Boundary Scan Test Coverage Calculations and Fault Diagnostics in High Density Boards
Boundary scan test coverage calculations require explicit PCOLA-SOAMI net weighting to yield verifiable structural fault isolation in HDI board assemblies.
Microvia Open
Interconnect Discontinuity
A laser-drilled vertical channel fails to establish electrical contact between stacked copper layers in a printed circuit board because the plating chemistry fails to reach the base of the bore. This microvia open exists when the metallic bridge fails to form or fractures under thermal stress during the soldering cycle. Residual contaminants from the ablation phase block the conductive path at the landing pad interface.
Chemical cleaners often fail to remove charred debris from the blind hole geometry. High aspect ratios increase the probability that the vertical structure contains a complete break in the internal cylinder.
Fabrication Diagnostics
Automated optical inspection and electrical continuity testing detect the absence of a signal path through the copper structure. Cross-sectional analysis verifies the presence of a separation between the copper cap and the target pad by grinding through the center of the structure to reveal the fault. X-ray inspection identifies voiding within the hole that indicates a lack of solid copper deposition.
Boards pass continuity checks if the break sits within a redundant structure or if an alternate path hides the failure. Verification of the defect requires destructive sectioning to observe the internal copper morphology.
Operational Reliability
Thermal cycling induces stress on the barrel of the microvia due to the difference in the coefficient of thermal expansion between the copper plating and the dielectric substrate material. Repeated expansion causes a crack to propagate through the thin wall of the metal cylinder when the geometry experiences high temperatures. Vibration during product usage increases the mechanical load on the weak vertical connection.
Failure occurs when the electrical path disconnects during service despite passing initial room temperature continuity testing. A complete break in the vertical channel renders the affected signal line non-functional.