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42 PCB007 MAGAZINE I OCTOBER 2025 Adoption of ENEPIG and Development of IPC-4556 for Improved Corrosion Performance Surface finishes are protective coatings applied to PCBs, providing a solderable surface that with- stands shipping, storage, and multiple reflow assembly operations. Numerous options are avail- able for the final finishing of PCBs, including organic solderability preservatives (OSP), immersion tin, immersion silver, electroless nickel/immersion gold (ENIG), and electroless nickel/electroless palla- dium/immersion gold (ENEPIG). Selection is based on the lowest cost option that meets the end users' application criteria. Fabricators must consider the PCB's application, assembly complexity, frequency requirements, and environmental goals. ENIG and ENEPIG surface finishes are renowned for their high corrosion resistance, long shelf life, and durability through multiple reflow assemblies, making them ideal for mission-critical applica- tions that demand long-term reliability. Despite the increased costs associated with ENEPIG, the sys- tem has gained in popularity over the past decade. This is due to both an increasing need for wire bond capability (for example, in LED 1 manufacturing) as well as the perception that adding the electroless palladium layer will eliminate the corrosion defects typically associated with ENIG. ENEPIG first gained traction in the mid-to-late 1990s as a solution to hyper-corrosion issues, such as black pad defects, commonly seen with ENIG. However, adding a palladium layer introduced new challenges, sometimes leading to more localized and aggressive corrosion events. The first edition of IPC-4556, released in January 2013, only speci- fied a minimum gold thickness of 1.2 μin. Many fabricators assumed that depositing thicker gold would improve wire bond performance. How- ever, while longer immersion times drove higher gold thickness, this increased localized corrosion. To address this, Amendment 1 (March 2016) intro- duced a maximum gold thickness limit of 2.8 μin. With corrosion remaining a critical industry con- cern, chemical suppliers responded by developing alternative gold deposition technologies aimed at reducing these risks. These reduction-assisted or hybrid gold systems utilize a chemical reducing agent to supply a portion of the electrons needed to facilitate gold deposition, thereby minimizing the amount of gold deposited by galvanic displacement and, as a result, the amount of nickel dissolution (or corrosion) required. Reduction- assisted gold systems were initially targeted for ENIG, but the development of traditional galvanic displace- ment immersion gold was sufficient to easily exceed the IPC-4552A and then B corrosion requirements. It is ENEPIG where real corrosion improvements are observed when using hybrid gold technology. These developments, and a deeper understand- ing of what constitutes a reliable ENEPIG deposit, led to the 4556A update. IPC-4556A requires PCB manufacturers to show they can accurately mea- sure and control the thickness of electroless nickel, palladium, and gold layers, as well as the phosphorus content in the nickel layer, and meet this new corro- sion standard. IPC-4556A covers traditional ENEPIG Ty p e 1 X R F g a g e st u d y fo r g o l d t h i c k n e s s w i t h C o l l i m ato r X R F. ▼ Ty p e 1 X R F g a g e st u d y fo r g o l d t h i c k n e s s w i t h p o l y- c a p i l l a r y X R F. ▼