Understanding cross-sectioning

By Tony Musall

Cross-sectioning is a common reliability test method used to expose inner details of circuit boards normally hidden to the eye. It is most commonly used in the electronics industry to identify manufacturing defects.

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How it works

A cross-section sample is prepared using a destructive technique, which requires that a portion of the circuit board be cut or ground away to expose the internal plane for analysis. A dry polishing process is then used on the exposed surface to prepare it for inspection. This process adds no meaningful elemental or ionic contamination, which allows for an accurate assessment of the original surface with all its layers.

 

Once the sample is cross-sectioned, it will typically be presented for optical analysis and measurement. Foresite’s test equipment includes two SEMs (Scanning Electron Microscopes), one fitted with EDS (Energy-dispersive X-ray Spectroscopy) and the other equipped with integrated XRF (iXRF). A Keyence VHX-5000 allows for visual inspection at magnification up to 2,000x.

 

PCB testing

At the PCB level, cross-sectioning can be used to evaluate a variety of factors. These include through-hole plating thickness, plating quality, solder mask thickness, step coverage, surface-plating stack-up and surface-plating thickness. IPC-A-600 (Acceptability of Printed Boards) contains an entire section on internally observable characteristics perfectly suited to a cross-sectional test.

 

PCBA testing

Several scenarios could elicit a need for cross-sectioning of PCBAs. For example, investigating the appearance and thickness of intermetallic formations in solder joints can yield valuable insight into the reflow process. Additionally, post-temperature cycle or thermal shock evaluation sheds light on compromised solder joints. Virtually any solder interconnect can be evaluated for wetting, intermetallic formation, intermetallic thickness, cracking and voiding.

 

Components

Foresite has refined the way cross-sectioning is used in the electronics industry to simplify root-cause analysis of failures—in manufacturing and beyond.

 

Our proprietary mounting, potting and polishing process allows a level of precision unavailable elsewhere. This means cracked capacitors, open resistors and other failures can be confirmed easily. Using x-ray and high-magnification imaging, we can perform evaluation of individual ball bonds or other integrated-circuit, internal features with a high degree of positional accuracy.

 

Examples of various cross-sections

Below: SEM image - 33x MAG - compliant PIN cross-section

 

SEM IMAGE - 33x MAG - COMPLIANT PIN CROSS-SECTION 

Below: Optical image - 2,000x MAG - gold wire bond

 

OPTICAL IMAGE - 2,000x MAG - GOLD WIRE BOND-1

 

Below: SEM image - 500x MAG - QFN field failure

 

SEM IMAGE - 500x MAG - QFN FIELD FAILURE

 

Below: SEM image - 40x MAG - BGA with solder thieving

 

SEM IMAGE - 40x MAG - BGA WITH SOLDER THIEVING

 

 

 

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