Meaning
Sample preparation for microstructural analysis of printed circuit boards requires the selective removal of material to reveal internal boundaries and layers. Technicians execute cross-section delineation by exposing a polished specimen to specific chemical reagents that react at different rates with metals, polymers, and glass fibres. This difference in reaction rates creates a three-dimensional topography on the surface of the specimen.
The procedure finishes when the grain structures and plating boundaries become visible under a microscope.
Etching Reaction
Chemical mixtures must be chosen to match the target metals present in the board stackup. For copper layers, a mixture of ammonium hydroxide and hydrogen peroxide is applied to the polished surface during cross-section delineation to etch the grain boundaries. This liquid acts rapidly, and an exposure of three to five seconds is sufficient to reveal the microstructural details without destroying the specimen.
Different compositions are required for nickel or gold platings.
Microscopic Inspection
Enhanced surface topography allows precise measurements of plating thickness and solder joint integrity. Once cross-section delineation has occurred, high-magnification optical microscopes or scanning electron microscopes can easily distinguish between distinct plating layers. Defects such as voids, cracks, or intermetallic compound growth stand out clearly against the etched metal.
The measurements obtained through this step are used to verify compliance with industrial quality standards, ensuring that internal connectivity is reliable and free from thermal stress fractures.
Process Restriction
Over-etching represents a major risk during the preparation of delicate multi-layer samples. If cross-section delineation proceeds too long, the reagent dissolves the features of interest and leaves a degraded surface that cannot be measured. This destruction is irreversible and requires the operator to start the mounting and polishing steps again from a new sample.
