SMT BGA Solder Joint Quality Inspection Technology (Part 1)

2025-04-25

BGA in the development of assembly processes or as an audit mechanism during production, and the inspection techniques used at different stages. The following table provides some recommendations on the applICability of inspection methods.

Method	Process Development	In - line Production	Failure Analysis	Process Audit	NPI or Small - batch Production
Optical Inspection	Excellent	Good	Excellent	Good	Good
Manual X - ray	Excellent	Good	Excellent	Good	Excellent
Automatic Transmission X - ray	Excellent	Excellent	Good	Good	Good
Automatic Cross - sectional X - ray	Excellent	Excellent	Excellent	Good	Good
Scanning Acoustic Microscope	Excellent	Fair	Good	Good	Fair
Gap Height Measurement	Fair	Fair	Good	Good	Fair
Automatic Optical Inspection	Solder Paste Volume	Component Identification, Solder Paste Volume	Not Applicable	Component Identification, Solder Paste Volume	Component Identification, Solder Paste Volume
Destructive Analysis	Good	Poor	Excellent	Fair	Fair

I. X - ray

X - ray inspection is typically used when solder joints are highly obscured and out of sight, and when a large number of solder joints are untestable. Examples of untestable solder joints are redundant connections and back - to - back BGAs, where the vias in the BGA fan - out are difficult to access and there is not enough space to set additional test points. The X - ray method can serve as a supplement to the selected testing process and provide rapid feedback to the production line. Depending on the capabilities of the X - ray system used, X - rays can detect soldering - related defects such as bridging, open solder joints, insufficient solder, and excessive solder. Other defects such as missing solder balls, misalignment, and the popcorn effect in the package can also be identified. In addition to defect detection, X - rays can also provide trend analysis of solder volume and solder joint shape. X - ray is the only non - destructive method for detecting voids in BGA solder joints.

Figure 1 below shows the principle of an X - ray device using X - rays above the sample under test; in some devices, the X - ray tube is below the sample or at an angle to the sample. The general features given in this figure are applicable to most X - ray systems. X - ray inspection has become a recognized tool for evaluating and analyzing solder joints and for monitoring the reflow soldering process. X - ray inspection technology can be most effectively applied by understanding the principles of X - ray image acquisition.

principle of an X - ray device.png

Figure 1: Principle of X - ray Device
X - rays are effective in confirming the integrity of BGA solder bonds and in monitoring the reflow soldering process. The following knowledge can be used to most effectively apply X - ray inspection technology:

Principles of X - ray image acquisition
X - ray image analysis (based on the reflow soldering process)

When using X - rays, it is necessary to pay attention to over - exposure of vulnerable materials or components. Figure 2 below shows the X - ray image characteristics of voids in the BGA connection inteRFace or missing solder balls.

BGA Connection Interface.png

Figure 2: X - ray Image Characteristics of Voids in BGA Connection Interface or Missing Solder Balls