ENIG & ENEPIG PCB Surface Finishing Prepress Gerber/DXF Vector Guide
Master ENIG chemical kinetics, solder mask expansion offsets, and HDI PCB prepress rules.
1. ENIG and ENEPIG Surface Finishing Technology
Electroless Nickel Immersion Gold (ENIG - IPC-4552) and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG - IPC-4556) are the gold standards for flat SMT pads, fine-pitch BGA interconnections, and wire bonding pads. Unlike HASL, ENIG delivers a sub-micron planar surface essential for 0.3mm micro-BGA components.
2. Chemical Reaction Kinetics & Black Pad Prevention
In ENIG plating, hypophosphite reduces nickel ions into an amorphous Ni-P alloy matrix ($7-10\text{ wt}\% \text{ P}$). During subsequent immersion gold plating, galvanic displacement occurs:
Ni^0 + 2 Au(CN)_2^- \rightarrow Ni^{2+} + 2 Au^0 + 4 CN^-
If the gold bath is overly aggressive or the phosphorus content exceeds 11%, preferential grain boundary attack occurs, forming high-phosphorus spikes ($\text{Ni}_3\text{P}$) that cause brittle solder joint failure (Black Pad).
3. Surface Finish Comparison Matrix
| Surface Finish | Ni Thickness (µm) | Pd Thickness (µm) | Au Thickness (µm) | Wire Bond Capability | Shelf Life |
|---|---|---|---|---|---|
| Standard ENIG (IPC-4552B) | 3.0 - 6.0 | None | 0.05 - 0.10 | Al Wedge Only | 12 - 18 mos |
| ENEPIG Universal (IPC-4556) | 3.0 - 6.0 | 0.05 - 0.15 | 0.03 - 0.08 | Au & Al & Cu Wire | 24+ mos |
| Immersion Tin (ISn) | None | None | None (Sn 1.0 µm) | None | 6 - 9 mos |
| Hard Gold (Electrolytic) | 3.0 - 8.0 | None | 0.50 - 1.25 | Gold Ball Bonding | 36+ mos |
4. Prepress Gerber & DXF Rules for Solder Mask Clearance
- Solder Mask Expansion (SME): Set solder mask aperture expansion to $+35\text{ to }+50\text{ µm}$ per side over copper pad geometry to eliminate mask overlap on solderable gold surfaces.
- Solder Mask Dam: Maintain minimum $75\text{ µm}$ green mask web between adjacent pads on $\le 0.5\text{ mm}$ pitch BGAs to prevent solder bridging during reflow.
- SpotItLive Vector Studio Gerber Conversion: Convert mechanical layout DXFs into verified 1-bit binary raster masks and Gerber RS-274X aperture layers with accurate clearance swelling.
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