The Price Step between Four Layers and Six
Moving from four to six layers adds 30 to 55 percent to bare board cost through double core usage, lamination press overhead, and tighter registration yields.
Hole Aspect Ratio
Drill Geometry
Plated through hole aspect ratio is the proportional numeric calculation relating finished board thickness to drilled barrel diameter inside printed circuit board manufacturing. Mechanical drills experience high deflection forces when plunging into thick laminate stacks at standard feed rates. Thinner barrels demand specialized tungsten carbide tooling with reinforced flute geometry to prevent bit breakage during the high speed mechanical removal of glass reinforced epoxy resin.
High density interconnect designs push this dimensional boundary downward because smaller pad lands require proportionally narrower holes to maintain reliable internal copper layer clearances. Copper electroplating baths struggle to deposit uniform thickness onto the interior walls when this dimensional proportion exceeds standard throwing power limits.
Plating Deposition
Chemical reduction baths and direct metallization systems deposit a thin seed layer across nonconductive dielectrics before acid copper plating solutions build the final barrel thickness. Deep cylindrical geometries restrict mass transport of cupric ions and brightener additives down the narrow barrel during high current density electroplating cycles. Insufficient agitation inside restricted channels causes starved deposition rates near the center plane of the printed circuit board.
Acid copper solutions require organic leveling agents and optimized bath chemistry to prevent premature bridging at the outer annular rings before the center reaches minimum thickness requirements.
Reliability Verification
Microsection analysis performed after thermal stress testing evaluates internal barrel integrity by exposing cross sectional planes of plated through holes under high magnification microscopy. Thermal expansion mismatches between the z axis of the laminate material and the electrodeposited copper barrel induce severe tensile stresses during solder reflow cycles. Cracking typically initiates at the center of the barrel or along the knee where the barrel transitions to the outer surface copper foil.
Acceptance standards demand continuous copper coverage without voids, resin recession or microcracks exceeding specified dimensional limits after thermal excursion testing.