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46 PCB007 MAGAZINE I OCTOBER 2025 gold is essential for ENEPIG, it increases gold thick- ness control complexity: • Palladium thickness variation increases within-panel gold variation • Excessive palladium thickness restricts gold build-up • Palladium defects can expose nickel, concen- trating galvanic displacement and causing localised corrosion Hybrid gold technologies reduce reliance on nickel ions, enabling gold build independent of pal- ladium thickness and with significantly less cor- rosion risk, making them an attractive option for ENEPIG fabricators seeking tighter process control. Introduction to Hybrid Gold Systems Hybrid gold plating technology has been on the market for several years, but with the inclusion of corrosion evaluation and compli- ance in IPC-4556A, hybrid gold systems are now becoming increasingly attractive to fabricators to reduce the number of ENEPIG pro- duction challenges. Traditional immersion gold relies heavily on the galvanic displace- ment reaction, meaning fabricators must tightly control multiple factors, such as EN percentage of phosphorus and palladium thick- ness, to meet gold thickness specifications and pre- vent corrosion. Hybrid gold systems work differently. By incorpo- rating a chemical reduction step, they reduce reli- ance on nickel ions for deposition. This makes gold thickness less sensitive to EN percentage of phos- phorus variation and palladium uniformity, giving fabricators more process flexibility and simplifying production while still meeting specification require- ments. IPC-4556A is the first specification to for- mally recognize hybrid gold, validating its role as a viable alternative for ENEPIG fabrication. Corrosion Evaluation in IPC-4556A For the first time, ENEPIG will be evaluated using the same guidance as ENIG's corrosion grading system. In IPC-4552B for ENIG, the corrosion spec- ifications widened to four levels—0, 1, 2, and 3— allowing for a product rating of 0 and defect-free ENIG with zero evidence of corrosion. In this revi- sion, IPC-4556 has used the same guidelines from ENIG in IPC-4552B to create the first specification for corrosion on ENEPIG finishes. Under IPC-4556 section 3.6, ENEPIG corrosion acceptability and reliability follow similar points as ENIG: • Single occurrences of hyper corrosion: Not rejectable. (It is expected that if enough sam- ples are taken from a PCB, some occurrences may be observed.) • Hypercorrosion present, but does not inter- fere with solder joint formation (intermetallic compound (IMC) formation): Not rejectable. I l l u st rat i o n of i m m e rs i o n g o l d p l at i n g by g a l va n i c d i s p l a c e m e nt . ▼ I l l u st rat i o n of hy b r i d g o l d p l at i n g by c h e m i c a l re d u ct i o n . ▼ Frank Xu