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.
Barrel Wall Cracking
Structural Defect
A structural fracture within the electrodeposited copper plating of a printed circuit board through-hole interrupts electrical continuity along the vertical axis. Stress during thermal cycling causes barrel wall cracking when Z-axis thermal expansion of the surrounding resin exceeds the ductility limit of deposited copper. The crack separates the plated wall, creating intermittent open circuits or total electrical failure during operating cycles.
Standard quality evaluations measure this separation in cross-sectional microsections after assembly soldering or thermal stress testing.
Stress Mechanism
Differences in thermal expansion coefficients between epoxy resin systems and electrodeposited copper generate cyclic tensile strain along the plated cylinder. Copper deposits with low elongation percentages or thin wall plating yield under lower stress thresholds. Acid copper plating baths with organic contaminant accumulation produce brittle deposits vulnerable to thermal mechanical fatigue.
Hole drilling roughness causes localized stress concentrations that initiate microscopic fissures under thermal expansion pressures.
Failure Boundary
Microsection analysis under optical magnification confirms wall fractures within internal hole structures. Any physical rupture in the copper barrel violates acceptance criteria for rigid printed wiring boards across all IPC performance classes. Visual detection requires polishing microsection coupons to the precise hole centerline.
Thermal stress pre-conditioning accelerates latent barrel fatigue before final board acceptance testing.