Structural Validation
Optical cross section verification is the laboratory validation protocol that confirms internal printed circuit board architecture meets specification limits through destructive physical analysis. Independent testing laboratories execute microsection qualification to prove that multi layer registration, barrel plating thickness, and dielectric spacing withstand thermal stress without cracking. Suppliers submit coupon specimens cut from production panels, and metallographic examination reveals defects that surface inspection cannot detect.
The procedure subjects test coupons to simulated reflow soldering cycles before potting in epoxy, grinding to the target plane, and polishing to a mirror finish. Acid etching follows the polishing step to reveal grain boundaries in copper barrels and intermetallic compound formation at soldered interfaces.
Thermal Resistance
Plated through hole barrels undergo severe dimensional change during assembly soldering because the coefficient of thermal expansion along the z axis exceeds the expansion rate of copper. Microsection qualification measures barrel wall thickness and plating voids under high magnification to verify that electrodeposited copper possesses adequate ductility. Thermal stress testing at dual temperatures simulates multiple assembly passes, and subsequent cross sectioning exposes barrel cracking, corner thinning, or laminate delamination caused by excessive strain.
Laboratories evaluate the grain structure of the copper deposit to detect columnar growth or inclusion clusters that reduce elongation below acceptable minimums. Acceptance criteria demand continuous copper walls without radial cracks or excessive reduction at hole corners, ensuring electrical continuity survives repeated thermal excursions.
Acceptance Boundary
Metallographic evaluation establishes the physical limits for dielectric spacing between adjacent internal copper planes and inner layer pull back distances at drilled holes. Etch back requirements govern resin removal from glass fiber bundles, and microsection qualification confirms that chemical desmear processes leave no residual glass cloth exposure or wedge voids. Acceptance standards permit restricted amounts of inner layer separation or plating roughness, provided measured values remain within defined percentage thresholds of nominal thickness.
Rejection of the production lot occurs when structural anomalies exceed maximum allowable dimension limits, terminating further processing before component assembly begins.
