Extracting Out-Of-Plane Dielectric Tensor in Heterogeneous Packaging Core
Extracting out-of-plane permittivity requires guarded parallel-plate or re-entrant cavity tests to isolate vertical flux fields from in-plane glass weave effects.
Microvia Impedance
Transition Discontinuity
The characteristic impedance of a small-diameter laser-drilled blind connection depends on the diameter of the hole, the dielectric thickness between the layers, and the surrounding clearance in the reference planes. In multi-layer high-speed circuit boards, microvia impedance must be carefully controlled to prevent signal reflections at the transition between routing layers. Because these microvias are physically much shorter and narrower than traditional through-hole vias, they present a smaller capacitive discontinuity to the signal path.
They are widely used in high-density interconnect designs to routing multi-gigabit signals with minimal signal degradation.
Geometric Tuning
Optimizing the diameter of the microvia pad and the clearance size in the adjacent ground planes allows engineers to tune the capacitive and inductive properties of the vertical transition. If the clearance in the ground reference plane is too small, the excess parasitic capacitance will lower the impedance of the transition below the target value of the transmission line, which causes a signal reflection. Conversely, a clearance that is too large can disrupt the continuous return current path, which increases parasitic inductance and generates electromagnetic interference.
Fabricators utilize automated optical inspection and time-domain reflectometry on test coupons to verify that these transitions are drilled and plated with high dimensional precision across the entire production run.
Assembly Reliability
Repeated thermal cycles during lead-free assembly can induce high mechanical stress on the copper plating within these tiny transitions, which risks crack formation at the joint. Maintaining the target impedance depends on the structural integrity of this plated copper layer, which must remain uniform and free from voids. Design standards require specific aspect ratios to ensure complete chemical plating of the microvia walls.