Micro-sectioning is a powerful failure analysis technique when coupled with Optical and/or Scanning Electron Microscopy (SEM) because it allows the examination of a variety of failures which could not been seen without this destructive analysis. Some of the electronic assembly areas that can be investigated through micro-sectional analysis include component defects, thermo-mechanical failures, processing failures related to solder reflow, opens or shorts, voiding and raw material evaluations. The process of microsectioning is delineated in the following steps:

Step 1: If the sample is an assembly, some adjacent components may need to be removed as their presence may physically hinder access to the area of interest. A common tool used to remove components and section the area is a variable speed Dremel® rotary tool. A cross-section slightly larger than the area of interest is cut out to reduce damage to the adjacent areas.

Step 2: The sample is mounted (potted) to provide a matrix from which a highly polished surface can be obtained without damaging critical areas. The type of potting material will be dependant upon the sample. The EMPF uses both epoxy and acrylic mounting materials. These systems both incorporate a two part combination of resin and hardener, the ratio of which will affect the final characteristics of the mount. Epoxy is used whenever possible because of its lower shrinkage during curing and because the final mount is clear to allow viewing the internal features of the mount during polishing. The drawback of epoxy is that it requires an overnight cure as compared to the acrylic which cures in 2-4 hours.

Step 3: The section or sample is held in the appropriate orientation with either a plastic or metal clip to prevent movement of the sample during curing. The supported sample is placed into a molding cup pre-sprayed with a dry release agent to facilitate removal of the mount after curing. The potting material is carefully poured into the cup making sure to completely surround the sample and minimize entrapped air.

Step 4: The sample is placed into a vacuum for approximately five (5) minutes to assure the mounting material pulls into all the cracks and crevices and to remove any entrapped air. (Figure 6-2).

Step 5: The sample is progressively ground and polished with silicon carbide (SiC) paper of progressively smaller grit sizes that is fixed to a rotating platen. Polishing techniques can vary, but the objective is the same, to remove the scratches from the previously larger grit paper through a successively smaller grit paper. Final polishing is done with a rotating felt pad saturated with either alumina or diamond slurry.

Step 6: Optical and/or SEM imaging is used depending on the failure. (Figure 6-4).

Micro-sectioning requires development of good technique to be able to section exactly into the desired area of interest. During failure analysis, only a single sample is available for analysis. Since micro-sectioning is a destructive test, care must be taken not to destroy critical evidence.

The EMPF has the capability and experience to perform microsectional analysis of both boards and components in addition to non-destructive testing, cleanliness testing, and reliability testing. If you would like additional information please contact the EMPF helpline at 610-362-1320 or log onto the EMPF website at www.empf.org.