Geometric Ratio
Geometric proportions define the relationship between the thickness of a base laminate and the diameter of its smallest drilled feature. Designers calculate the core aspect ratio to determine the feasibility of plating copper into the blind or through holes of a rigid substrate. A higher ratio indicates a deeper and narrower hole, which presents greater challenges for the fluid dynamics of the plating bath.
Plating Integrity
Electrochemical deposition relies on the consistent flow of ions into every cavity on the production panel. If the core aspect ratio is too aggressive, the copper plating becomes thinner in the center of the hole than on the surface, leading to a condition known as dog-boning. This variation in thickness creates weak points that may fracture during the thermal expansion of the board.
Manufacturing standards often limit the ratio to 10 to 1 for standard production, though specialized facilities can achieve 12 to 1 or higher with advanced pulse plating techniques. Successful plating requires the chemistry to reach the bottom of the hole without being blocked by trapped air or depleted solution.
Design Constraint
Reliability during assembly depends on the robustness of these vertical interconnects. When a core aspect ratio exceeds the capabilities of the fabricator, the resulting vias may suffer from barrel cracks or knee cracks during lead-free soldering. Engineers must select a core thickness and drill size that stay within the verified process window of the supplier.
This selection ensures that the final circuit can withstand the mechanical stresses of its operating environment without electrical failure.
