Material Cross Section Analyses
Material cross section analyses involve the extraction of components, usually by physical cutting with an abrasive tool in a fashion that does not disturb any areas of interest. The extracted pieces are encapsulated in epoxy resin, which retains the true structure of the sample during subsequent processing.
The encapsulated samples are subjected to a series of grinding steps ranging from coarse to very fine, targeting features of interest. Care must be taken to remove damage from previous steps before hitting the target location.
Once the target location is reached, e.g. the center of a solder joint, intermediate and final polishing steps are used to achieve a sub-micron finish. A light chemical etch brings out the detail of the cross section layer structure—the sample is now ready for SEM cross section analyses and other analytical processes.
USES & LIMITATIONS OF MATERIAL CROSS SECTION ANALYSES:
- What it is great for:
- Confirming cracks, non-wetting, bridging, or other anomalies in solder joints per IPC-A-610G.
- Measuring stand-off heights and Intermetallic (IMC) layer thicknesses in BGA solder joints.
- Measuring solder joint fillet heights in Surface Mount (SMT) solder joints per IPC-A-610G.
- Measuring percent hole-fill in Through-Pin solder joints per IPC-A-610G.
- Confirming cracks, delamination, registration of internal conductors, copper plating, copper foil, and dielectric layer thickness measurements in printed circuit boards per IPC-A-600J and 6012B.
- Evaluating the internal structure of any component, substrate, or assembly.
- Can be combined with SEM/EDX for high-resolution imaging, elemental ID, and other cross sectioning failure analyses.
- Destructive test, boards cannot be returned to service.
- Only one row of solder joints or board features can be viewed in a single slice.
- Multiple slices are required to evaluate all pins on a suspect component.
- Manual technique requiring a high level of expertise for proper sample preparation. Improper preparation can produce artifacts that are easily misinterpreted.
- What it is great for:
Covalent Metrology’s technical staff has over 30 years of experience in cross section analyses.
BGA device showing minimal warpage. Solder joints are acceptable per IPC-A-610G, section 8.3.12.
BGA solder joint showing wetting and solder joint shape are acceptable per IPC-A-610G, section 8.3.12. This mixed alloy solder joint shows full mixing of Pb from the solder paste into the Pb-free solder ball.
Through-Pin solder joint showing hole-fill is <75%, which is a defect per IPC-A-610G, section 18.104.22.168.
Heel fillet crack, which is a defect per IPC-A-610G, section 5.2.9.
Inner Layer Separation, which is non-conforming per IPC-A-600J, section 3.3.6.
Plated Through-Hole (PTH) showing copper plating thickness in the center region is below the minimum requirement of 18um in IPC-6012B, Table 3-4.