Achieving Smooth Hole Walls to Safeguard Plating Reliability in Multi Layer PCBs

Nail heading and burr formation were highlighted as major entry-related defects in high speed PCB drilling. Yet even when entry integrity is controlled, another critical issue remains inside the drilled hole: wall smoothness. The inner wall surface is the foundation for copper plating, which creates the conductive pathways between PCB layers. If the wall is rough, smeared, or delaminated, plating coverage becomes inconsistent, resulting in voids, weak joints, and reliability failures that may not appear until late in testing or field use.

Problem: Hole Wall Roughness and Resin Smear

As drills operate at spindle speeds exceeding 100,000 RPM, tool edges gradually lose sharpness. Instead of slicing cleanly, worn edges scrape or chip the laminate, leaving uneven surfaces. These conditions cause:

• Surface delamination, where glass fibers are torn out of the resin matrix
• Resin smear, which blocks copper layers and prevents proper interconnect formation
• Plating voids, as irregular wall textures reduce copper coverage
• Impedance inconsistency and hidden defects, which only appear under thermal or electrical stress

Unlike nail heading or burrs, these problems occur deep inside the hole, making them harder to detect during routine visual inspection. Often, they are only revealed during microsection analysis or reliability testing, when correcting the defect is already costly.

How Rainbow TAC Improves Hole Wall Quality

NTI Nanofilm’s Rainbow TAC, developed with proprietary Filtered taC Coating Technology using FCVA, directly addresses the mechanical and thermal conditions that degrade hole walls.

1. Sharper Cutting Action Maintained
   With hardness near 50 GPa (~5000Hv), Rainbow TAC preserves the sharpness of cutting flutes. This allows drills to slice smoothly through resin and glass, delivering uniform wall textures with Ra values consistently below 0.1 µm.
2. Reduced Plating Voids
   Smooth, consistent hole walls create ideal surfaces for copper deposition. This ensures plating spreads evenly and eliminates weak spots, lowering the risk of voids or thin copper coverage.
3. Suppressed Resin Smear
   By keeping the friction coefficient below 0.1, Rainbow TAC minimizes localized heat. Resin remains stable and does not flow across copper layers, reducing blockage and improving plating contact.
4. Stable Coating Integrity
   FCVA deposition produces a dense, crack-free, and strongly adherent taC layer. Unlike conventional TiN or DLC coatings that can degrade mid-run, Rainbow TAC sustains wall quality throughout the drill’s full operating life.

Observed in Customer Applications Testing

Comparative drilling trials on multi-layer FR4 boards at 120,000 RPM demonstrated Rainbow TAC’s ability to deliver ultra-smooth hole walls with minimal plating risk.

Results confirmed that Rainbow TAC drills consistently produced ultra-smooth hole walls, safeguarding plating adhesion and ensuring stable performance throughout extended production runs.

The Value for PCB Fabricators

By addressing hole wall quality, Rainbow TAC provides benefits that go beyond immediate drilling performance. Manufacturers achieve:

• Stronger copper plating adhesion and continuity
• Lower risk of hidden defects that emerge during testing or in the field
• Greater confidence in IPC Class 3 compliance for high-reliability boards
• Longer tool life with consistent wall quality throughout production cycles
• Lower overall cost per hole through fewer rejects and reduced rework

Conclusion

Hole wall roughness is one of the most serious but least visible risks in PCB manufacturing. Poor wall quality leads directly to plating voids, reliability failures, and costly downstream issues. Rainbow TAC, validated through joint evaluations with customers, ensures smoother walls, cleaner plating, and more consistent electrical performance. For applications where reliability cannot be compromised, such as automotive, aerospace, and telecom, Rainbow TAC delivers the stable drilling performance needed to meet the highest standards in multilayer PCB production.