Standard Microsection Inspection Procedures for Class Two and Class Three Annular Rings
Class 3 microsections require zero inner layer breakout and 0.050 mm minimum ring width, whereas Class 2 permits 90-degree breakout if conductor spacing holds.
Brightfield Optical Metrology
Lighting Configuration
Industrial microscopy techniques often rely on perpendicular illumination where light travels along the optical axis to strike the sample directly. In this arrangement, brightfield optical metrology utilizes a semi-reflective mirror to direct light downward through the objective lens onto the printed circuit board surface. Reflected light from flat features travels back up the same path to create a bright background, while angled or scattered rays are lost to the objective.
This contrast allows automated optical inspection systems to measure the widths of conductors and the diameters of drilled vias with high precision. Measurement algorithms then compute the boundaries based on the abrupt transitions between bright planar surfaces and dark slopes.
Defect Discrimination
Surface defects on copper pads and solder joints become visible as dark regions against a highly reflective background. When evaluating board fabrication, brightfield optical metrology exposes scratches, pits, voids and trace narrowings by highlighting deviations from the flat planar surface of the metal. This high-contrast illumination provides rapid recognition of dimensional boundaries during automated inspection.
Process Limitation
Highly reflective materials with complex three-dimensional shapes can produce severe glare that saturates digital imaging sensors. Because of this, brightfield optical metrology is less effective for inspecting highly curved solder fillets or domed ball grid array connections. Alternative illumination methods must be employed when the height profile is the primary parameter of interest.