First flat section (just below the solder, looking up through the copper traces)
Intermediate flat section (notice solder joint geometry)
Last flat section (near the component, looking at the bottom of the part)
Most of us think instantly of cross-sectioning when sample examination is mentioned. ‘Flat’ or ‘lap’ sectioning can be a great alternative in certain situations, and is usually a good supplement to cross-section views. Flat or lap sections are essentially cross-sections that are polished in the same plane as a BGA or die, and are used to view all interconnections in relationship to each-other. As shown in the picture here, a flat section can also reveal oddly shaped solder joints that may have gone unnoticed otherwise. I like to use this technique when I suspect problems with solder mask openings. Often, you can clearly see when solder wets a trace instead of being contained on a pad properly. Flat sectioning is a notable technique, and perhaps another trick you can use to learn more about solder joint geometry in your own application.