With the rapid development of high-frequency and high-reliability electronic products, the limitations of traditional ENIG (Electroless Nickel Immersion Gold) surface finishes have become increasingly evident. EPIG (Electroless Palladium Immersion Gold) has emerged as an advanced alternative, especially for RF, microwave, and fine-pitch PCB applications.
This article, optimized and technically interpreted by KKPCB, presents a comparative analysis of ENIG and EPIG coated printed circuit boards after steam aging treatment, focusing on solderability, joint reliability, and storage performance. The results clearly demonstrate the technical advantages of EPIG over conventional ENIG.
1. Background: Why Compare ENIG and EPIG?
ENIG has been widely used for more than 20 years due to its flat surface, excellent planarity, and compatibility with lead-free soldering. However, ENIG relies on a nickel underlayer, which introduces several challenges:
- Nickel magnetism affects high-frequency and RF signal integrity
- Formation of Ni–Sn intermetallic compounds, which may reduce solder joint reliability
- Rapid oxidation of the nickel layer under high temperature and humidity
To address these limitations, EPIG replaces the nickel layer with electroless palladium, eliminating nickel-related risks while maintaining excellent surface flatness and solderability.
2. Key Advantages of EPIG Surface Finish
Based on experimental results and industry validation, EPIG offers the following advantages over ENIG:
- Nickel-free structure – ideal for RF, microwave, and high-speed digital PCBs
- More reliable solder joints – formation of Cu–Sn intermetallics instead of brittle Ni–Sn phases
- Superior solderability retention under harsh storage and aging conditions
- Excellent planarity – suitable for fine-pitch components and BGA assembly
These characteristics make EPIG a strong candidate for next-generation PCB surface finishes.
3. Steam Aging as an Accelerated Solderability Test
Steam aging is a proven method for accelerating PCB aging and predicting solderability shelf life.
According to Garron Morris et al.:
- 8 hours of steam aging ≈ 12 months of natural storage for lead-tin systems
- Most PCB surface finishes fail to maintain acceptable solderability after 1 hour of steam exposure
Steam aging provides a fast, economical, and reliable indicator of long-term solderability performance compared with real-time environmental testing.
4. Test Methodology (IPC J-STD-002 / J-STD-003)
4.1 Test Materials
- Surface finishes: ENIG and EPIG
- Flux composition:
- 25 ± 0.5% rosin
- 0.39 ± 0.01% diethylammonium chloride
- Isopropanol solvent
- Solder alloy: SAC305 (Sn–3.0Ag–0.5Cu)
- Soldering temperature: 255 °C
4.2 Coating Structures
ENIG Structure
- 3.75 µm electroless Ni–P layer
- 0.075–0.10 µm immersion gold layer
EPIG Structure
- 0.375 µm electroless Pd–P layer
- 0.025 µm immersion gold layer
The coating thickness was strictly controlled and kept constant throughout the testing process.
5. Solderability Evaluation Procedure
- Samples were tested as-coated and after 1–8 hours of steam aging
- Solderability was evaluated using the wetting balance (wetting equilibrium) method
- Parameters measured:
- Wetting time
- Maximum wetting force (solder joint strength)
Solder failure was defined as insufficient wetting caused by surface oxidation that could not be removed by flux.
6. Test Results and Performance Comparison
6.1 ENIG Performance After Steam Aging
- Rapid degradation after short steam exposure
- Nickel layer oxidizes quickly under high temperature and humidity
- Solderability loss observed well before 8 hours of steam aging
6.2 EPIG Performance After Steam Aging
- Minimal solderability degradation even after extended steam exposure
- Palladium layer exhibits excellent oxidation resistance
- Stable wetting behavior throughout the full 8-hour test duration
Conclusion: EPIG demonstrates a clear and significant advantage over ENIG in steam aging resistance and solderability retention.
7. Technical Explanation: Why EPIG Performs Better
The performance difference between ENIG and EPIG is mainly due to electrochemical properties:
- Nickel–Gold potential difference: Δ ≈ 1.748 V
- Palladium–Gold potential difference: Δ ≈ 0.511 V
The larger potential difference in ENIG accelerates nickel corrosion and oxidation during aging, while the smaller Pd–Au potential gap in EPIG results in a more stable and corrosion-resistant interface.
8. Implications for PCB Design and Manufacturing
For applications requiring:
- Long shelf life
- Multiple reflow cycles
- High-frequency or RF performance
- High solder joint reliability
EPIG surface finish is a superior choice compared to traditional ENIG.
9. KKPCB Engineering Recommendation
At KKPCB, we recommend EPIG surface finishing for:
- RF and microwave PCBs
- High-speed digital designs
- Fine-pitch BGA and HDI boards
- Applications exposed to harsh storage environments
Our engineering team provides ENIG, EPIG, ENEPIG, and other advanced surface finishes, supported by IPC-standard testing and strict process control.
Steam aging test results clearly confirm that EPIG coating outperforms ENIG in solderability retention, environmental resistance, and joint reliability. As electronics continue to move toward higher frequencies and higher reliability requirements, EPIG is positioned as a next-generation PCB surface finish.
KKPCB is committed to delivering technically optimized PCB solutions that meet the most demanding industry standards.
