects. From the medical industry to industrial control and automation, X-Ray inspection is being used in a wide range of industries today, printed electronics notwithstanding. This growing inspection technique is used specifically to inspect main PCB assembly for better quality control.
 

1. The Fundamental Principles of X-Ray Inspection

X-Rays have an exceptional edge – they are absorbed by the objects in proportion to their atomic weights. Therefore, different objects would absorb these rays differently. So heavier objects are easily processed and imaged accordingly. Lighter objects, however, are more transparent. This absorption technique of X-Rays is helpful in determining defects like short or open circuits, misalignments missing PCB electronic components.

The device for X-Ray inspection is composed of three main elements:
1. A tube that generates X-Rays
2. An operation platform that examines the sample at different angles
3. A detector that detects X-Rays and convert them into an understandable image
The X-Rays pass through the main PCB assemblies and the projection of the rays is made on the detectors. An ideal inspection system must be able to produce clear images and should have different magnification settings and ability to perform oblique angle inspection for the analysis of CSPs and BGAs. Without this functionality, the solder balls can only be inspected from top, therefore, more detailed analysis about the joints cannot be made.
 

2. Technological Advancements Propel the Need to Use Automated Inspection

Nowadays, the X-Ray Inspection or Automated X-Ray Inspection (AXI) is becoming more popular than optical inspection techniques because of the extensive usage of chip scale packages (CSP), ball grid arrays (BGA), quad flip no lead (QFNs) and flip chips which have hidden solder contacts. These unique packages are being used in various fields like aerospace, automation and industrial control. Traditional inspection methods such as thermal imaging, ultrasonic and optical inspections are rendered useless with these packages because of the main assembly of PCBs have greater density as their joints are obscured from sight. X-rays have the ability to penetrate deep into the inner surface of these packages to observe the quality of the hidden solder joints.

The soldering imperfections can be classified as:
• Dry joints because of inadequate solder
• Misalignment of the solder with respect to the pads
• Debris on the balls of BGAs
• Surplus solder
• Gas bubbles in the solder
 

3. High Reliability – Best for Harsh Environments

As we already know, PCBs are being used in harsh environmental conditions and has applications in industries that demand the utmost reliability in all kinds of equipment – like aerospace. This makes X-Ray inspection a necessity. Simply knowing the missing solder joints is not enough; ensuring quality of the main PCB assemblies is equally necessary. Additionally, X-Ray inspection is optimal for PCBs installed under the hood of a car because it does not cause mechanical or thermal stress that may cause solder joints to fail.
 

The Dire Need for 3D X-Rays

Even though X-Ray inspection devices are categorized into 2D and 3D systems, it is really the 3D X-Ray technology that is really important. This is because engineers are designing boards with multiple layers in order to tackle with the space and performance issues. For single-sided PCBs, 2D imaging works well, but because of the complexity and density of the main PCB assemblies of multilayer PCBs circuit boards, 3D X-Ray inspection is necessary for the imaging of the concealed features of all the sides.