Circuit Boundary
Routing traces terminate at internal voltage planes without surface test points, creating partially accessible nets that defy standard bed of nails probing during bare board electrical verification. Automated optical equipment inspects trace width and spacing on inner layers before lamination, but functional continuity remains unverified until subsequent drilling exposes underlying copper.
Test Access
Flying probe systems navigate this limitation by targeting available vias near buried node transitions, though physical clearance limits prevent full signal tracing across high density regions. Manufacturers rely on boundary scan architectures embedded within silicon devices to shift internal node states through serial protocols, bypassing the absence of physical contact points on the outer board perimeter.
Assembly Verification
Functional test fixtures apply stimulus through edge connectors while monitoring response signatures, confirming board integrity without probing internal traces directly. Signal degradation across hidden nodes exposes manufacturing defects like micro voids in via barrels during thermal shock screening.
