AI computing servers set strict standards for PCB signal integrity, impedance tolerance and long-term reliability. Most computing boards adopt the structure of heavy copper, POFV (plated over filled via), backdrilling and resin plugging. Non-plated through hole (NPTH) residual copper is a stubborn manufacturing defect on high-aspect-ratio PCBs.

Residual copper inside NPTH holes will trigger short circuit and SMT mounting failure. It also changes the dielectric environment around holes, leading to impedance deviation and excessive RF loss of high-speed traces.

JUEHUI WEIYE Circuit carried out full failure analysis on NPTH residual copper (hole diameter 0.6–10.0 mm) and verified mass-production improvement solutions. Finally, the defect is fully eliminated.

1. Defect Observation and Failure Analysis

NPTH (Non-Plated Through Hole) is designed for screw mounting and component positioning on server PCBs. No copper residue is allowed on the barrel wall. Different from PTH, NPTH relies on outer layer negative etching to remove copper.

CCD inspection found circular residual copper inside NPTH holes, and the copper could not be removed even after re-etching. It proves that barrier contaminants already existed on copper surface before etching.

Cross-section and EDS test confirm that the black contaminants inside holes are organic resin films, not copper debris or dust.

Trace the production sequence: contaminants are generated during the high-temperature curing stage after resin plugging, before outer layer patterning and etching.

AI computing PCBs are thick boards (2.0–10.0 mm) with aspect ratio up to 40:1. Pulse plating is mandatory for uniform heavy copper. Pulse-plated copper has higher dyne level and lower surface tension. Resin small molecules easily spread and flow down into NPTH barrels during baking, forming a thin organic barrier film and blocking etching.

2. Optimized Process & Mass Verification

Two improvement plans were tested:

Plan 1: Replace high-temperature curing resin with low-temperature curing resin. The reject rate dropped from 18.72% to 0.32%, but could not reach zero defect.

Plan 2 (Final mass solution): Low-temperature curing resin + add **horizontal desmear process right after resin plugging and before outer patterning**, to remove organic resin contaminants on hole barrels.

Optimized flow:

Panel Cutting → Inner Layer → AOI → Lamination → Drilling → Desmear → Heavy Copper Plating → Backdrilling → Resin Plugging → Horizontal Desmear → Outer Pattern → Outer Etching → AOI → Solder Mask → Profiling

Mass trial: nearly 30,000 NPTH holes from 3 part numbers were validated. The reject rate of NPTH residual copper reduced to 0%. Impedance, RF performance and reliability fully meet server customer specs.

3. Conclusion

The root cause of NPTH residual copper on AI computing PCBs: small resin molecules migrate and drip into NPTH holes during high-temperature curing after resin plugging, forming organic barrier film on pulse-plated copper surface.

Our combined solution uses low-temperature resin and extra horizontal desmear step after resin plugging, which removes organic contamination and guarantees uniform etching.

This process is suitable for heavy copper, high-aspect-ratio AI server PCBs with resin plugging, to stabilize yield.

JUEHUI WEIYE Circuit provides custom high-speed PCB, AI computing PCB, HDI and backdrilled PCB manufacturing with impedance control & IPC Class 3 capability. Contact us for quotation and technical support.