Meaning
Destructive sample preparation techniques involving mounting and grinding enable the high resolution inspection of internal structures and defects within a multilayer electronic assembly. This metallographic cross sectioning is the gold standard for failure analysis in the printed circuit board industry, as it allows for the direct observation of the solder joint interface and the internal copper traces. The process involves cutting a representative piece of the assembly, embedding it in a hard resin, and then grinding it down to the exact plane of interest.
This surface is then polished to a mirror-like finish, often using diamond or alumina slurries, to remove any scratches that could obscure microstructural details. Once prepared, the sample is examined under an optical microscope or a scanning electron microscope to measure intermetallic layers, check for cracks, and identify voids. This method provides the empirical evidence needed to determine the root cause of electrical failures that cannot be seen from the outside of the package.
Sample Preparation
Success in the analytical process depends heavily on the quality of the initial mounting and grinding steps. The sample must be cut carefully to avoid introducing mechanical damage, such as cracking or delamination, which could be mistaken for a manufacturing defect. Once cut, the piece is placed in a mold and covered with a liquid epoxy or acrylic resin that hardens into a solid block.
This mounting material provides the necessary support for the thin copper layers and brittle intermetallic compounds during the subsequent grinding steps. Grinding is performed using a series of silicon carbide papers with increasingly fine grit sizes to gradually approach the target area. Each step must remove the damage from the previous one, and the sample must be thoroughly cleaned between stages to prevent contamination.
Professional laboratories in manufacturing centers like Shenzhen follow strict protocols to ensure that every cross section is a true representation of the original assembly.
Microscopic Inspection
Evaluation of the polished surface involves a detailed study of the various layers and interfaces that make up the electronic interconnect. Metallographic cross sectioning allows the technician to see the Cu6Sn5 and Cu3Sn intermetallic phases and measure their thickness with high precision. It also reveals the quality of the wetting between the solder and the pad, showing any signs of non-wetting or “head-in-pillow” defects.
For multi-layer boards, this technique is used to inspect the integrity of the plated through-holes and the connection between the inner copper planes and the vias. Cracks that develop during thermal cycling or mechanical shock are clearly visible, and their path can be traced to see if they follow the intermetallic layer or pass through the bulk solder. In some cases, a chemical etchant is applied to the surface to highlight specific grain boundaries or phases, providing even more detail about the microstructural state of the metal.
Analysis Accuracy
Reliability of the findings from this technique is governed by the precision of the polishing process and the skill of the operator. A common error in metallographic cross sectioning is the “smearing” of soft solder over the harder intermetallic layers, which can hide small cracks or voids. To avoid this, the final polishing steps must be done with very light pressure and high quality abrasives.
Another issue is the “tilt error,” where the sample is not ground perfectly perpendicular to the interface, leading to an overestimation of the layer thickness. Skilled technicians use specialized fixtures and digital measuring tools to minimize these artifacts and provide accurate data for engineering reports. Despite being a destructive and time-consuming process, it remains an essential tool for the validation of new manufacturing processes and the investigation of field failures.
The results of a well executed cross section provide the definitive proof required to resolve disputes between suppliers and customers regarding the quality of a production batch.
