Inner Layer Process
(IV): AOI Inspection and Brown Oxide Treatment

After etching and stripping, the inner layer circuits are formed, but the process does not end there. The next step involves Automated Optical Inspection (AOI).
Using high-resolution cameras and image comparison software, AOI automatically detects defects such as short circuits, open circuits, nicks, or copper residues.
This inspection ensures that defects are identified before lamination, significantly reducing scrap risk.

Once the inspection is passed, the inner layer panels undergo brown oxide (or black oxide) treatment. This process forms a micro-roughened oxide layer on the copper surface to enhance adhesion with the resin during multilayer lamination.
Proper control of the chemical composition and treatment time is essential; insufficient oxidation may result in poor adhesion, while excessive oxidation can damage the copper surface.
Although AOI and brown oxide treatment appear to be concluding steps, they are in fact critical safeguards for multilayer board reliability.

These processes ensure strong interlayer bonding, prevent delamination or voids, and are indispensable parts of the inner layer fabrication process.