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.
Internal Annular Ring
Internal Geometry
Inner-layer conductive lands surround drilled barrel holes within multilayer printed circuit structures to establish inter-layer interconnects. The internal annular ring defines the remaining width of internal copper foil between the etched hole edge and the outer perimeter of the internal land. Solid mechanical capture ensures that plated copper barrels maintain low-resistance contact with buried circuit traces.
Layer Registration
Multilayer lamination stresses and dimensional material movement shift inner copper pattern locations relative to mechanical drill points. When drill wander or artwork misregistration degrades internal annular ring dimensions, hole breakout occurs on internal signal layers. Breakout separates the plated copper barrel from the inner conductive plane, causing intermittent open circuits or high-resistance connections under operational thermal cycling.
Desmear processing weakens damaged internal junctions further by over-etching epoxy resin around compromised pad margins. Microvoids in hole wall plating form where internal copper breakout exposes bare laminate glass fibers to plating chemistry, leading to conductive anodic filament growth over long field exposure.
Microsection Evaluation
Destructive cross-section testing verifies internal pad integrity and copper barrel attachment across multilayer coupons. Measurement of internal annular ring thickness follows standardized microsection preparation and optical microscopy protocols. Quality standards set explicit minimum ring dimensions to prevent trace disconnection in harsh operating environments.