Destructive Metallurgy
Metallurgical examination begins by cutting a printed circuit board cross section to expose internal copper layers and plated through holes. Destructive physical analysis requires grinding and polishing the specimen until the plane of interest sits directly under a microscope. Microsectioning failure analysis reveals structural flaws hidden deep inside multilayer boards after thermal stress or mechanical loading occurs.
Voids in copper plating, resin recession, and barrel cracking appear under high magnification during this inspection step. Laboratory technicians etch the exposed cross section with chemical reagents to reveal grain structures in copper deposits and intermetallic compound layers at solder interfaces. Plating thickness measurements and annular ring evaluations rely entirely on this polished plane.
Destructive preparation ends the useful life of the sample board entirely.
Thermal Stress
Solder float testing precedes the grinding phase to simulate assembly conditions on bare laminate samples. Automated reflow ovens apply aggressive heat profiles that force entrapped moisture to expand inside vulnerable laminates. Delamination and inner layer separation manifest when moisture vapor pressure exceeds the cohesive strength of epoxy resins and fiberglass weaves.
Thermal shock induces localized strain that pulls plated copper barrels apart during rapid temperature transitions. Analysts examine the polished cross section specifically for barrel cracks generated by mismatched coefficients of thermal expansion along the z axis. Evaluating these thermal anomalies distinguishes assembly process failures from base material manufacturing defects.
Acceptance Criteria
IPC standards govern minimum acceptable plating thicknesses and maximum allowable void percentages for high reliability electronics manufacturing. Plated copper barrels must maintain continuous wall thickness without localized thinning that violates specification limits. Acceptance decisions depend on quantitative measurements taken across multiple holes in the cross section plane.
Contract manufacturers face financial penalties or scrap orders when metallographic cross sections expose nonconforming internal features. Quality engineers review microsectioning failure analysis reports to verify that supplied printed circuit boards meet baseline IPC class three requirements before assembly operations proceed.
