Evaluating the Tripod as an Alternative Cross-Sectioning Method

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The electronics industry has been using the epoxy puck to process the vast majority of electronics micro-sections since the 1970s. Not much has changed in that cross-sectioning method, which is now used for precision micro-sections of PCBs, PCBAs, and device packages.

Encapsulating samples in a cylindrical "puck," using either thermal-cure or room-temperature-cure epoxies, has been the standard operating procedure for cross-sectioning electronic samples for years. However, this method can be somewhat limiting in the preparation of specimens to be analyzed where there is a need for a specific angle of attack and/or a requirement to stay parallel to a datum plane. Additional disadvantages or weaknesses in this method also exist.

One particularly advantageous alternative is the tripod method. Cross-sectioning using a tripod micro-section fixture dates back to the late 1980s, but has primarily been used for silicon wafer analysis. Considering the degree of control needed for wafer and die cross-sectioning, we wanted to bring the advantages of this method to our analysis of SMT and PCB sections. Those advantages include the following:

• Easier specimen positioning and repositioning, if desired
• Reduction of bulk material (epoxy), resulting in the following:
• A faster rate of removal/less processing time
• Less grit and polish consumption
• Less removed material (less waste and reduced potential to affect the specimen)
• Minimal light distortion
• Advantages during SEM imaging

Although this manual method limits the size of the specimen and requires dedicated fixtures throughout the analysis, we find that using the tripod fixtures, in most instances, proves beneficial.

Other cross-sectioning considerations

For other cross-sectioning considerations, Foresite lead investigator Manuel Solis presented a technical article titled "Alternative Methods for Cross-Sectioning of SMT and PCB-Related Architectures" at last year's SMTA International conference. In that paper, he outlined more details on cross-sectioning techniques, including hints and recommendations related to:

• Bulk material removal
• Sample encapsulation (pre, micro and mini)
• Handling of flux residues and conformal coatings
• Grinding and lapping
• Sample cleaning
• SEM imaging