Plating Requirement
IPC-6012 specification tables define minimum wall plating dimensions inside drilled vias to ensure electrical continuity under thermal stress. In printed circuit board fabrication, hole barrel copper thickness measures the electrodeposited copper film along the interior wall of a drilled through-hole. Primary electroless copper deposition creates an initial conductive seed layer across non-conductive glass-epoxy hole walls prior to electrolytic plating.
Plating tanks deliver copper ions that deposit onto the barrel surface, building metallic wall strength to withstand Z-axis thermal expansion during assembly reflow. Average wall plating requirements typically mandate twenty micrometers for Class 2 electronics and twenty-five micrometers for Class 3 high-reliability applications.
Deposition Control
Current density distribution inside narrow vias limits metal deposition rate deep within high aspect ratio structures. Fluid agitation pumps drive acid copper electrolyte through small holes, replacing depleted copper ions during electrolytic processing. Chemical additives including brighteners and suppressors modulate local deposition rates, suppressing dog-boning at hole knees.
Micro-voids in barrel copper arise when gas bubbles stick to drilled hole walls during electroless seeding.
Microsection Acceptance
Cross-sectional metallographic preparation provides direct physical measurement of plating thickness across multiple hole locations. Micro-section samples cut from quality coupons undergo grinding and polishing to expose copper grain boundaries under optical microscopes. Thermal stress testing subjects test coupons to float cycles in molten solder at two hundred eighty-eight degrees Celsius before micro-sectioning.
Cracks in thin barrel walls form when thermal expansion mismatch forces glass-epoxy laminates to expand vertically against weak copper deposits. Acceptance criteria reject printed wiring boards displaying copper thickness below specified absolute minimums at any single point along the hole barrel. Standard quality plans require separate measurements taken at top and bottom locations on both sides of the barrel wall.
Thin plating zones increase circuit resistance and reduce long-term fatigue life under operational temperature cycling.
